Auto merge of #120576 - nnethercote:merge-Diagnostic-DiagnosticBuilder, r=davidtwco

Overhaul `Diagnostic` and `DiagnosticBuilder`

Implements the first part of https://github.com/rust-lang/compiler-team/issues/722, which moves functionality and use away from `Diagnostic`, onto `DiagnosticBuilder`.

Likely follow-ups:
- Move things around, because this PR was written to minimize diff size, so some things end up in sub-optimal places. E.g. `DiagnosticBuilder` has impls in both `diagnostic.rs` and `diagnostic_builder.rs`.
- Rename `Diagnostic` as `DiagInner` and `DiagnosticBuilder` as `Diag`.

r? `@davidtwco`

compiler/rustc_ast_lowering/src/errors.rs

@@ -1,5 +1,6 @@
 use rustc_errors::{
-    codes::*, AddToDiagnostic, Diagnostic, DiagnosticArgFromDisplay, SubdiagnosticMessageOp,
+    codes::*, AddToDiagnostic, DiagnosticArgFromDisplay, DiagnosticBuilder, EmissionGuarantee,
+    SubdiagnosticMessageOp,
 };
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{symbol::Ident, Span, Symbol};
@@ -41,7 +42,11 @@ pub struct InvalidAbi {
 pub struct InvalidAbiReason(pub &'static str);
 
 impl AddToDiagnostic for InvalidAbiReason {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _: F,
+    ) {
         #[allow(rustc::untranslatable_diagnostic)]
         diag.note(self.0);
     }

compiler/rustc_ast_passes/src/errors.rs

@@ -1,7 +1,10 @@
 //! Errors emitted by ast_passes.
 
 use rustc_ast::ParamKindOrd;
-use rustc_errors::{codes::*, AddToDiagnostic, Applicability, Diagnostic, SubdiagnosticMessageOp};
+use rustc_errors::{
+    codes::*, AddToDiagnostic, Applicability, DiagnosticBuilder, EmissionGuarantee,
+    SubdiagnosticMessageOp,
+};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{symbol::Ident, Span, Symbol};
 
@@ -372,7 +375,11 @@ pub struct EmptyLabelManySpans(pub Vec<Span>);
 
 // The derive for `Vec<Span>` does multiple calls to `span_label`, adding commas between each
 impl AddToDiagnostic for EmptyLabelManySpans {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _: F,
+    ) {
         diag.span_labels(self.0, "");
     }
 }
@@ -729,7 +736,11 @@ pub struct StableFeature {
 }
 
 impl AddToDiagnostic for StableFeature {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _: F,
+    ) {
         diag.arg("name", self.name);
         diag.arg("since", self.since);
         diag.help(fluent::ast_passes_stable_since);

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -22,6 +22,9 @@ macro_rules! gate {
     }};
     ($visitor:expr, $feature:ident, $span:expr, $explain:expr, $help:expr) => {{
         if !$visitor.features.$feature && !$span.allows_unstable(sym::$feature) {
+            // FIXME: make this translatable
+            #[allow(rustc::diagnostic_outside_of_impl)]
+            #[allow(rustc::untranslatable_diagnostic)]
             feature_err(&$visitor.sess, sym::$feature, $span, $explain).with_help($help).emit();
         }
     }};

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -6,9 +6,7 @@
 use either::Either;
 use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::FxIndexSet;
-use rustc_errors::{
+use rustc_errors::{codes::*, struct_span_code_err, Applicability, DiagnosticBuilder, MultiSpan};
-    codes::*, struct_span_code_err, Applicability, Diagnostic, DiagnosticBuilder, MultiSpan,
-};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::intravisit::{walk_block, walk_expr, Visitor};
@@ -635,7 +633,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
     fn suggest_assign_value(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         moved_place: PlaceRef<'tcx>,
         sugg_span: Span,
     ) {
@@ -674,7 +672,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
     fn suggest_borrow_fn_like(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         ty: Ty<'tcx>,
         move_sites: &[MoveSite],
         value_name: &str,
@@ -742,7 +740,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
     fn suggest_cloning(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         ty: Ty<'tcx>,
         expr: &hir::Expr<'_>,
         span: Span,
@@ -778,7 +776,12 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         }
     }
 
-    fn suggest_adding_copy_bounds(&self, err: &mut Diagnostic, ty: Ty<'tcx>, span: Span) {
+    fn suggest_adding_copy_bounds(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        ty: Ty<'tcx>,
+        span: Span,
+    ) {
         let tcx = self.infcx.tcx;
         let generics = tcx.generics_of(self.mir_def_id());
 
@@ -1225,7 +1228,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     #[instrument(level = "debug", skip(self, err))]
     fn suggest_using_local_if_applicable(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         location: Location,
         issued_borrow: &BorrowData<'tcx>,
         explanation: BorrowExplanation<'tcx>,
@@ -1321,7 +1324,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
     fn suggest_slice_method_if_applicable(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         place: Place<'tcx>,
         borrowed_place: Place<'tcx>,
     ) {
@@ -1430,7 +1433,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     /// ```
     pub(crate) fn explain_iterator_advancement_in_for_loop_if_applicable(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         span: Span,
         issued_spans: &UseSpans<'tcx>,
     ) {
@@ -1617,7 +1620,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
     /// ```
     fn suggest_using_closure_argument_instead_of_capture(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         borrowed_place: Place<'tcx>,
         issued_spans: &UseSpans<'tcx>,
     ) {
@@ -1751,7 +1754,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
     fn suggest_binding_for_closure_capture_self(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         issued_spans: &UseSpans<'tcx>,
     ) {
         let UseSpans::ClosureUse { capture_kind_span, .. } = issued_spans else { return };
@@ -2997,7 +3000,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         self.buffer_error(err);
     }
 
-    fn explain_deref_coercion(&mut self, loan: &BorrowData<'tcx>, err: &mut Diagnostic) {
+    fn explain_deref_coercion(&mut self, loan: &BorrowData<'tcx>, err: &mut DiagnosticBuilder<'_>) {
         let tcx = self.infcx.tcx;
         if let (
             Some(Terminator {
@@ -3532,7 +3535,11 @@ enum AnnotatedBorrowFnSignature<'tcx> {
 impl<'tcx> AnnotatedBorrowFnSignature<'tcx> {
     /// Annotate the provided diagnostic with information about borrow from the fn signature that
     /// helps explain.
-    pub(crate) fn emit(&self, cx: &mut MirBorrowckCtxt<'_, 'tcx>, diag: &mut Diagnostic) -> String {
+    pub(crate) fn emit(
+        &self,
+        cx: &mut MirBorrowckCtxt<'_, 'tcx>,
+        diag: &mut DiagnosticBuilder<'_>,
+    ) -> String {
         match self {
             &AnnotatedBorrowFnSignature::Closure { argument_ty, argument_span } => {
                 diag.span_label(

compiler/rustc_borrowck/src/diagnostics/explain_borrow.rs

@@ -3,7 +3,7 @@
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
 
-use rustc_errors::{Applicability, Diagnostic};
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_hir::intravisit::Visitor;
 use rustc_index::IndexSlice;
@@ -65,7 +65,7 @@ impl<'tcx> BorrowExplanation<'tcx> {
         tcx: TyCtxt<'tcx>,
         body: &Body<'tcx>,
         local_names: &IndexSlice<Local, Option<Symbol>>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         borrow_desc: &str,
         borrow_span: Option<Span>,
         multiple_borrow_span: Option<(Span, Span)>,
@@ -306,7 +306,7 @@ impl<'tcx> BorrowExplanation<'tcx> {
     fn add_object_lifetime_default_note(
         &self,
         tcx: TyCtxt<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         unsize_ty: Ty<'tcx>,
     ) {
         if let ty::Adt(def, args) = unsize_ty.kind() {
@@ -359,7 +359,7 @@ impl<'tcx> BorrowExplanation<'tcx> {
 
     fn add_lifetime_bound_suggestion_to_diagnostic(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         category: &ConstraintCategory<'tcx>,
         span: Span,
         region_name: &RegionName,

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -5,7 +5,7 @@ use crate::session_diagnostics::{
     CaptureVarKind, CaptureVarPathUseCause, OnClosureNote,
 };
 use itertools::Itertools;
-use rustc_errors::{Applicability, Diagnostic};
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, Namespace};
 use rustc_hir::CoroutineKind;
@@ -80,7 +80,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
         &self,
         location: Location,
         place: PlaceRef<'tcx>,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
     ) -> bool {
         debug!("add_moved_or_invoked_closure_note: location={:?} place={:?}", location, place);
         let mut target = place.local_or_deref_local();
@@ -588,7 +588,7 @@ impl UseSpans<'_> {
     pub(super) fn args_subdiag(
         self,
         dcx: &rustc_errors::DiagCtxt,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         f: impl FnOnce(Span) -> CaptureArgLabel,
     ) {
         if let UseSpans::ClosureUse { args_span, .. } = self {
@@ -601,7 +601,7 @@ impl UseSpans<'_> {
     pub(super) fn var_path_only_subdiag(
         self,
         dcx: &rustc_errors::DiagCtxt,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         action: crate::InitializationRequiringAction,
     ) {
         use crate::InitializationRequiringAction::*;
@@ -638,7 +638,7 @@ impl UseSpans<'_> {
     pub(super) fn var_subdiag(
         self,
         dcx: &rustc_errors::DiagCtxt,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         kind: Option<rustc_middle::mir::BorrowKind>,
         f: impl FnOnce(hir::ClosureKind, Span) -> CaptureVarCause,
     ) {
@@ -1010,7 +1010,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
 
     fn explain_captures(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         span: Span,
         move_span: Span,
         move_spans: UseSpans<'tcx>,

compiler/rustc_borrowck/src/diagnostics/move_errors.rs

@@ -1,7 +1,7 @@
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
 
-use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder};
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, Ty};
 use rustc_mir_dataflow::move_paths::{LookupResult, MovePathIndex};
@@ -437,7 +437,12 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         err
     }
 
-    fn add_move_hints(&self, error: GroupedMoveError<'tcx>, err: &mut Diagnostic, span: Span) {
+    fn add_move_hints(
+        &self,
+        error: GroupedMoveError<'tcx>,
+        err: &mut DiagnosticBuilder<'_>,
+        span: Span,
+    ) {
         match error {
             GroupedMoveError::MovesFromPlace { mut binds_to, move_from, .. } => {
                 self.add_borrow_suggestions(err, span);
@@ -500,7 +505,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         }
     }
 
-    fn add_borrow_suggestions(&self, err: &mut Diagnostic, span: Span) {
+    fn add_borrow_suggestions(&self, err: &mut DiagnosticBuilder<'_>, span: Span) {
         match self.infcx.tcx.sess.source_map().span_to_snippet(span) {
             Ok(snippet) if snippet.starts_with('*') => {
                 err.span_suggestion_verbose(
@@ -521,7 +526,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         }
     }
 
-    fn add_move_error_suggestions(&self, err: &mut Diagnostic, binds_to: &[Local]) {
+    fn add_move_error_suggestions(&self, err: &mut DiagnosticBuilder<'_>, binds_to: &[Local]) {
         let mut suggestions: Vec<(Span, String, String)> = Vec::new();
         for local in binds_to {
             let bind_to = &self.body.local_decls[*local];
@@ -573,7 +578,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         }
     }
 
-    fn add_move_error_details(&self, err: &mut Diagnostic, binds_to: &[Local]) {
+    fn add_move_error_details(&self, err: &mut DiagnosticBuilder<'_>, binds_to: &[Local]) {
         for (j, local) in binds_to.iter().enumerate() {
             let bind_to = &self.body.local_decls[*local];
             let binding_span = bind_to.source_info.span;
@@ -610,7 +615,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
     /// expansion of a packed struct.
     /// Such errors happen because derive macro expansions shy away from taking
     /// references to the struct's fields since doing so would be undefined behaviour
-    fn add_note_for_packed_struct_derive(&self, err: &mut Diagnostic, local: Local) {
+    fn add_note_for_packed_struct_derive(&self, err: &mut DiagnosticBuilder<'_>, local: Local) {
         let local_place: PlaceRef<'tcx> = local.into();
         let local_ty = local_place.ty(self.body.local_decls(), self.infcx.tcx).ty.peel_refs();
 

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -2,7 +2,7 @@
 #![allow(rustc::untranslatable_diagnostic)]
 
 use hir::ExprKind;
-use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder};
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_hir::intravisit::Visitor;
 use rustc_hir::Node;
@@ -540,7 +540,12 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         }
     }
 
-    fn suggest_map_index_mut_alternatives(&self, ty: Ty<'tcx>, err: &mut Diagnostic, span: Span) {
+    fn suggest_map_index_mut_alternatives(
+        &self,
+        ty: Ty<'tcx>,
+        err: &mut DiagnosticBuilder<'tcx>,
+        span: Span,
+    ) {
         let Some(adt) = ty.ty_adt_def() else { return };
         let did = adt.did();
         if self.infcx.tcx.is_diagnostic_item(sym::HashMap, did)
@@ -548,7 +553,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         {
             struct V<'a, 'tcx> {
                 assign_span: Span,
-                err: &'a mut Diagnostic,
+                err: &'a mut DiagnosticBuilder<'tcx>,
                 ty: Ty<'tcx>,
                 suggested: bool,
             }
@@ -790,7 +795,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         tcx: TyCtxt<'_>,
         closure_local_def_id: hir::def_id::LocalDefId,
         the_place_err: PlaceRef<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
     ) {
         let tables = tcx.typeck(closure_local_def_id);
         if let Some((span, closure_kind_origin)) = tcx.closure_kind_origin(closure_local_def_id) {
@@ -852,7 +857,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 
     // Attempt to search similar mutable associated items for suggestion.
     // In the future, attempt in all path but initially for RHS of for_loop
-    fn suggest_similar_mut_method_for_for_loop(&self, err: &mut Diagnostic, span: Span) {
+    fn suggest_similar_mut_method_for_for_loop(&self, err: &mut DiagnosticBuilder<'_>, span: Span) {
         use hir::{
             BorrowKind, Expr,
             ExprKind::{AddrOf, Block, Call, MethodCall},
@@ -936,7 +941,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
     }
 
     /// Targeted error when encountering an `FnMut` closure where an `Fn` closure was expected.
-    fn expected_fn_found_fn_mut_call(&self, err: &mut Diagnostic, sp: Span, act: &str) {
+    fn expected_fn_found_fn_mut_call(&self, err: &mut DiagnosticBuilder<'_>, sp: Span, act: &str) {
         err.span_label(sp, format!("cannot {act}"));
 
         let hir = self.infcx.tcx.hir();

compiler/rustc_borrowck/src/diagnostics/outlives_suggestion.rs

@@ -5,7 +5,7 @@
 #![allow(rustc::untranslatable_diagnostic)]
 
 use rustc_data_structures::fx::FxIndexSet;
-use rustc_errors::Diagnostic;
+use rustc_errors::DiagnosticBuilder;
 use rustc_middle::ty::RegionVid;
 use smallvec::SmallVec;
 use std::collections::BTreeMap;
@@ -158,13 +158,13 @@ impl OutlivesSuggestionBuilder {
         self.constraints_to_add.entry(fr).or_default().push(outlived_fr);
     }
 
-    /// Emit an intermediate note on the given `Diagnostic` if the involved regions are
+    /// Emit an intermediate note on the given `DiagnosticBuilder` if the involved regions are
     /// suggestable.
     pub(crate) fn intermediate_suggestion(
         &mut self,
         mbcx: &MirBorrowckCtxt<'_, '_>,
         errci: &ErrorConstraintInfo<'_>,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
     ) {
         // Emit an intermediate note.
         let fr_name = self.region_vid_to_name(mbcx, errci.fr);

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -1,7 +1,7 @@
 //! Error reporting machinery for lifetime errors.
 
 use rustc_data_structures::fx::FxIndexSet;
-use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, MultiSpan};
+use rustc_errors::{Applicability, DiagnosticBuilder, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def::Res::Def;
 use rustc_hir::def_id::DefId;
@@ -251,6 +251,9 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 
         hrtb_bounds.iter().for_each(|bound| {
             let Trait(PolyTraitRef { trait_ref, span: trait_span, .. }, _) = bound else { return; };
+            // FIXME: make this translatable
+            #[allow(rustc::diagnostic_outside_of_impl)]
+            #[allow(rustc::untranslatable_diagnostic)]
             diag.span_note(
                 *trait_span,
                 "due to current limitations in the borrow checker, this implies a `'static` lifetime"
@@ -421,6 +424,9 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
     /// ```
     ///
     /// Here we would be invoked with `fr = 'a` and `outlived_fr = 'b`.
+    // FIXME: make this translatable
+    #[allow(rustc::diagnostic_outside_of_impl)]
+    #[allow(rustc::untranslatable_diagnostic)]
     pub(crate) fn report_region_error(
         &mut self,
         fr: RegionVid,
@@ -685,12 +691,18 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             borrowck_errors::borrowed_data_escapes_closure(self.infcx.tcx, *span, escapes_from);
 
         if let Some((Some(outlived_fr_name), outlived_fr_span)) = outlived_fr_name_and_span {
+            // FIXME: make this translatable
+            #[allow(rustc::diagnostic_outside_of_impl)]
+            #[allow(rustc::untranslatable_diagnostic)]
             diag.span_label(
                 outlived_fr_span,
                 format!("`{outlived_fr_name}` declared here, outside of the {escapes_from} body",),
             );
         }
 
+        // FIXME: make this translatable
+        #[allow(rustc::diagnostic_outside_of_impl)]
+        #[allow(rustc::untranslatable_diagnostic)]
         if let Some((Some(fr_name), fr_span)) = fr_name_and_span {
             diag.span_label(
                 fr_span,
@@ -714,6 +726,9 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                 let outlived_fr_region_name = self.give_region_a_name(errci.outlived_fr).unwrap();
                 outlived_fr_region_name.highlight_region_name(&mut diag);
 
+                // FIXME: make this translatable
+                #[allow(rustc::diagnostic_outside_of_impl)]
+                #[allow(rustc::untranslatable_diagnostic)]
                 diag.span_label(
                     *span,
                     format!(
@@ -808,7 +823,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
     /// ```
     fn add_static_impl_trait_suggestion(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         fr: RegionVid,
         // We need to pass `fr_name` - computing it again will label it twice.
         fr_name: RegionName,
@@ -897,7 +912,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 
     fn maybe_suggest_constrain_dyn_trait_impl(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         f: Region<'tcx>,
         o: Region<'tcx>,
         category: &ConstraintCategory<'tcx>,
@@ -959,7 +974,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
     #[instrument(skip(self, err), level = "debug")]
     fn suggest_constrain_dyn_trait_in_impl(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         found_dids: &FxIndexSet<DefId>,
         ident: Ident,
         self_ty: &hir::Ty<'_>,
@@ -994,7 +1009,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 
     fn suggest_adding_lifetime_params(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         sub: RegionVid,
         sup: RegionVid,
     ) {
@@ -1023,7 +1038,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
         suggest_adding_lifetime_params(self.infcx.tcx, sub, ty_sup, ty_sub, diag);
     }
 
-    fn suggest_move_on_borrowing_closure(&self, diag: &mut Diagnostic) {
+    fn suggest_move_on_borrowing_closure(&self, diag: &mut DiagnosticBuilder<'_>) {
         let map = self.infcx.tcx.hir();
         let body_id = map.body_owned_by(self.mir_def_id());
         let expr = &map.body(body_id).value.peel_blocks();

compiler/rustc_borrowck/src/diagnostics/region_name.rs

@@ -5,7 +5,7 @@ use std::fmt::{self, Display};
 use std::iter;
 
 use rustc_data_structures::fx::IndexEntry;
-use rustc_errors::Diagnostic;
+use rustc_errors::DiagnosticBuilder;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_middle::ty::print::RegionHighlightMode;
@@ -106,7 +106,7 @@ impl RegionName {
         }
     }
 
-    pub(crate) fn highlight_region_name(&self, diag: &mut Diagnostic) {
+    pub(crate) fn highlight_region_name(&self, diag: &mut DiagnosticBuilder<'_>) {
         match &self.source {
             RegionNameSource::NamedLateParamRegion(span)
             | RegionNameSource::NamedEarlyParamRegion(span) => {

compiler/rustc_borrowck/src/lib.rs

@@ -2497,6 +2497,8 @@ mod diags {
             }
             for (_, (mut diag, count)) in std::mem::take(&mut self.diags.buffered_mut_errors) {
                 if count > 10 {
+                    #[allow(rustc::diagnostic_outside_of_impl)]
+                    #[allow(rustc::untranslatable_diagnostic)]
                     diag.note(format!("...and {} other attempted mutable borrows", count - 10));
                 }
                 self.diags.buffered_diags.push(BufferedDiag::Error(diag));

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -5,7 +5,7 @@ use rustc_data_structures::binary_search_util;
 use rustc_data_structures::frozen::Frozen;
 use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_data_structures::graph::scc::Sccs;
-use rustc_errors::Diagnostic;
+use rustc_errors::DiagnosticBuilder;
 use rustc_hir::def_id::CRATE_DEF_ID;
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_infer::infer::outlives::test_type_match;
@@ -592,7 +592,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     }
 
     /// Adds annotations for `#[rustc_regions]`; see `UniversalRegions::annotate`.
-    pub(crate) fn annotate(&self, tcx: TyCtxt<'tcx>, err: &mut Diagnostic) {
+    pub(crate) fn annotate(&self, tcx: TyCtxt<'tcx>, err: &mut DiagnosticBuilder<'_, ()>) {
         self.universal_regions.annotate(tcx, err)
     }
 

compiler/rustc_borrowck/src/universal_regions.rs

@@ -16,7 +16,7 @@
 #![allow(rustc::untranslatable_diagnostic)]
 
 use rustc_data_structures::fx::FxIndexMap;
-use rustc_errors::Diagnostic;
+use rustc_errors::DiagnosticBuilder;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::BodyOwnerKind;
@@ -343,7 +343,7 @@ impl<'tcx> UniversalRegions<'tcx> {
     /// that this region imposes on others. The methods in this file
     /// handle the part about dumping the inference context internal
     /// state.
-    pub(crate) fn annotate(&self, tcx: TyCtxt<'tcx>, err: &mut Diagnostic) {
+    pub(crate) fn annotate(&self, tcx: TyCtxt<'tcx>, err: &mut DiagnosticBuilder<'_, ()>) {
         match self.defining_ty {
             DefiningTy::Closure(def_id, args) => {
                 let v = with_no_trimmed_paths!(

compiler/rustc_builtin_macros/src/errors.rs

@@ -1,6 +1,6 @@
 use rustc_errors::{
-    codes::*, AddToDiagnostic, DiagCtxt, Diagnostic, DiagnosticBuilder, EmissionGuarantee,
+    codes::*, AddToDiagnostic, DiagCtxt, DiagnosticBuilder, EmissionGuarantee, IntoDiagnostic,
-    IntoDiagnostic, Level, MultiSpan, SingleLabelManySpans, SubdiagnosticMessageOp,
+    Level, MultiSpan, SingleLabelManySpans, SubdiagnosticMessageOp,
 };
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{symbol::Ident, Span, Symbol};
@@ -611,7 +611,11 @@ pub(crate) struct FormatUnusedArg {
 // Allow the singular form to be a subdiagnostic of the multiple-unused
 // form of diagnostic.
 impl AddToDiagnostic for FormatUnusedArg {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, f: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        f: F,
+    ) {
         diag.arg("named", self.named);
         let msg = f(diag, crate::fluent_generated::builtin_macros_format_unused_arg.into());
         diag.span_label(self.span, msg);

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -1856,9 +1856,7 @@ impl SharedEmitterMain {
                 Ok(SharedEmitterMessage::Diagnostic(diag)) => {
                     let dcx = sess.dcx();
                     let mut d = rustc_errors::Diagnostic::new_with_messages(diag.lvl, diag.msgs);
-                    if let Some(code) = diag.code {
+                    d.code = diag.code; // may be `None`, that's ok
-                        d.code(code);
-                    }
                     d.replace_args(diag.args);
                     dcx.emit_diagnostic(d);
                 }

compiler/rustc_const_eval/src/transform/check_consts/ops.rs

@@ -93,6 +93,9 @@ pub struct FnCallNonConst<'tcx> {
 }
 
 impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
+    // FIXME: make this translatable
+    #[allow(rustc::diagnostic_outside_of_impl)]
+    #[allow(rustc::untranslatable_diagnostic)]
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, _: Span) -> DiagnosticBuilder<'tcx> {
         let FnCallNonConst { caller, callee, args, span, call_source, feature } = *self;
         let ConstCx { tcx, param_env, .. } = *ccx;
@@ -321,6 +324,8 @@ impl<'tcx> NonConstOp<'tcx> for FnCallUnstable {
             .dcx()
             .create_err(errors::UnstableConstFn { span, def_path: ccx.tcx.def_path_str(def_id) });
 
+        // FIXME: make this translatable
+        #[allow(rustc::untranslatable_diagnostic)]
         if ccx.is_const_stable_const_fn() {
             err.help("const-stable functions can only call other const-stable functions");
         } else if ccx.tcx.sess.is_nightly_build() {
@@ -591,6 +596,8 @@ impl<'tcx> NonConstOp<'tcx> for StaticAccess {
             span,
             format!("referencing statics in {}s is unstable", ccx.const_kind(),),
         );
+        // FIXME: make this translatable
+        #[allow(rustc::untranslatable_diagnostic)]
         err
             .note("`static` and `const` variables can refer to other `const` variables. A `const` variable, however, cannot refer to a `static` variable.")
             .help("to fix this, the value can be extracted to a `const` and then used.");

compiler/rustc_errors/src/diagnostic.rs

@@ -12,6 +12,7 @@ use rustc_span::{Span, DUMMY_SP};
 use std::borrow::Cow;
 use std::fmt::{self, Debug};
 use std::hash::{Hash, Hasher};
+use std::ops::{Deref, DerefMut};
 use std::panic::Location;
 
 /// Error type for `Diagnostic`'s `suggestions` field, indicating that
@@ -71,17 +72,21 @@ where
     Self: Sized,
 {
     /// Add a subdiagnostic to an existing diagnostic.
-    fn add_to_diagnostic(self, diag: &mut Diagnostic) {
+    fn add_to_diagnostic<G: EmissionGuarantee>(self, diag: &mut DiagnosticBuilder<'_, G>) {
         self.add_to_diagnostic_with(diag, |_, m| m);
     }
 
     /// Add a subdiagnostic to an existing diagnostic where `f` is invoked on every message used
     /// (to optionally perform eager translation).
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, f: F);
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        f: F,
+    );
 }
 
-pub trait SubdiagnosticMessageOp =
+pub trait SubdiagnosticMessageOp<G> =
-    Fn(&mut Diagnostic, SubdiagnosticMessage) -> SubdiagnosticMessage;
+    Fn(&mut DiagnosticBuilder<'_, G>, SubdiagnosticMessage) -> SubdiagnosticMessage;
 
 /// Trait implemented by lint types. This should not be implemented manually. Instead, use
 /// `#[derive(LintDiagnostic)]` -- see [rustc_macros::LintDiagnostic].
@@ -93,6 +98,10 @@ pub trait DecorateLint<'a, G: EmissionGuarantee> {
     fn msg(&self) -> DiagnosticMessage;
 }
 
+/// The main part of a diagnostic. Note that `DiagnosticBuilder`, which wraps
+/// this type, is used for most operations, and should be used instead whenever
+/// possible. This type should only be used when `DiagnosticBuilder`'s lifetime
+/// causes difficulties, e.g. when storing diagnostics within `DiagCtxt`.
 #[must_use]
 #[derive(Clone, Debug, Encodable, Decodable)]
 pub struct Diagnostic {
@@ -289,6 +298,90 @@ impl Diagnostic {
         }
     }
 
+    // See comment on `DiagnosticBuilder::subdiagnostic_message_to_diagnostic_message`.
+    pub(crate) fn subdiagnostic_message_to_diagnostic_message(
+        &self,
+        attr: impl Into<SubdiagnosticMessage>,
+    ) -> DiagnosticMessage {
+        let msg =
+            self.messages.iter().map(|(msg, _)| msg).next().expect("diagnostic with no messages");
+        msg.with_subdiagnostic_message(attr.into())
+    }
+
+    pub(crate) fn sub(
+        &mut self,
+        level: Level,
+        message: impl Into<SubdiagnosticMessage>,
+        span: MultiSpan,
+    ) {
+        let sub = SubDiagnostic {
+            level,
+            messages: vec![(
+                self.subdiagnostic_message_to_diagnostic_message(message),
+                Style::NoStyle,
+            )],
+            span,
+        };
+        self.children.push(sub);
+    }
+
+    pub(crate) fn arg(&mut self, name: impl Into<DiagnosticArgName>, arg: impl IntoDiagnosticArg) {
+        self.args.insert(name.into(), arg.into_diagnostic_arg());
+    }
+
+    pub fn args(&self) -> impl Iterator<Item = DiagnosticArg<'_>> {
+        self.args.iter()
+    }
+
+    pub fn replace_args(&mut self, args: FxIndexMap<DiagnosticArgName, DiagnosticArgValue>) {
+        self.args = args;
+    }
+}
+
+/// `DiagnosticBuilder` impls many `&mut self -> &mut Self` methods. Each one
+/// modifies an existing diagnostic, either in a standalone fashion, e.g.
+/// `err.code(code);`, or in a chained fashion to make multiple modifications,
+/// e.g. `err.code(code).span(span);`.
+///
+/// This macro creates an equivalent `self -> Self` method, with a `with_`
+/// prefix. This can be used in a chained fashion when making a new diagnostic,
+/// e.g. `let err = struct_err(msg).with_code(code);`, or emitting a new
+/// diagnostic, e.g. `struct_err(msg).with_code(code).emit();`.
+///
+/// Although the latter method can be used to modify an existing diagnostic,
+/// e.g. `err = err.with_code(code);`, this should be avoided because the former
+/// method gives shorter code, e.g. `err.code(code);`.
+///
+/// Note: the `with_` methods are added only when needed. If you want to use
+/// one and it's not defined, feel free to add it.
+///
+/// Note: any doc comments must be within the `with_fn!` call.
+macro_rules! with_fn {
+    {
+        $with_f:ident,
+        $(#[$attrs:meta])*
+        pub fn $f:ident(&mut $self:ident, $($name:ident: $ty:ty),* $(,)?) -> &mut Self {
+            $($body:tt)*
+        }
+    } => {
+        // The original function.
+        $(#[$attrs])*
+        #[doc = concat!("See [`DiagnosticBuilder::", stringify!($f), "()`].")]
+        pub fn $f(&mut $self, $($name: $ty),*) -> &mut Self {
+            $($body)*
+        }
+
+        // The `with_*` variant.
+        $(#[$attrs])*
+        #[doc = concat!("See [`DiagnosticBuilder::", stringify!($f), "()`].")]
+        pub fn $with_f(mut $self, $($name: $ty),*) -> Self {
+            $self.$f($($name),*);
+            $self
+        }
+    };
+}
+
+impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
     /// Delay emission of this diagnostic as a bug.
     ///
     /// This can be useful in contexts where an error indicates a bug but
@@ -309,6 +402,7 @@ impl Diagnostic {
         self.level = Level::DelayedBug;
     }
 
+    with_fn! { with_span_label,
     /// Appends a labeled span to the diagnostic.
     ///
     /// Labels are used to convey additional context for the diagnostic's primary span. They will
@@ -323,10 +417,12 @@ impl Diagnostic {
     /// primary.
     #[rustc_lint_diagnostics]
     pub fn span_label(&mut self, span: Span, label: impl Into<SubdiagnosticMessage>) -> &mut Self {
-        self.span.push_span_label(span, self.subdiagnostic_message_to_diagnostic_message(label));
+        let msg = self.subdiagnostic_message_to_diagnostic_message(label);
+        self.span.push_span_label(span, msg);
         self
-    }
+    } }
 
+    with_fn! { with_span_labels,
     /// Labels all the given spans with the provided label.
     /// See [`Self::span_label()`] for more information.
     pub fn span_labels(&mut self, spans: impl IntoIterator<Item = Span>, label: &str) -> &mut Self {
@@ -334,7 +430,7 @@ impl Diagnostic {
             self.span_label(span, label.to_string());
         }
         self
-    }
+    } }
 
     pub fn replace_span_with(&mut self, after: Span, keep_label: bool) -> &mut Self {
         let before = self.span.clone();
@@ -412,39 +508,40 @@ impl Diagnostic {
         self
     }
 
+    with_fn! { with_note,
     /// Add a note attached to this diagnostic.
     #[rustc_lint_diagnostics]
     pub fn note(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self {
         self.sub(Level::Note, msg, MultiSpan::new());
         self
-    }
+    } }
 
     fn highlighted_note(&mut self, msg: Vec<StringPart>) -> &mut Self {
         self.sub_with_highlights(Level::Note, msg, MultiSpan::new());
         self
     }
 
-    /// Prints the span with a note above it.
+    /// This is like [`DiagnosticBuilder::note()`], but it's only printed once.
-    /// This is like [`Diagnostic::note()`], but it gets its own span.
     pub fn note_once(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self {
         self.sub(Level::OnceNote, msg, MultiSpan::new());
         self
     }
 
+    with_fn! { with_span_note,
     /// Prints the span with a note above it.
-    /// This is like [`Diagnostic::note()`], but it gets its own span.
+    /// This is like [`DiagnosticBuilder::note()`], but it gets its own span.
     #[rustc_lint_diagnostics]
-    pub fn span_note<S: Into<MultiSpan>>(
+    pub fn span_note(
         &mut self,
-        sp: S,
+        sp: impl Into<MultiSpan>,
         msg: impl Into<SubdiagnosticMessage>,
     ) -> &mut Self {
         self.sub(Level::Note, msg, sp.into());
         self
-    }
+    } }
 
     /// Prints the span with a note above it.
-    /// This is like [`Diagnostic::note()`], but it gets its own span.
+    /// This is like [`DiagnosticBuilder::note_once()`], but it gets its own span.
     pub fn span_note_once<S: Into<MultiSpan>>(
         &mut self,
         sp: S,
@@ -454,15 +551,16 @@ impl Diagnostic {
         self
     }
 
+    with_fn! { with_warn,
     /// Add a warning attached to this diagnostic.
     #[rustc_lint_diagnostics]
     pub fn warn(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self {
         self.sub(Level::Warning, msg, MultiSpan::new());
         self
-    }
+    } }
 
     /// Prints the span with a warning above it.
-    /// This is like [`Diagnostic::warn()`], but it gets its own span.
+    /// This is like [`DiagnosticBuilder::warn()`], but it gets its own span.
     #[rustc_lint_diagnostics]
     pub fn span_warn<S: Into<MultiSpan>>(
         &mut self,
@@ -473,15 +571,15 @@ impl Diagnostic {
         self
     }
 
+    with_fn! { with_help,
     /// Add a help message attached to this diagnostic.
     #[rustc_lint_diagnostics]
     pub fn help(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self {
         self.sub(Level::Help, msg, MultiSpan::new());
         self
-    }
+    } }
 
-    /// Prints the span with a help above it.
+    /// This is like [`DiagnosticBuilder::help()`], but it's only printed once.
-    /// This is like [`Diagnostic::help()`], but it gets its own span.
     pub fn help_once(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self {
         self.sub(Level::OnceHelp, msg, MultiSpan::new());
         self
@@ -494,7 +592,7 @@ impl Diagnostic {
     }
 
     /// Prints the span with some help above it.
-    /// This is like [`Diagnostic::help()`], but it gets its own span.
+    /// This is like [`DiagnosticBuilder::help()`], but it gets its own span.
     #[rustc_lint_diagnostics]
     pub fn span_help<S: Into<MultiSpan>>(
         &mut self,
@@ -531,6 +629,7 @@ impl Diagnostic {
         }
     }
 
+    with_fn! { with_multipart_suggestion,
     /// Show a suggestion that has multiple parts to it.
     /// In other words, multiple changes need to be applied as part of this suggestion.
     pub fn multipart_suggestion(
@@ -545,7 +644,7 @@ impl Diagnostic {
             applicability,
             SuggestionStyle::ShowCode,
         )
-    }
+    } }
 
     /// Show a suggestion that has multiple parts to it, always as it's own subdiagnostic.
     /// In other words, multiple changes need to be applied as part of this suggestion.
@@ -562,7 +661,8 @@ impl Diagnostic {
             SuggestionStyle::ShowAlways,
         )
     }
-    /// [`Diagnostic::multipart_suggestion()`] but you can set the [`SuggestionStyle`].
+
+    /// [`DiagnosticBuilder::multipart_suggestion()`] but you can set the [`SuggestionStyle`].
     pub fn multipart_suggestion_with_style(
         &mut self,
         msg: impl Into<SubdiagnosticMessage>,
@@ -619,6 +719,7 @@ impl Diagnostic {
         )
     }
 
+    with_fn! { with_span_suggestion,
     /// Prints out a message with a suggested edit of the code.
     ///
     /// In case of short messages and a simple suggestion, rustc displays it as a label:
@@ -651,9 +752,9 @@ impl Diagnostic {
             SuggestionStyle::ShowCode,
         );
         self
-    }
+    } }
 
-    /// [`Diagnostic::span_suggestion()`] but you can set the [`SuggestionStyle`].
+    /// [`DiagnosticBuilder::span_suggestion()`] but you can set the [`SuggestionStyle`].
     pub fn span_suggestion_with_style(
         &mut self,
         sp: Span,
@@ -677,6 +778,7 @@ impl Diagnostic {
         self
     }
 
+    with_fn! { with_span_suggestion_verbose,
     /// Always show the suggested change.
     pub fn span_suggestion_verbose(
         &mut self,
@@ -693,10 +795,11 @@ impl Diagnostic {
             SuggestionStyle::ShowAlways,
         );
         self
-    }
+    } }
 
+    with_fn! { with_span_suggestions,
     /// Prints out a message with multiple suggested edits of the code.
-    /// See also [`Diagnostic::span_suggestion()`].
+    /// See also [`DiagnosticBuilder::span_suggestion()`].
     pub fn span_suggestions(
         &mut self,
         sp: Span,
@@ -711,9 +814,8 @@ impl Diagnostic {
             applicability,
             SuggestionStyle::ShowCode,
         )
-    }
+    } }
 
-    /// [`Diagnostic::span_suggestions()`] but you can set the [`SuggestionStyle`].
     pub fn span_suggestions_with_style(
         &mut self,
         sp: Span,
@@ -743,7 +845,7 @@ impl Diagnostic {
 
     /// Prints out a message with multiple suggested edits of the code, where each edit consists of
     /// multiple parts.
-    /// See also [`Diagnostic::multipart_suggestion()`].
+    /// See also [`DiagnosticBuilder::multipart_suggestion()`].
     pub fn multipart_suggestions(
         &mut self,
         msg: impl Into<SubdiagnosticMessage>,
@@ -785,6 +887,7 @@ impl Diagnostic {
         self
     }
 
+    with_fn! { with_span_suggestion_short,
     /// Prints out a message with a suggested edit of the code. If the suggestion is presented
     /// inline, it will only show the message and not the suggestion.
     ///
@@ -804,7 +907,7 @@ impl Diagnostic {
             SuggestionStyle::HideCodeInline,
         );
         self
-    }
+    } }
 
     /// Prints out a message for a suggestion without showing the suggested code.
     ///
@@ -829,6 +932,7 @@ impl Diagnostic {
         self
     }
 
+    with_fn! { with_tool_only_span_suggestion,
     /// Adds a suggestion to the JSON output that will not be shown in the CLI.
     ///
     /// This is intended to be used for suggestions that are *very* obvious in what the changes
@@ -849,7 +953,7 @@ impl Diagnostic {
             SuggestionStyle::CompletelyHidden,
         );
         self
-    }
+    } }
 
     /// Add a subdiagnostic from a type that implements `Subdiagnostic` (see
     /// [rustc_macros::Subdiagnostic]). Performs eager translation of any translatable messages
@@ -868,45 +972,45 @@ impl Diagnostic {
         self
     }
 
-    pub fn span<S: Into<MultiSpan>>(&mut self, sp: S) -> &mut Self {
+    with_fn! { with_span,
+    /// Add a span.
+    pub fn span(&mut self, sp: impl Into<MultiSpan>) -> &mut Self {
         self.span = sp.into();
         if let Some(span) = self.span.primary_span() {
             self.sort_span = span;
         }
         self
-    }
+    } }
 
     pub fn is_lint(&mut self, name: String, has_future_breakage: bool) -> &mut Self {
         self.is_lint = Some(IsLint { name, has_future_breakage });
         self
     }
 
+    with_fn! { with_code,
+    /// Add an error code.
     pub fn code(&mut self, code: ErrCode) -> &mut Self {
         self.code = Some(code);
         self
-    }
+    } }
 
+    with_fn! { with_primary_message,
+    /// Add a primary message.
     pub fn primary_message(&mut self, msg: impl Into<DiagnosticMessage>) -> &mut Self {
         self.messages[0] = (msg.into(), Style::NoStyle);
         self
-    }
+    } }
-
-    pub fn args(&self) -> impl Iterator<Item = DiagnosticArg<'_>> {
-        self.args.iter()
-    }
 
+    with_fn! { with_arg,
+    /// Add an argument.
     pub fn arg(
         &mut self,
         name: impl Into<DiagnosticArgName>,
         arg: impl IntoDiagnosticArg,
     ) -> &mut Self {
-        self.args.insert(name.into(), arg.into_diagnostic_arg());
+        self.deref_mut().arg(name, arg);
         self
-    }
+    } }
-
-    pub fn replace_args(&mut self, args: FxIndexMap<DiagnosticArgName, DiagnosticArgValue>) {
-        self.args = args;
-    }
 
     /// Helper function that takes a `SubdiagnosticMessage` and returns a `DiagnosticMessage` by
     /// combining it with the primary message of the diagnostic (if translatable, otherwise it just
@@ -915,9 +1019,7 @@ impl Diagnostic {
         &self,
         attr: impl Into<SubdiagnosticMessage>,
     ) -> DiagnosticMessage {
-        let msg =
+        self.deref().subdiagnostic_message_to_diagnostic_message(attr)
-            self.messages.iter().map(|(msg, _)| msg).next().expect("diagnostic with no messages");
-        msg.with_subdiagnostic_message(attr.into())
     }
 
     /// Convenience function for internal use, clients should use one of the
@@ -925,15 +1027,7 @@ impl Diagnostic {
     ///
     /// Used by `proc_macro_server` for implementing `server::Diagnostic`.
     pub fn sub(&mut self, level: Level, message: impl Into<SubdiagnosticMessage>, span: MultiSpan) {
-        let sub = SubDiagnostic {
+        self.deref_mut().sub(level, message, span);
-            level,
-            messages: vec![(
-                self.subdiagnostic_message_to_diagnostic_message(message),
-                Style::NoStyle,
-            )],
-            span,
-        };
-        self.children.push(sub);
     }
 
     /// Convenience function for internal use, clients should use one of the
@@ -946,7 +1040,9 @@ impl Diagnostic {
         let sub = SubDiagnostic { level, messages, span };
         self.children.push(sub);
     }
+}
 
+impl Diagnostic {
     /// Fields used for Hash, and PartialEq trait
     fn keys(
         &self,

compiler/rustc_errors/src/diagnostic_builder.rs

@@ -1,14 +1,8 @@
-use crate::diagnostic::IntoDiagnosticArg;
-use crate::{DiagCtxt, Level, MultiSpan, StashKey};
 use crate::{
-    Diagnostic, DiagnosticMessage, DiagnosticStyledString, ErrCode, ErrorGuaranteed, ExplicitBug,
+    DiagCtxt, Diagnostic, DiagnosticMessage, ErrorGuaranteed, ExplicitBug, Level, StashKey,
-    SubdiagnosticMessage,
 };
-use rustc_lint_defs::Applicability;
 use rustc_span::source_map::Spanned;
-
 use rustc_span::Span;
-use std::borrow::Cow;
 use std::fmt::{self, Debug};
 use std::marker::PhantomData;
 use std::ops::{Deref, DerefMut};
@@ -35,6 +29,11 @@ where
 }
 
 /// Used for emitting structured error messages and other diagnostic information.
+/// Wraps a `Diagnostic`, adding some useful things.
+/// - The `dcx` field, allowing it to (a) emit itself, and (b) do a drop check
+///   that it has been emitted or cancelled.
+/// - The `EmissionGuarantee`, which determines the type returned from `emit`.
+///
 /// Each constructed `DiagnosticBuilder` must be consumed by a function such as
 /// `emit`, `cancel`, `delay_as_bug`, or `into_diagnostic`. A panic occurrs if a
 /// `DiagnosticBuilder` is dropped without being consumed by one of these
@@ -56,9 +55,11 @@ pub struct DiagnosticBuilder<'a, G: EmissionGuarantee = ErrorGuaranteed> {
     /// often used as a return value, especially within the frequently-used
     /// `PResult` type. In theory, return value optimization (RVO) should avoid
     /// unnecessary copying. In practice, it does not (at the time of writing).
-    diag: Option<Box<Diagnostic>>,
+    // FIXME(nnethercote) Make private once this moves to diagnostic.rs.
+    pub(crate) diag: Option<Box<Diagnostic>>,
 
-    _marker: PhantomData<G>,
+    // FIXME(nnethercote) Make private once this moves to diagnostic.rs.
+    pub(crate) _marker: PhantomData<G>,
 }
 
 // Cloning a `DiagnosticBuilder` is a recipe for a diagnostic being emitted
@@ -88,18 +89,21 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
     /// Takes the diagnostic. For use by methods that consume the
     /// DiagnosticBuilder: `emit`, `cancel`, etc. Afterwards, `drop` is the
     /// only code that will be run on `self`.
-    fn take_diag(&mut self) -> Diagnostic {
+    // FIXME(nnethercote) Make private once this moves to diagnostic.rs.
+    pub(crate) fn take_diag(&mut self) -> Diagnostic {
         Box::into_inner(self.diag.take().unwrap())
     }
 
     /// Most `emit_producing_guarantee` functions use this as a starting point.
-    fn emit_producing_nothing(mut self) {
+    // FIXME(nnethercote) Make private once this moves to diagnostic.rs.
+    pub(crate) fn emit_producing_nothing(mut self) {
         let diag = self.take_diag();
         self.dcx.emit_diagnostic(diag);
     }
 
     /// `ErrorGuaranteed::emit_producing_guarantee` uses this.
-    fn emit_producing_error_guaranteed(mut self) -> ErrorGuaranteed {
+    // FIXME(nnethercote) Make private once this moves to diagnostic.rs.
+    pub(crate) fn emit_producing_error_guaranteed(mut self) -> ErrorGuaranteed {
         let diag = self.take_diag();
 
         // The only error levels that produce `ErrorGuaranteed` are
@@ -168,40 +172,6 @@ impl EmissionGuarantee for rustc_span::fatal_error::FatalError {
     }
 }
 
-/// `DiagnosticBuilder` impls `DerefMut`, which allows access to the fields and
-/// methods of the embedded `Diagnostic`. However, that doesn't allow method
-/// chaining at the `DiagnosticBuilder` level. Each use of this macro defines
-/// two builder methods at that level, both of which wrap the equivalent method
-/// in `Diagnostic`.
-/// - A `&mut self -> &mut Self` method, with the same name as the underlying
-///   `Diagnostic` method. It is mostly to modify existing diagnostics, either
-///   in a standalone fashion, e.g. `err.code(code)`, or in a chained fashion
-///   to make multiple modifications, e.g. `err.code(code).span(span)`.
-/// - A `self -> Self` method, which has a `with_` prefix added.
-///   It is mostly used in a chained fashion when producing a new diagnostic,
-///   e.g. `let err = struct_err(msg).with_code(code)`, or when emitting a new
-///   diagnostic , e.g. `struct_err(msg).with_code(code).emit()`.
-///
-/// Although the latter method can be used to modify an existing diagnostic,
-/// e.g. `err = err.with_code(code)`, this should be avoided because the former
-/// method gives shorter code, e.g. `err.code(code)`.
-macro_rules! forward {
-    (
-        ($f:ident, $with_f:ident)($($name:ident: $ty:ty),* $(,)?)
-    ) => {
-        #[doc = concat!("See [`Diagnostic::", stringify!($f), "()`].")]
-        pub fn $f(&mut self, $($name: $ty),*) -> &mut Self {
-            self.diag.as_mut().unwrap().$f($($name),*);
-            self
-        }
-        #[doc = concat!("See [`Diagnostic::", stringify!($f), "()`].")]
-        pub fn $with_f(mut self, $($name: $ty),*) -> Self {
-            self.diag.as_mut().unwrap().$f($($name),*);
-            self
-        }
-    };
-}
-
 impl<G: EmissionGuarantee> Deref for DiagnosticBuilder<'_, G> {
     type Target = Diagnostic;
 
@@ -278,135 +248,6 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
         self.downgrade_to_delayed_bug();
         self.emit()
     }
-
-    forward!((span_label, with_span_label)(
-        span: Span,
-        label: impl Into<SubdiagnosticMessage>,
-    ));
-    forward!((span_labels, with_span_labels)(
-        spans: impl IntoIterator<Item = Span>,
-        label: &str,
-    ));
-    forward!((note_expected_found, with_note_expected_found)(
-        expected_label: &dyn fmt::Display,
-        expected: DiagnosticStyledString,
-        found_label: &dyn fmt::Display,
-        found: DiagnosticStyledString,
-    ));
-    forward!((note_expected_found_extra, with_note_expected_found_extra)(
-        expected_label: &dyn fmt::Display,
-        expected: DiagnosticStyledString,
-        found_label: &dyn fmt::Display,
-        found: DiagnosticStyledString,
-        expected_extra: &dyn fmt::Display,
-        found_extra: &dyn fmt::Display,
-    ));
-    forward!((note, with_note)(
-        msg: impl Into<SubdiagnosticMessage>,
-    ));
-    forward!((note_once, with_note_once)(
-        msg: impl Into<SubdiagnosticMessage>,
-    ));
-    forward!((span_note, with_span_note)(
-        sp: impl Into<MultiSpan>,
-        msg: impl Into<SubdiagnosticMessage>,
-    ));
-    forward!((span_note_once, with_span_note_once)(
-        sp: impl Into<MultiSpan>,
-        msg: impl Into<SubdiagnosticMessage>,
-    ));
-    forward!((warn, with_warn)(
-        msg: impl Into<SubdiagnosticMessage>,
-    ));
-    forward!((span_warn, with_span_warn)(
-        sp: impl Into<MultiSpan>,
-        msg: impl Into<SubdiagnosticMessage>,
-    ));
-    forward!((help, with_help)(
-        msg: impl Into<SubdiagnosticMessage>,
-    ));
-    forward!((help_once, with_help_once)(
-        msg: impl Into<SubdiagnosticMessage>,
-    ));
-    forward!((span_help, with_span_help_once)(
-        sp: impl Into<MultiSpan>,
-        msg: impl Into<SubdiagnosticMessage>,
-    ));
-    forward!((multipart_suggestion, with_multipart_suggestion)(
-        msg: impl Into<SubdiagnosticMessage>,
-        suggestion: Vec<(Span, String)>,
-        applicability: Applicability,
-    ));
-    forward!((multipart_suggestion_verbose, with_multipart_suggestion_verbose)(
-        msg: impl Into<SubdiagnosticMessage>,
-        suggestion: Vec<(Span, String)>,
-        applicability: Applicability,
-    ));
-    forward!((tool_only_multipart_suggestion, with_tool_only_multipart_suggestion)(
-        msg: impl Into<SubdiagnosticMessage>,
-        suggestion: Vec<(Span, String)>,
-        applicability: Applicability,
-    ));
-    forward!((span_suggestion, with_span_suggestion)(
-        sp: Span,
-        msg: impl Into<SubdiagnosticMessage>,
-        suggestion: impl ToString,
-        applicability: Applicability,
-    ));
-    forward!((span_suggestions, with_span_suggestions)(
-        sp: Span,
-        msg: impl Into<SubdiagnosticMessage>,
-        suggestions: impl IntoIterator<Item = String>,
-        applicability: Applicability,
-    ));
-    forward!((multipart_suggestions, with_multipart_suggestions)(
-        msg: impl Into<SubdiagnosticMessage>,
-        suggestions: impl IntoIterator<Item = Vec<(Span, String)>>,
-        applicability: Applicability,
-    ));
-    forward!((span_suggestion_short, with_span_suggestion_short)(
-        sp: Span,
-        msg: impl Into<SubdiagnosticMessage>,
-        suggestion: impl ToString,
-        applicability: Applicability,
-    ));
-    forward!((span_suggestion_verbose, with_span_suggestion_verbose)(
-        sp: Span,
-        msg: impl Into<SubdiagnosticMessage>,
-        suggestion: impl ToString,
-        applicability: Applicability,
-    ));
-    forward!((span_suggestion_hidden, with_span_suggestion_hidden)(
-        sp: Span,
-        msg: impl Into<SubdiagnosticMessage>,
-        suggestion: impl ToString,
-        applicability: Applicability,
-    ));
-    forward!((tool_only_span_suggestion, with_tool_only_span_suggestion)(
-        sp: Span,
-        msg: impl Into<SubdiagnosticMessage>,
-        suggestion: impl ToString,
-        applicability: Applicability,
-    ));
-    forward!((primary_message, with_primary_message)(
-        msg: impl Into<DiagnosticMessage>,
-    ));
-    forward!((span, with_span)(
-        sp: impl Into<MultiSpan>,
-    ));
-    forward!((is_lint, with_is_lint)(
-        name: String, has_future_breakage: bool,
-    ));
-    forward!((code, with_code)(
-        code: ErrCode,
-    ));
-    forward!((arg, with_arg)(
-        name: impl Into<Cow<'static, str>>, arg: impl IntoDiagnosticArg,
-    ));
-    forward!((subdiagnostic, with_subdiagnostic)(
-        dcx: &DiagCtxt,
-        subdiagnostic: impl crate::AddToDiagnostic,
-    ));
 }
 
 impl<G: EmissionGuarantee> Debug for DiagnosticBuilder<'_, G> {

compiler/rustc_errors/src/diagnostic_impls.rs

@@ -299,7 +299,11 @@ pub struct SingleLabelManySpans {
     pub label: &'static str,
 }
 impl AddToDiagnostic for SingleLabelManySpans {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut crate::Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _: F,
+    ) {
         diag.span_labels(self.spans, self.label);
     }
 }
@@ -312,23 +316,6 @@ pub struct ExpectedLifetimeParameter {
     pub count: usize,
 }
 
-#[derive(Subdiagnostic)]
-#[note(errors_delayed_at_with_newline)]
-pub struct DelayedAtWithNewline {
-    #[primary_span]
-    pub span: Span,
-    pub emitted_at: DiagnosticLocation,
-    pub note: Backtrace,
-}
-#[derive(Subdiagnostic)]
-#[note(errors_delayed_at_without_newline)]
-pub struct DelayedAtWithoutNewline {
-    #[primary_span]
-    pub span: Span,
-    pub emitted_at: DiagnosticLocation,
-    pub note: Backtrace,
-}
-
 impl IntoDiagnosticArg for DiagnosticLocation {
     fn into_diagnostic_arg(self) -> DiagnosticArgValue {
         DiagnosticArgValue::Str(Cow::from(self.to_string()))
@@ -341,13 +328,6 @@ impl IntoDiagnosticArg for Backtrace {
     }
 }
 
-#[derive(Subdiagnostic)]
-#[note(errors_invalid_flushed_delayed_diagnostic_level)]
-pub struct InvalidFlushedDelayedDiagnosticLevel {
-    #[primary_span]
-    pub span: Span,
-    pub level: Level,
-}
 impl IntoDiagnosticArg for Level {
     fn into_diagnostic_arg(self) -> DiagnosticArgValue {
         DiagnosticArgValue::Str(Cow::from(self.to_string()))

compiler/rustc_errors/src/emitter.rs

@@ -599,7 +599,7 @@ impl Emitter for SilentEmitter {
 
     fn emit_diagnostic(&mut self, mut diag: Diagnostic) {
         if diag.level == Level::Fatal {
-            diag.note(self.fatal_note.clone());
+            diag.sub(Level::Note, self.fatal_note.clone(), MultiSpan::new());
             self.fatal_dcx.emit_diagnostic(diag);
         }
     }

compiler/rustc_errors/src/lib.rs

@@ -46,7 +46,7 @@ pub use diagnostic_builder::{
 };
 pub use diagnostic_impls::{
     DiagnosticArgFromDisplay, DiagnosticSymbolList, ExpectedLifetimeParameter,
-    IndicateAnonymousLifetime, InvalidFlushedDelayedDiagnosticLevel, SingleLabelManySpans,
+    IndicateAnonymousLifetime, SingleLabelManySpans,
 };
 pub use emitter::ColorConfig;
 pub use rustc_error_messages::{
@@ -62,7 +62,6 @@ pub use snippet::Style;
 // See https://github.com/rust-lang/rust/pull/115393.
 pub use termcolor::{Color, ColorSpec, WriteColor};
 
-use crate::diagnostic_impls::{DelayedAtWithNewline, DelayedAtWithoutNewline};
 use emitter::{is_case_difference, DynEmitter, Emitter, HumanEmitter};
 use registry::Registry;
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap, FxIndexSet};
@@ -1395,9 +1394,8 @@ impl DiagCtxtInner {
                 };
                 diagnostic.children.extract_if(already_emitted_sub).for_each(|_| {});
                 if already_emitted {
-                    diagnostic.note(
+                    let msg = "duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`";
-                        "duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`",
+                    diagnostic.sub(Level::Note, msg, MultiSpan::new());
-                    );
                 }
 
                 if is_error {
@@ -1483,6 +1481,16 @@ impl DiagCtxtInner {
         self.emitter.translate_message(&message, &args).map_err(Report::new).unwrap().to_string()
     }
 
+    fn eagerly_translate_for_subdiag(
+        &self,
+        diag: &Diagnostic,
+        msg: impl Into<SubdiagnosticMessage>,
+    ) -> SubdiagnosticMessage {
+        let args = diag.args();
+        let msg = diag.subdiagnostic_message_to_diagnostic_message(msg);
+        self.eagerly_translate(msg, args)
+    }
+
     fn flush_delayed(&mut self) {
         if self.delayed_bugs.is_empty() {
             return;
@@ -1527,17 +1535,14 @@ impl DiagCtxtInner {
             if bug.level != DelayedBug {
                 // NOTE(eddyb) not panicking here because we're already producing
                 // an ICE, and the more information the merrier.
-                let subdiag = InvalidFlushedDelayedDiagnosticLevel {
+                //
-                    span: bug.span.primary_span().unwrap(),
+                // We are at the `Diagnostic`/`DiagCtxtInner` level rather than
-                    level: bug.level,
+                // the usual `DiagnosticBuilder`/`DiagCtxt` level, so we must
-                };
+                // augment `bug` in a lower-level fashion.
-                // FIXME: Cannot use `Diagnostic::subdiagnostic` which takes `DiagCtxt`, but it
+                bug.arg("level", bug.level);
-                // just uses `DiagCtxtInner` functions.
+                let msg = crate::fluent_generated::errors_invalid_flushed_delayed_diagnostic_level;
-                subdiag.add_to_diagnostic_with(&mut bug, |diag, msg| {
+                let msg = self.eagerly_translate_for_subdiag(&bug, msg); // after the `arg` call
-                    let args = diag.args();
+                bug.sub(Level::Note, msg, bug.span.primary_span().unwrap().into());
-                    let msg = diag.subdiagnostic_message_to_diagnostic_message(msg);
-                    self.eagerly_translate(msg, args)
-                });
             }
             bug.level = Bug;
 
@@ -1571,39 +1576,22 @@ impl DelayedDiagnostic {
         DelayedDiagnostic { inner: diagnostic, note: backtrace }
     }
 
-    fn decorate(mut self, dcx: &DiagCtxtInner) -> Diagnostic {
+    fn decorate(self, dcx: &DiagCtxtInner) -> Diagnostic {
-        // FIXME: Cannot use `Diagnostic::subdiagnostic` which takes `DiagCtxt`, but it
+        // We are at the `Diagnostic`/`DiagCtxtInner` level rather than the
-        // just uses `DiagCtxtInner` functions.
+        // usual `DiagnosticBuilder`/`DiagCtxt` level, so we must construct
-        let subdiag_with = |diag: &mut Diagnostic, msg| {
+        // `diag` in a lower-level fashion.
-            let args = diag.args();
+        let mut diag = self.inner;
-            let msg = diag.subdiagnostic_message_to_diagnostic_message(msg);
+        let msg = match self.note.status() {
-            dcx.eagerly_translate(msg, args)
+            BacktraceStatus::Captured => crate::fluent_generated::errors_delayed_at_with_newline,
-        };
-
-        match self.note.status() {
-            BacktraceStatus::Captured => {
-                let inner = &self.inner;
-                let subdiag = DelayedAtWithNewline {
-                    span: inner.span.primary_span().unwrap_or(DUMMY_SP),
-                    emitted_at: inner.emitted_at.clone(),
-                    note: self.note,
-                };
-                subdiag.add_to_diagnostic_with(&mut self.inner, subdiag_with);
-            }
             // Avoid the needless newline when no backtrace has been captured,
             // the display impl should just be a single line.
-            _ => {
+            _ => crate::fluent_generated::errors_delayed_at_without_newline,
-                let inner = &self.inner;
-                let subdiag = DelayedAtWithoutNewline {
-                    span: inner.span.primary_span().unwrap_or(DUMMY_SP),
-                    emitted_at: inner.emitted_at.clone(),
-                    note: self.note,
         };
-                subdiag.add_to_diagnostic_with(&mut self.inner, subdiag_with);
+        diag.arg("emitted_at", diag.emitted_at.clone());
-            }
+        diag.arg("note", self.note);
-        }
+        let msg = dcx.eagerly_translate_for_subdiag(&diag, msg); // after the `arg` calls
-
+        diag.sub(Level::Note, msg, diag.span.primary_span().unwrap_or(DUMMY_SP).into());
-        self.inner
+        diag
     }
 }
 
@@ -1745,9 +1733,9 @@ impl Level {
 }
 
 // FIXME(eddyb) this doesn't belong here AFAICT, should be moved to callsite.
-pub fn add_elided_lifetime_in_path_suggestion<E: EmissionGuarantee>(
+pub fn add_elided_lifetime_in_path_suggestion<G: EmissionGuarantee>(
     source_map: &SourceMap,
-    diag: &mut DiagnosticBuilder<'_, E>,
+    diag: &mut DiagnosticBuilder<'_, G>,
     n: usize,
     path_span: Span,
     incl_angl_brckt: bool,

compiler/rustc_expand/src/base.rs

@@ -1176,6 +1176,8 @@ impl<'a> ExtCtxt<'a> {
         for (span, notes) in self.expansions.iter() {
             let mut db = self.dcx().create_note(errors::TraceMacro { span: *span });
             for note in notes {
+                // FIXME: make this translatable
+                #[allow(rustc::untranslatable_diagnostic)]
                 db.note(note.clone());
             }
             db.emit();

compiler/rustc_expand/src/config.rs

@@ -384,6 +384,7 @@ impl<'a> StripUnconfigured<'a> {
             );
 
             if attr.is_doc_comment() {
+                #[allow(rustc::untranslatable_diagnostic)]
                 err.help("`///` is for documentation comments. For a plain comment, use `//`.");
             }
 

compiler/rustc_expand/src/mbe/diagnostics.rs

@@ -7,7 +7,7 @@ use crate::mbe::{
 use rustc_ast::token::{self, Token, TokenKind};
 use rustc_ast::tokenstream::TokenStream;
 use rustc_ast_pretty::pprust;
-use rustc_errors::{Applicability, DiagCtxt, Diagnostic, DiagnosticBuilder, DiagnosticMessage};
+use rustc_errors::{Applicability, DiagCtxt, DiagnosticBuilder, DiagnosticMessage};
 use rustc_parse::parser::{Parser, Recovery};
 use rustc_span::source_map::SourceMap;
 use rustc_span::symbol::Ident;
@@ -285,7 +285,11 @@ pub(super) fn emit_frag_parse_err(
     e.emit();
 }
 
-pub(crate) fn annotate_err_with_kind(err: &mut Diagnostic, kind: AstFragmentKind, span: Span) {
+pub(crate) fn annotate_err_with_kind(
+    err: &mut DiagnosticBuilder<'_>,
+    kind: AstFragmentKind,
+    span: Span,
+) {
     match kind {
         AstFragmentKind::Ty => {
             err.span_label(span, "this macro call doesn't expand to a type");
@@ -313,7 +317,7 @@ enum ExplainDocComment {
 
 pub(super) fn annotate_doc_comment(
     dcx: &DiagCtxt,
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_>,
     sm: &SourceMap,
     span: Span,
 ) {

compiler/rustc_expand/src/proc_macro_server.rs

@@ -10,7 +10,7 @@ use rustc_ast::util::literal::escape_byte_str_symbol;
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sync::Lrc;
-use rustc_errors::{ErrorGuaranteed, MultiSpan, PResult};
+use rustc_errors::{DiagnosticBuilder, ErrorGuaranteed, MultiSpan, PResult};
 use rustc_parse::lexer::nfc_normalize;
 use rustc_parse::parse_stream_from_source_str;
 use rustc_session::parse::ParseSess;
@@ -509,13 +509,14 @@ impl server::FreeFunctions for Rustc<'_, '_> {
     }
 
     fn emit_diagnostic(&mut self, diagnostic: Diagnostic<Self::Span>) {
-        let mut diag =
+        let message = rustc_errors::DiagnosticMessage::from(diagnostic.message);
-            rustc_errors::Diagnostic::new(diagnostic.level.to_internal(), diagnostic.message);
+        let mut diag: DiagnosticBuilder<'_, rustc_errors::ErrorGuaranteed> =
+            DiagnosticBuilder::new(&self.sess().dcx, diagnostic.level.to_internal(), message);
         diag.span(MultiSpan::from_spans(diagnostic.spans));
         for child in diagnostic.children {
             diag.sub(child.level.to_internal(), child.message, MultiSpan::from_spans(child.spans));
         }
-        self.sess().dcx.emit_diagnostic(diag);
+        diag.emit();
     }
 }
 

compiler/rustc_hir_analysis/src/astconv/errors.rs

@@ -9,7 +9,7 @@ use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_data_structures::sorted_map::SortedMap;
 use rustc_data_structures::unord::UnordMap;
 use rustc_errors::{
-    codes::*, pluralize, struct_span_code_err, Applicability, Diagnostic, ErrorGuaranteed,
+    codes::*, pluralize, struct_span_code_err, Applicability, DiagnosticBuilder, ErrorGuaranteed,
 };
 use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
@@ -371,7 +371,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     // FIXME(fmease): Heavily adapted from `rustc_hir_typeck::method::suggest`. Deduplicate.
     fn note_ambiguous_inherent_assoc_type(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         candidates: Vec<DefId>,
         span: Span,
     ) {
@@ -429,7 +429,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
         let tcx = self.tcx();
 
         let adt_did = self_ty.ty_adt_def().map(|def| def.did());
-        let add_def_label = |err: &mut Diagnostic| {
+        let add_def_label = |err: &mut DiagnosticBuilder<'_>| {
             if let Some(did) = adt_did {
                 err.span_label(
                     tcx.def_span(did),

compiler/rustc_hir_analysis/src/astconv/generics.rs

@@ -6,7 +6,7 @@ use crate::astconv::{
 use crate::structured_errors::{GenericArgsInfo, StructuredDiagnostic, WrongNumberOfGenericArgs};
 use rustc_ast::ast::ParamKindOrd;
 use rustc_errors::{
-    codes::*, struct_span_code_err, Applicability, Diagnostic, ErrorGuaranteed, MultiSpan,
+    codes::*, struct_span_code_err, Applicability, DiagnosticBuilder, ErrorGuaranteed, MultiSpan,
 };
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
@@ -47,7 +47,7 @@ fn generic_arg_mismatch_err(
         }
     }
 
-    let add_braces_suggestion = |arg: &GenericArg<'_>, err: &mut Diagnostic| {
+    let add_braces_suggestion = |arg: &GenericArg<'_>, err: &mut DiagnosticBuilder<'_>| {
         let suggestions = vec![
             (arg.span().shrink_to_lo(), String::from("{ ")),
             (arg.span().shrink_to_hi(), String::from(" }")),

compiler/rustc_hir_analysis/src/astconv/lint.rs

@@ -1,5 +1,5 @@
 use rustc_ast::TraitObjectSyntax;
-use rustc_errors::{codes::*, Diagnostic, StashKey};
+use rustc_errors::{codes::*, DiagnosticBuilder, EmissionGuarantee, StashKey};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_lint_defs::{builtin::BARE_TRAIT_OBJECTS, Applicability};
@@ -10,10 +10,10 @@ use super::AstConv;
 
 impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     /// Make sure that we are in the condition to suggest the blanket implementation.
-    pub(super) fn maybe_lint_blanket_trait_impl(
+    pub(super) fn maybe_lint_blanket_trait_impl<G: EmissionGuarantee>(
         &self,
         self_ty: &hir::Ty<'_>,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_, G>,
     ) {
         let tcx = self.tcx();
         let parent_id = tcx.hir().get_parent_item(self_ty.hir_id).def_id;
@@ -75,7 +75,11 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     }
 
     /// Make sure that we are in the condition to suggest `impl Trait`.
-    fn maybe_lint_impl_trait(&self, self_ty: &hir::Ty<'_>, diag: &mut Diagnostic) -> bool {
+    fn maybe_lint_impl_trait(
+        &self,
+        self_ty: &hir::Ty<'_>,
+        diag: &mut DiagnosticBuilder<'_>,
+    ) -> bool {
         let tcx = self.tcx();
         let parent_id = tcx.hir().get_parent_item(self_ty.hir_id).def_id;
         let (sig, generics, owner) = match tcx.hir_node_by_def_id(parent_id) {

compiler/rustc_hir_analysis/src/astconv/mod.rs

@@ -18,8 +18,8 @@ use crate::require_c_abi_if_c_variadic;
 use rustc_ast::TraitObjectSyntax;
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
 use rustc_errors::{
-    codes::*, struct_span_code_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
+    codes::*, struct_span_code_err, Applicability, DiagnosticBuilder, ErrorGuaranteed, FatalError,
-    FatalError, MultiSpan,
+    MultiSpan,
 };
 use rustc_hir as hir;
 use rustc_hir::def::{CtorOf, DefKind, Namespace, Res};
@@ -1724,7 +1724,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
     pub fn prohibit_generics<'a>(
         &self,
         segments: impl Iterator<Item = &'a hir::PathSegment<'a>> + Clone,
-        extend: impl Fn(&mut Diagnostic),
+        extend: impl Fn(&mut DiagnosticBuilder<'_>),
     ) -> bool {
         let args = segments.clone().flat_map(|segment| segment.args().args);
 

compiler/rustc_hir_analysis/src/check/check.rs

@@ -1248,7 +1248,7 @@ fn check_enum(tcx: TyCtxt<'_>, def_id: LocalDefId) {
 fn detect_discriminant_duplicate<'tcx>(tcx: TyCtxt<'tcx>, adt: ty::AdtDef<'tcx>) {
     // Helper closure to reduce duplicate code. This gets called everytime we detect a duplicate.
     // Here `idx` refers to the order of which the discriminant appears, and its index in `vs`
-    let report = |dis: Discr<'tcx>, idx, err: &mut Diagnostic| {
+    let report = |dis: Discr<'tcx>, idx, err: &mut DiagnosticBuilder<'_>| {
         let var = adt.variant(idx); // HIR for the duplicate discriminant
         let (span, display_discr) = match var.discr {
             ty::VariantDiscr::Explicit(discr_def_id) => {

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -78,7 +78,7 @@ use std::num::NonZero;
 
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
 use rustc_errors::ErrorGuaranteed;
-use rustc_errors::{pluralize, struct_span_code_err, Diagnostic, DiagnosticBuilder};
+use rustc_errors::{pluralize, struct_span_code_err, DiagnosticBuilder};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::intravisit::Visitor;
 use rustc_index::bit_set::BitSet;

compiler/rustc_hir_analysis/src/structured_errors/wrong_number_of_generic_args.rs

@@ -1,5 +1,5 @@
 use crate::structured_errors::StructuredDiagnostic;
-use rustc_errors::{codes::*, pluralize, Applicability, Diagnostic, DiagnosticBuilder, MultiSpan};
+use rustc_errors::{codes::*, pluralize, Applicability, DiagnosticBuilder, MultiSpan};
 use rustc_hir as hir;
 use rustc_middle::ty::{self as ty, AssocItems, AssocKind, TyCtxt};
 use rustc_session::Session;
@@ -525,7 +525,7 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
     }
 
     /// Builds the `expected 1 type argument / supplied 2 type arguments` message.
-    fn notify(&self, err: &mut Diagnostic) {
+    fn notify(&self, err: &mut DiagnosticBuilder<'_>) {
         let (quantifier, bound) = self.get_quantifier_and_bound();
         let provided_args = self.num_provided_args();
 
@@ -577,7 +577,7 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
         }
     }
 
-    fn suggest(&self, err: &mut Diagnostic) {
+    fn suggest(&self, err: &mut DiagnosticBuilder<'_>) {
         debug!(
             "suggest(self.provided {:?}, self.gen_args.span(): {:?})",
             self.num_provided_args(),
@@ -605,7 +605,7 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
     /// ```text
     /// type Map = HashMap<String>;
     /// ```
-    fn suggest_adding_args(&self, err: &mut Diagnostic) {
+    fn suggest_adding_args(&self, err: &mut DiagnosticBuilder<'_>) {
         if self.gen_args.parenthesized != hir::GenericArgsParentheses::No {
             return;
         }
@@ -624,7 +624,7 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
         }
     }
 
-    fn suggest_adding_lifetime_args(&self, err: &mut Diagnostic) {
+    fn suggest_adding_lifetime_args(&self, err: &mut DiagnosticBuilder<'_>) {
         debug!("suggest_adding_lifetime_args(path_segment: {:?})", self.path_segment);
         let num_missing_args = self.num_missing_lifetime_args();
         let num_params_to_take = num_missing_args;
@@ -678,7 +678,7 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
         }
     }
 
-    fn suggest_adding_type_and_const_args(&self, err: &mut Diagnostic) {
+    fn suggest_adding_type_and_const_args(&self, err: &mut DiagnosticBuilder<'_>) {
         let num_missing_args = self.num_missing_type_or_const_args();
         let msg = format!("add missing {} argument{}", self.kind(), pluralize!(num_missing_args));
 
@@ -738,7 +738,7 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
     /// ```compile_fail
     /// Into::into::<Option<_>>(42) // suggests considering `Into::<Option<_>>::into(42)`
     /// ```
-    fn suggest_moving_args_from_assoc_fn_to_trait(&self, err: &mut Diagnostic) {
+    fn suggest_moving_args_from_assoc_fn_to_trait(&self, err: &mut DiagnosticBuilder<'_>) {
         let trait_ = match self.tcx.trait_of_item(self.def_id) {
             Some(def_id) => def_id,
             None => return,
@@ -794,7 +794,7 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
 
     fn suggest_moving_args_from_assoc_fn_to_trait_for_qualified_path(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         qpath: &'tcx hir::QPath<'tcx>,
         msg: String,
         num_assoc_fn_excess_args: usize,
@@ -827,7 +827,7 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
 
     fn suggest_moving_args_from_assoc_fn_to_trait_for_method_call(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         trait_def_id: DefId,
         expr: &'tcx hir::Expr<'tcx>,
         msg: String,
@@ -881,7 +881,7 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
     /// ```text
     /// type Map = HashMap<String, String, String, String>;
     /// ```
-    fn suggest_removing_args_or_generics(&self, err: &mut Diagnostic) {
+    fn suggest_removing_args_or_generics(&self, err: &mut DiagnosticBuilder<'_>) {
         let num_provided_lt_args = self.num_provided_lifetime_args();
         let num_provided_type_const_args = self.num_provided_type_or_const_args();
         let unbound_types = self.get_unbound_associated_types();
@@ -899,7 +899,7 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
         let provided_args_matches_unbound_traits =
             unbound_types.len() == num_redundant_type_or_const_args;
 
-        let remove_lifetime_args = |err: &mut Diagnostic| {
+        let remove_lifetime_args = |err: &mut DiagnosticBuilder<'_>| {
             let mut lt_arg_spans = Vec::new();
             let mut found_redundant = false;
             for arg in self.gen_args.args {
@@ -940,7 +940,7 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
             );
         };
 
-        let remove_type_or_const_args = |err: &mut Diagnostic| {
+        let remove_type_or_const_args = |err: &mut DiagnosticBuilder<'_>| {
             let mut gen_arg_spans = Vec::new();
             let mut found_redundant = false;
             for arg in self.gen_args.args {
@@ -1037,7 +1037,7 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
     }
 
     /// Builds the `type defined here` message.
-    fn show_definition(&self, err: &mut Diagnostic) {
+    fn show_definition(&self, err: &mut DiagnosticBuilder<'_>) {
         let mut spans: MultiSpan = if let Some(def_span) = self.tcx.def_ident_span(self.def_id) {
             if self.tcx.sess.source_map().is_span_accessible(def_span) {
                 def_span.into()
@@ -1088,7 +1088,7 @@ impl<'a, 'tcx> WrongNumberOfGenericArgs<'a, 'tcx> {
     }
 
     /// Add note if `impl Trait` is explicitly specified.
-    fn note_synth_provided(&self, err: &mut Diagnostic) {
+    fn note_synth_provided(&self, err: &mut DiagnosticBuilder<'_>) {
         if !self.is_synth_provided() {
             return;
         }

compiler/rustc_hir_typeck/src/_match.rs

@@ -1,6 +1,6 @@
 use crate::coercion::{AsCoercionSite, CoerceMany};
 use crate::{Diverges, Expectation, FnCtxt, Needs};
-use rustc_errors::{Applicability, Diagnostic};
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir::{
     self as hir,
     def::{CtorOf, DefKind, Res},
@@ -177,7 +177,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn explain_never_type_coerced_to_unit(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         arm: &hir::Arm<'tcx>,
         arm_ty: Ty<'tcx>,
         prior_arm: Option<(Option<hir::HirId>, Ty<'tcx>, Span)>,
@@ -209,7 +209,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn suggest_removing_semicolon_for_coerce(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'tcx>,
         arm_ty: Ty<'tcx>,
         prior_arm: Option<(Option<hir::HirId>, Ty<'tcx>, Span)>,
@@ -303,7 +303,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// `if let PAT = EXPR {}` expressions that could be turned into `let PAT = EXPR;`.
     fn explain_if_expr(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         ret_reason: Option<(Span, String)>,
         if_span: Span,
         cond_expr: &'tcx hir::Expr<'tcx>,

compiler/rustc_hir_typeck/src/callee.rs

@@ -4,7 +4,7 @@ use super::{Expectation, FnCtxt, TupleArgumentsFlag};
 
 use crate::errors;
 use rustc_ast::util::parser::PREC_POSTFIX;
-use rustc_errors::{Applicability, Diagnostic, ErrorGuaranteed, StashKey};
+use rustc_errors::{Applicability, DiagnosticBuilder, ErrorGuaranteed, StashKey};
 use rustc_hir as hir;
 use rustc_hir::def::{self, CtorKind, Namespace, Res};
 use rustc_hir::def_id::DefId;
@@ -347,7 +347,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// likely intention is to call the closure, suggest `(||{})()`. (#55851)
     fn identify_bad_closure_def_and_call(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         hir_id: hir::HirId,
         callee_node: &hir::ExprKind<'_>,
         callee_span: Span,
@@ -410,7 +410,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// likely intention is to create an array containing tuples.
     fn maybe_suggest_bad_array_definition(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         call_expr: &'tcx hir::Expr<'tcx>,
         callee_expr: &'tcx hir::Expr<'tcx>,
     ) -> bool {
@@ -601,7 +601,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// and suggesting the fix if the method probe is successful.
     fn suggest_call_as_method(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_, ()>,
         segment: &'tcx hir::PathSegment<'tcx>,
         arg_exprs: &'tcx [hir::Expr<'tcx>],
         call_expr: &'tcx hir::Expr<'tcx>,

compiler/rustc_hir_typeck/src/cast.rs

@@ -33,7 +33,7 @@ use super::FnCtxt;
 use crate::errors;
 use crate::type_error_struct;
 use hir::ExprKind;
-use rustc_errors::{codes::*, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_errors::{codes::*, Applicability, DiagnosticBuilder, ErrorGuaranteed};
 use rustc_hir as hir;
 use rustc_macros::{TypeFoldable, TypeVisitable};
 use rustc_middle::mir::Mutability;
@@ -983,7 +983,11 @@ impl<'a, 'tcx> CastCheck<'tcx> {
 
     /// Attempt to suggest using `.is_empty` when trying to cast from a
     /// collection type to a boolean.
-    fn try_suggest_collection_to_bool(&self, fcx: &FnCtxt<'a, 'tcx>, err: &mut Diagnostic) {
+    fn try_suggest_collection_to_bool(
+        &self,
+        fcx: &FnCtxt<'a, 'tcx>,
+        err: &mut DiagnosticBuilder<'_>,
+    ) {
         if self.cast_ty.is_bool() {
             let derefed = fcx
                 .autoderef(self.expr_span, self.expr_ty)

compiler/rustc_hir_typeck/src/coercion.rs

@@ -36,9 +36,7 @@
 //! ```
 
 use crate::FnCtxt;
-use rustc_errors::{
+use rustc_errors::{codes::*, struct_span_code_err, Applicability, DiagnosticBuilder, MultiSpan};
-    codes::*, struct_span_code_err, Applicability, Diagnostic, DiagnosticBuilder, MultiSpan,
-};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::{self, Visitor};
@@ -1439,7 +1437,7 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
         &mut self,
         fcx: &FnCtxt<'a, 'tcx>,
         cause: &ObligationCause<'tcx>,
-        augment_error: impl FnOnce(&mut Diagnostic),
+        augment_error: impl FnOnce(&mut DiagnosticBuilder<'_>),
         label_unit_as_expected: bool,
     ) {
         self.coerce_inner(
@@ -1462,7 +1460,7 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
         cause: &ObligationCause<'tcx>,
         expression: Option<&'tcx hir::Expr<'tcx>>,
         mut expression_ty: Ty<'tcx>,
-        augment_error: impl FnOnce(&mut Diagnostic),
+        augment_error: impl FnOnce(&mut DiagnosticBuilder<'_>),
         label_expression_as_expected: bool,
     ) {
         // Incorporate whatever type inference information we have
@@ -1673,7 +1671,7 @@ impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
 
     fn note_unreachable_loop_return(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         tcx: TyCtxt<'tcx>,
         expr: &hir::Expr<'tcx>,
         ret_exprs: &Vec<&'tcx hir::Expr<'tcx>>,

compiler/rustc_hir_typeck/src/demand.rs

@@ -1,6 +1,6 @@
 use crate::FnCtxt;
 use rustc_errors::MultiSpan;
-use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder};
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_hir::def::Res;
 use rustc_hir::intravisit::Visitor;
@@ -19,7 +19,7 @@ use super::method::probe;
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub fn emit_type_mismatch_suggestions(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'tcx>,
         expr_ty: Ty<'tcx>,
         expected: Ty<'tcx>,
@@ -70,7 +70,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub fn emit_coerce_suggestions(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'tcx>,
         expr_ty: Ty<'tcx>,
         expected: Ty<'tcx>,
@@ -280,7 +280,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// with some expectation given by `source`.
     pub fn note_source_of_type_mismatch_constraint(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         source: TypeMismatchSource<'tcx>,
     ) -> bool {
@@ -550,7 +550,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     // expected type.
     pub fn annotate_loop_expected_due_to_inference(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         error: Option<TypeError<'tcx>>,
     ) {
@@ -673,7 +673,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn annotate_expected_due_to_let_ty(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         error: Option<TypeError<'tcx>>,
     ) {
@@ -782,7 +782,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn annotate_alternative_method_deref(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         error: Option<TypeError<'tcx>>,
     ) {
@@ -1029,7 +1029,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn explain_self_literal(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'tcx>,
         expected: Ty<'tcx>,
         found: Ty<'tcx>,
@@ -1082,7 +1082,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn note_wrong_return_ty_due_to_generic_arg(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         checked_ty: Ty<'tcx>,
     ) {

compiler/rustc_hir_typeck/src/errors.rs

@@ -3,8 +3,8 @@ use std::borrow::Cow;
 
 use crate::fluent_generated as fluent;
 use rustc_errors::{
-    codes::*, AddToDiagnostic, Applicability, Diagnostic, DiagnosticArgValue, IntoDiagnosticArg,
+    codes::*, AddToDiagnostic, Applicability, DiagnosticArgValue, DiagnosticBuilder,
-    MultiSpan, SubdiagnosticMessageOp,
+    EmissionGuarantee, IntoDiagnosticArg, MultiSpan, SubdiagnosticMessageOp,
 };
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
 use rustc_middle::ty::Ty;
@@ -195,7 +195,11 @@ pub struct TypeMismatchFruTypo {
 }
 
 impl AddToDiagnostic for TypeMismatchFruTypo {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         diag.arg("expr", self.expr.as_deref().unwrap_or("NONE"));
 
         // Only explain that `a ..b` is a range if it's split up
@@ -370,7 +374,11 @@ pub struct RemoveSemiForCoerce {
 }
 
 impl AddToDiagnostic for RemoveSemiForCoerce {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         let mut multispan: MultiSpan = self.semi.into();
         multispan.push_span_label(self.expr, fluent::hir_typeck_remove_semi_for_coerce_expr);
         multispan.push_span_label(self.ret, fluent::hir_typeck_remove_semi_for_coerce_ret);
@@ -541,8 +549,12 @@ pub enum CastUnknownPointerSub {
     From(Span),
 }
 
-impl AddToDiagnostic for CastUnknownPointerSub {
+impl rustc_errors::AddToDiagnostic for CastUnknownPointerSub {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, f: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        f: F,
+    ) {
         match self {
             CastUnknownPointerSub::To(span) => {
                 let msg = f(diag, crate::fluent_generated::hir_typeck_label_to);

compiler/rustc_hir_typeck/src/expr.rs

@@ -26,8 +26,8 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_data_structures::unord::UnordMap;
 use rustc_errors::{
-    codes::*, pluralize, struct_span_code_err, AddToDiagnostic, Applicability, Diagnostic,
+    codes::*, pluralize, struct_span_code_err, AddToDiagnostic, Applicability, DiagnosticBuilder,
-    DiagnosticBuilder, ErrorGuaranteed, StashKey,
+    ErrorGuaranteed, StashKey,
 };
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, DefKind, Res};
@@ -69,7 +69,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         &self,
         expr: &'tcx hir::Expr<'tcx>,
         expected_ty: Ty<'tcx>,
-        extend_err: impl FnOnce(&mut Diagnostic),
+        extend_err: impl FnOnce(&mut DiagnosticBuilder<'_>),
     ) -> Ty<'tcx> {
         let mut ty = self.check_expr_with_expectation(expr, ExpectHasType(expected_ty));
 
@@ -958,7 +958,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         lhs: &'tcx hir::Expr<'tcx>,
         code: ErrCode,
         op_span: Span,
-        adjust_err: impl FnOnce(&mut Diagnostic),
+        adjust_err: impl FnOnce(&mut DiagnosticBuilder<'_>),
     ) {
         if lhs.is_syntactic_place_expr() {
             return;
@@ -1223,7 +1223,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
         let lhs_ty = self.check_expr_with_needs(lhs, Needs::MutPlace);
 
-        let suggest_deref_binop = |err: &mut Diagnostic, rhs_ty: Ty<'tcx>| {
+        let suggest_deref_binop = |err: &mut DiagnosticBuilder<'_>, rhs_ty: Ty<'tcx>| {
             if let Some(lhs_deref_ty) = self.deref_once_mutably_for_diagnostic(lhs_ty) {
                 // Can only assign if the type is sized, so if `DerefMut` yields a type that is
                 // unsized, do not suggest dereferencing it.
@@ -2008,7 +2008,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         last_expr_field: &hir::ExprField<'tcx>,
         variant: &ty::VariantDef,
         args: GenericArgsRef<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
     ) {
         // I don't use 'is_range_literal' because only double-sided, half-open ranges count.
         if let ExprKind::Struct(QPath::LangItem(LangItem::Range, ..), [range_start, range_end], _) =
@@ -2524,7 +2524,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn suggest_await_on_field_access(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         field_ident: Ident,
         base: &'tcx hir::Expr<'tcx>,
         ty: Ty<'tcx>,
@@ -2723,7 +2723,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         err.emit()
     }
 
-    fn point_at_param_definition(&self, err: &mut Diagnostic, param: ty::ParamTy) {
+    fn point_at_param_definition(&self, err: &mut DiagnosticBuilder<'_>, param: ty::ParamTy) {
         let generics = self.tcx.generics_of(self.body_id);
         let generic_param = generics.type_param(&param, self.tcx);
         if let ty::GenericParamDefKind::Type { synthetic: true, .. } = generic_param.kind {
@@ -2742,7 +2742,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn maybe_suggest_array_indexing(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         base: &hir::Expr<'_>,
         field: Ident,
@@ -2766,7 +2766,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn suggest_first_deref_field(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         base: &hir::Expr<'_>,
         field: Ident,

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -5,7 +5,7 @@ use crate::rvalue_scopes;
 use crate::{BreakableCtxt, Diverges, Expectation, FnCtxt, LoweredTy};
 use rustc_data_structures::captures::Captures;
 use rustc_data_structures::fx::FxHashSet;
-use rustc_errors::{Applicability, Diagnostic, ErrorGuaranteed, MultiSpan, StashKey};
+use rustc_errors::{Applicability, DiagnosticBuilder, ErrorGuaranteed, MultiSpan, StashKey};
 use rustc_hir as hir;
 use rustc_hir::def::{CtorOf, DefKind, Res};
 use rustc_hir::def_id::DefId;
@@ -1020,7 +1020,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub(in super::super) fn note_internal_mutation_in_method(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         expected: Option<Ty<'tcx>>,
         found: Ty<'tcx>,

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -13,7 +13,7 @@ use itertools::Itertools;
 use rustc_ast as ast;
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_errors::{
-    codes::*, pluralize, Applicability, Diagnostic, ErrorGuaranteed, MultiSpan, StashKey,
+    codes::*, pluralize, Applicability, DiagnosticBuilder, ErrorGuaranteed, MultiSpan, StashKey,
 };
 use rustc_hir as hir;
 use rustc_hir::def::{CtorOf, DefKind, Res};
@@ -1936,7 +1936,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn label_fn_like(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         callable_def_id: Option<DefId>,
         callee_ty: Option<Ty<'tcx>>,
         call_expr: &'tcx hir::Expr<'tcx>,

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -12,7 +12,7 @@ use core::cmp::min;
 use core::iter;
 use rustc_ast::util::parser::{ExprPrecedence, PREC_POSTFIX};
 use rustc_data_structures::packed::Pu128;
-use rustc_errors::{Applicability, Diagnostic, MultiSpan};
+use rustc_errors::{Applicability, DiagnosticBuilder, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def::Res;
 use rustc_hir::def::{CtorKind, CtorOf, DefKind};
@@ -48,7 +48,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             .copied()
     }
 
-    pub(in super::super) fn suggest_semicolon_at_end(&self, span: Span, err: &mut Diagnostic) {
+    pub(in super::super) fn suggest_semicolon_at_end(
+        &self,
+        span: Span,
+        err: &mut DiagnosticBuilder<'_>,
+    ) {
         // This suggestion is incorrect for
         // fn foo() -> bool { match () { () => true } || match () { () => true } }
         err.span_suggestion_short(
@@ -66,7 +70,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// - Possible missing return type if the return type is the default, and not `fn main()`.
     pub fn suggest_mismatched_types_on_tail(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &'tcx hir::Expr<'tcx>,
         expected: Ty<'tcx>,
         found: Ty<'tcx>,
@@ -97,7 +101,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// ```
     pub(crate) fn suggest_fn_call(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         found: Ty<'tcx>,
         can_satisfy: impl FnOnce(Ty<'tcx>) -> bool,
@@ -179,7 +183,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub fn suggest_two_fn_call(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         lhs_expr: &'tcx hir::Expr<'tcx>,
         lhs_ty: Ty<'tcx>,
         rhs_expr: &'tcx hir::Expr<'tcx>,
@@ -253,7 +257,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub fn suggest_remove_last_method_call(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'tcx>,
         expected: Ty<'tcx>,
     ) -> bool {
@@ -282,7 +286,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub fn suggest_deref_ref_or_into(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'tcx>,
         expected: Ty<'tcx>,
         found: Ty<'tcx>,
@@ -540,7 +544,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// in the heap by calling `Box::new()`.
     pub(in super::super) fn suggest_boxing_when_appropriate(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         span: Span,
         hir_id: HirId,
         expected: Ty<'tcx>,
@@ -583,7 +587,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// suggest a non-capturing closure
     pub(in super::super) fn suggest_no_capture_closure(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expected: Ty<'tcx>,
         found: Ty<'tcx>,
     ) -> bool {
@@ -620,7 +624,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     #[instrument(skip(self, err))]
     pub(in super::super) fn suggest_calling_boxed_future_when_appropriate(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         expected: Ty<'tcx>,
         found: Ty<'tcx>,
@@ -732,7 +736,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// block is needed to be added too (`|| { expr; }`). This is denoted by `needs_block`.
     pub fn suggest_missing_semicolon(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expression: &'tcx hir::Expr<'tcx>,
         expected: Ty<'tcx>,
         needs_block: bool,
@@ -791,7 +795,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     #[instrument(level = "trace", skip(self, err))]
     pub(in super::super) fn suggest_missing_return_type(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         fn_decl: &hir::FnDecl<'tcx>,
         expected: Ty<'tcx>,
         found: Ty<'tcx>,
@@ -909,7 +913,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// ```
     fn try_suggest_return_impl_trait(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expected: Ty<'tcx>,
         found: Ty<'tcx>,
         fn_id: hir::HirId,
@@ -1014,7 +1018,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub(in super::super) fn suggest_missing_break_or_return_expr(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &'tcx hir::Expr<'tcx>,
         fn_decl: &hir::FnDecl<'tcx>,
         expected: Ty<'tcx>,
@@ -1118,7 +1122,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub(in super::super) fn suggest_missing_parentheses(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
     ) -> bool {
         let sp = self.tcx.sess.source_map().start_point(expr.span).with_parent(None);
@@ -1136,7 +1140,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// if it was possibly mistaken array syntax.
     pub(crate) fn suggest_block_to_brackets_peeling_refs(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         mut expr: &hir::Expr<'_>,
         mut expr_ty: Ty<'tcx>,
         mut expected_ty: Ty<'tcx>,
@@ -1163,7 +1167,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub(crate) fn suggest_clone_for_ref(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         expr_ty: Ty<'tcx>,
         expected_ty: Ty<'tcx>,
@@ -1198,7 +1202,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub(crate) fn suggest_copied_cloned_or_as_ref(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         expr_ty: Ty<'tcx>,
         expected_ty: Ty<'tcx>,
@@ -1248,7 +1252,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub(crate) fn suggest_into(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         expr_ty: Ty<'tcx>,
         expected_ty: Ty<'tcx>,
@@ -1311,7 +1315,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// When expecting a `bool` and finding an `Option`, suggests using `let Some(..)` or `.is_some()`
     pub(crate) fn suggest_option_to_bool(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         expr_ty: Ty<'tcx>,
         expected_ty: Ty<'tcx>,
@@ -1368,7 +1372,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// in case the block was mistaken array syntax, e.g. `{ 1 }` -> `[ 1 ]`.
     pub(crate) fn suggest_block_to_brackets(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         blk: &hir::Block<'_>,
         blk_ty: Ty<'tcx>,
         expected_ty: Ty<'tcx>,
@@ -1408,7 +1412,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     #[instrument(skip(self, err))]
     pub(crate) fn suggest_floating_point_literal(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         expected_ty: Ty<'tcx>,
     ) -> bool {
@@ -1479,7 +1483,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     #[instrument(skip(self, err))]
     pub(crate) fn suggest_null_ptr_for_literal_zero_given_to_ptr_arg(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         expected_ty: Ty<'tcx>,
     ) -> bool {
@@ -1517,7 +1521,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub(crate) fn suggest_associated_const(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'tcx>,
         expected_ty: Ty<'tcx>,
     ) -> bool {
@@ -1611,7 +1615,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// which is a side-effect of autoref.
     pub(crate) fn note_type_is_not_clone(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         expected_ty: Ty<'tcx>,
         found_ty: Ty<'tcx>,
         expr: &hir::Expr<'_>,
@@ -1811,7 +1815,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         &self,
         blk: &'tcx hir::Block<'tcx>,
         expected_ty: Ty<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
     ) -> bool {
         if let Some((span_semi, boxed)) = self.err_ctxt().could_remove_semicolon(blk, expected_ty) {
             if let StatementAsExpression::NeedsBoxing = boxed {
@@ -1856,7 +1860,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub(crate) fn suggest_missing_unwrap_expect(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'tcx>,
         expected: Ty<'tcx>,
         found: Ty<'tcx>,
@@ -1939,7 +1943,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub(crate) fn suggest_coercing_result_via_try_operator(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'tcx>,
         expected: Ty<'tcx>,
         found: Ty<'tcx>,
@@ -1986,7 +1990,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// sole field is of the found type, suggest such variants. (Issue #42764)
     pub(crate) fn suggest_compatible_variants(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         expected: Ty<'tcx>,
         expr_ty: Ty<'tcx>,
@@ -2175,7 +2179,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub(crate) fn suggest_non_zero_new_unwrap(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         expected: Ty<'tcx>,
         expr_ty: Ty<'tcx>,
@@ -2719,7 +2723,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub(crate) fn suggest_cast(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         checked_ty: Ty<'tcx>,
         expected_ty: Ty<'tcx>,
@@ -2847,7 +2851,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let in_const_context = self.tcx.hir().is_inside_const_context(expr.hir_id);
 
         let suggest_fallible_into_or_lhs_from =
-            |err: &mut Diagnostic, exp_to_found_is_fallible: bool| {
+            |err: &mut DiagnosticBuilder<'_>, exp_to_found_is_fallible: bool| {
                 // If we know the expression the expected type is derived from, we might be able
                 // to suggest a widening conversion rather than a narrowing one (which may
                 // panic). For example, given x: u8 and y: u32, if we know the span of "x",
@@ -2887,7 +2891,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             };
 
         let suggest_to_change_suffix_or_into =
-            |err: &mut Diagnostic,
+            |err: &mut DiagnosticBuilder<'_>,
              found_to_exp_is_fallible: bool,
              exp_to_found_is_fallible: bool| {
                 let exp_is_lhs = expected_ty_expr.is_some_and(|e| self.tcx.hir().is_lhs(e.hir_id));
@@ -3085,7 +3089,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// Identify when the user has written `foo..bar()` instead of `foo.bar()`.
     pub(crate) fn suggest_method_call_on_range_literal(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'tcx>,
         checked_ty: Ty<'tcx>,
         expected_ty: Ty<'tcx>,
@@ -3163,7 +3167,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// block without a tail expression.
     pub(crate) fn suggest_return_binding_for_missing_tail_expr(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         checked_ty: Ty<'tcx>,
         expected_ty: Ty<'tcx>,

compiler/rustc_hir_typeck/src/method/mod.rs

@@ -11,7 +11,7 @@ pub use self::suggest::SelfSource;
 pub use self::MethodError::*;
 
 use crate::FnCtxt;
-use rustc_errors::{Applicability, Diagnostic, SubdiagnosticMessage};
+use rustc_errors::{Applicability, DiagnosticBuilder, SubdiagnosticMessage};
 use rustc_hir as hir;
 use rustc_hir::def::{CtorOf, DefKind, Namespace};
 use rustc_hir::def_id::DefId;
@@ -126,7 +126,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     #[instrument(level = "debug", skip(self, err, call_expr))]
     pub(crate) fn suggest_method_call(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         msg: impl Into<SubdiagnosticMessage> + std::fmt::Debug,
         method_name: Ident,
         self_ty: Ty<'tcx>,

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -12,8 +12,8 @@ use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_data_structures::sorted_map::SortedMap;
 use rustc_data_structures::unord::UnordSet;
 use rustc_errors::{
-    codes::*, pluralize, struct_span_code_err, Applicability, Diagnostic, DiagnosticBuilder,
+    codes::*, pluralize, struct_span_code_err, Applicability, DiagnosticBuilder, MultiSpan,
-    MultiSpan, StashKey,
+    StashKey,
 };
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
@@ -1127,7 +1127,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
         }
 
-        let label_span_not_found = |err: &mut Diagnostic| {
+        let label_span_not_found = |err: &mut DiagnosticBuilder<'_>| {
             if unsatisfied_predicates.is_empty() {
                 err.span_label(span, format!("{item_kind} not found in `{ty_str}`"));
                 let is_string_or_ref_str = match rcvr_ty.kind() {
@@ -1438,7 +1438,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self_source: SelfSource<'tcx>,
         args: Option<&'tcx [hir::Expr<'tcx>]>,
         span: Span,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         sources: &mut Vec<CandidateSource>,
         sugg_span: Option<Span>,
     ) {
@@ -1584,7 +1584,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     /// Look at all the associated functions without receivers in the type's inherent impls
     /// to look for builders that return `Self`, `Option<Self>` or `Result<Self, _>`.
-    fn find_builder_fn(&self, err: &mut Diagnostic, rcvr_ty: Ty<'tcx>) {
+    fn find_builder_fn(&self, err: &mut DiagnosticBuilder<'_>, rcvr_ty: Ty<'tcx>) {
         let ty::Adt(adt_def, _) = rcvr_ty.kind() else {
             return;
         };
@@ -1665,7 +1665,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// doesn't take a `self` receiver.
     fn suggest_associated_call_syntax(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         static_candidates: &Vec<CandidateSource>,
         rcvr_ty: Ty<'tcx>,
         source: SelfSource<'tcx>,
@@ -1809,7 +1809,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         rcvr_ty: Ty<'tcx>,
         expr: &hir::Expr<'_>,
         item_name: Ident,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
     ) -> bool {
         let tcx = self.tcx;
         let field_receiver = self.autoderef(span, rcvr_ty).find_map(|(ty, _)| match ty.kind() {
@@ -2132,7 +2132,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// Suggest calling a method on a field i.e. `a.field.bar()` instead of `a.bar()`
     fn suggest_calling_method_on_field(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         source: SelfSource<'tcx>,
         span: Span,
         actual: Ty<'tcx>,
@@ -2212,7 +2212,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn suggest_unwrapping_inner_self(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         source: SelfSource<'tcx>,
         actual: Ty<'tcx>,
         item_name: Ident,
@@ -2401,7 +2401,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub(crate) fn note_unmet_impls_on_type(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         errors: Vec<FulfillmentError<'tcx>>,
         suggest_derive: bool,
     ) {
@@ -2484,7 +2484,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn note_predicate_source_and_get_derives(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         unsatisfied_predicates: &[(
             ty::Predicate<'tcx>,
             Option<ty::Predicate<'tcx>>,
@@ -2566,7 +2566,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub(crate) fn suggest_derive(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         unsatisfied_predicates: &[(
             ty::Predicate<'tcx>,
             Option<ty::Predicate<'tcx>>,
@@ -2602,7 +2602,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn note_derefed_ty_has_method(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         self_source: SelfSource<'tcx>,
         rcvr_ty: Ty<'tcx>,
         item_name: Ident,
@@ -2682,7 +2682,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn suggest_await_before_method(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         item_name: Ident,
         ty: Ty<'tcx>,
         call: &hir::Expr<'_>,
@@ -2705,7 +2705,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         }
     }
 
-    fn suggest_use_candidates(&self, err: &mut Diagnostic, msg: String, candidates: Vec<DefId>) {
+    fn suggest_use_candidates(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        msg: String,
+        candidates: Vec<DefId>,
+    ) {
         let parent_map = self.tcx.visible_parent_map(());
 
         // Separate out candidates that must be imported with a glob, because they are named `_`
@@ -2752,7 +2757,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn suggest_valid_traits(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         valid_out_of_scope_traits: Vec<DefId>,
         explain: bool,
     ) -> bool {
@@ -2793,7 +2798,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn suggest_traits_to_import(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         span: Span,
         rcvr_ty: Ty<'tcx>,
         item_name: Ident,
@@ -3332,7 +3337,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// FIXME: currently not working for suggesting `map_or_else`, see #102408
     pub(crate) fn suggest_else_fn_with_closure(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         expr: &hir::Expr<'_>,
         found: Ty<'tcx>,
         expected: Ty<'tcx>,
@@ -3458,7 +3463,7 @@ pub fn all_traits(tcx: TyCtxt<'_>) -> Vec<TraitInfo> {
 
 fn print_disambiguation_help<'tcx>(
     tcx: TyCtxt<'tcx>,
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_>,
     source: SelfSource<'tcx>,
     args: Option<&'tcx [hir::Expr<'tcx>]>,
     trait_ref: ty::TraitRef<'tcx>,

compiler/rustc_hir_typeck/src/op.rs

@@ -5,7 +5,7 @@ use super::FnCtxt;
 use crate::Expectation;
 use rustc_ast as ast;
 use rustc_data_structures::packed::Pu128;
-use rustc_errors::{codes::*, struct_span_code_err, Applicability, Diagnostic, DiagnosticBuilder};
+use rustc_errors::{codes::*, struct_span_code_err, Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_infer::traits::ObligationCauseCode;
@@ -695,7 +695,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         rhs_expr: &'tcx hir::Expr<'tcx>,
         lhs_ty: Ty<'tcx>,
         rhs_ty: Ty<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         is_assign: IsAssign,
         op: hir::BinOp,
     ) -> bool {

compiler/rustc_hir_typeck/src/pat.rs

@@ -3,8 +3,8 @@ use crate::{errors, FnCtxt, LoweredTy};
 use rustc_ast as ast;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::{
-    codes::*, pluralize, struct_span_code_err, Applicability, Diagnostic, DiagnosticBuilder,
+    codes::*, pluralize, struct_span_code_err, Applicability, DiagnosticBuilder, ErrorGuaranteed,
-    ErrorGuaranteed, MultiSpan,
+    MultiSpan,
 };
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, DefKind, Res};
@@ -529,7 +529,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         ty
     }
 
-    fn endpoint_has_type(&self, err: &mut Diagnostic, span: Span, ty: Ty<'_>) {
+    fn endpoint_has_type(&self, err: &mut DiagnosticBuilder<'_>, span: Span, ty: Ty<'_>) {
         if !ty.references_error() {
             err.span_label(span, format!("this is of type `{ty}`"));
         }
@@ -683,7 +683,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn suggest_adding_missing_ref_or_removing_ref(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         span: Span,
         expected: Ty<'tcx>,
         actual: Ty<'tcx>,
@@ -715,7 +715,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     }
 
     // Precondition: pat is a Ref(_) pattern
-    fn borrow_pat_suggestion(&self, err: &mut Diagnostic, pat: &Pat<'_>) {
+    fn borrow_pat_suggestion(&self, err: &mut DiagnosticBuilder<'_>, pat: &Pat<'_>) {
         let tcx = self.tcx;
         if let PatKind::Ref(inner, mutbl) = pat.kind
             && let PatKind::Binding(_, _, binding, ..) = inner.kind
@@ -933,7 +933,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn maybe_suggest_range_literal(
         &self,
-        e: &mut Diagnostic,
+        e: &mut DiagnosticBuilder<'_>,
         opt_def_id: Option<hir::def_id::DefId>,
         ident: Ident,
     ) -> bool {

compiler/rustc_infer/src/errors/mod.rs

@@ -1,7 +1,8 @@
 use hir::GenericParamKind;
 use rustc_errors::{
-    codes::*, AddToDiagnostic, Applicability, Diagnostic, DiagnosticMessage,
+    codes::*, AddToDiagnostic, Applicability, DiagnosticBuilder, DiagnosticMessage,
-    DiagnosticStyledString, IntoDiagnosticArg, MultiSpan, SubdiagnosticMessageOp,
+    DiagnosticStyledString, EmissionGuarantee, IntoDiagnosticArg, MultiSpan,
+    SubdiagnosticMessageOp,
 };
 use rustc_hir as hir;
 use rustc_hir::FnRetTy;
@@ -225,7 +226,11 @@ pub enum RegionOriginNote<'a> {
 }
 
 impl AddToDiagnostic for RegionOriginNote<'_> {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         let mut label_or_note = |span, msg: DiagnosticMessage| {
             let sub_count = diag.children.iter().filter(|d| d.span.is_dummy()).count();
             let expanded_sub_count = diag.children.iter().filter(|d| !d.span.is_dummy()).count();
@@ -286,7 +291,11 @@ pub enum LifetimeMismatchLabels {
 }
 
 impl AddToDiagnostic for LifetimeMismatchLabels {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         match self {
             LifetimeMismatchLabels::InRet { param_span, ret_span, span, label_var1 } => {
                 diag.span_label(param_span, fluent::infer_declared_different);
@@ -330,7 +339,11 @@ pub struct AddLifetimeParamsSuggestion<'a> {
 }
 
 impl AddToDiagnostic for AddLifetimeParamsSuggestion<'_> {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         let mut mk_suggestion = || {
             let (
                 hir::Ty { kind: hir::TyKind::Ref(lifetime_sub, _), .. },
@@ -428,7 +441,11 @@ pub struct IntroducesStaticBecauseUnmetLifetimeReq {
 }
 
 impl AddToDiagnostic for IntroducesStaticBecauseUnmetLifetimeReq {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(mut self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        mut self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         self.unmet_requirements
             .push_span_label(self.binding_span, fluent::infer_msl_introduces_static);
         diag.span_note(self.unmet_requirements, fluent::infer_msl_unmet_req);
@@ -743,7 +760,11 @@ pub struct ConsiderBorrowingParamHelp {
 }
 
 impl AddToDiagnostic for ConsiderBorrowingParamHelp {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, f: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        f: F,
+    ) {
         let mut type_param_span: MultiSpan = self.spans.clone().into();
         for &span in &self.spans {
             // Seems like we can't call f() here as Into<DiagnosticMessage> is required
@@ -784,7 +805,11 @@ pub struct DynTraitConstraintSuggestion {
 }
 
 impl AddToDiagnostic for DynTraitConstraintSuggestion {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, f: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        f: F,
+    ) {
         let mut multi_span: MultiSpan = vec![self.span].into();
         multi_span.push_span_label(self.span, fluent::infer_dtcs_has_lifetime_req_label);
         multi_span.push_span_label(self.ident.span, fluent::infer_dtcs_introduces_requirement);
@@ -827,7 +852,11 @@ pub struct ReqIntroducedLocations {
 }
 
 impl AddToDiagnostic for ReqIntroducedLocations {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(mut self, diag: &mut Diagnostic, f: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        mut self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        f: F,
+    ) {
         for sp in self.spans {
             self.span.push_span_label(sp, fluent::infer_ril_introduced_here);
         }
@@ -846,7 +875,11 @@ pub struct MoreTargeted {
 }
 
 impl AddToDiagnostic for MoreTargeted {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _f: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         diag.code(E0772);
         diag.primary_message(fluent::infer_more_targeted);
         diag.arg("ident", self.ident);
@@ -1265,7 +1298,11 @@ pub struct SuggestTuplePatternMany {
 }
 
 impl AddToDiagnostic for SuggestTuplePatternMany {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, f: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        f: F,
+    ) {
         diag.arg("path", self.path);
         let message = f(diag, crate::fluent_generated::infer_stp_wrap_many.into());
         diag.multipart_suggestions(

compiler/rustc_infer/src/errors/note_and_explain.rs

@@ -1,6 +1,9 @@
 use crate::fluent_generated as fluent;
 use crate::infer::error_reporting::nice_region_error::find_anon_type;
-use rustc_errors::{AddToDiagnostic, Diagnostic, IntoDiagnosticArg, SubdiagnosticMessageOp};
+use rustc_errors::{
+    AddToDiagnostic, DiagnosticBuilder, EmissionGuarantee, IntoDiagnosticArg,
+    SubdiagnosticMessageOp,
+};
 use rustc_middle::ty::{self, TyCtxt};
 use rustc_span::{symbol::kw, Span};
 
@@ -160,7 +163,11 @@ impl RegionExplanation<'_> {
 }
 
 impl AddToDiagnostic for RegionExplanation<'_> {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, f: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        f: F,
+    ) {
         diag.arg("pref_kind", self.prefix);
         diag.arg("suff_kind", self.suffix);
         diag.arg("desc_kind", self.desc.kind);

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -60,8 +60,8 @@ use crate::traits::{
 
 use rustc_data_structures::fx::{FxIndexMap, FxIndexSet};
 use rustc_errors::{
-    codes::*, pluralize, struct_span_code_err, Applicability, DiagCtxt, Diagnostic,
+    codes::*, pluralize, struct_span_code_err, Applicability, DiagCtxt, DiagnosticBuilder,
-    DiagnosticBuilder, DiagnosticStyledString, ErrorGuaranteed, IntoDiagnosticArg,
+    DiagnosticStyledString, ErrorGuaranteed, IntoDiagnosticArg,
 };
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
@@ -155,7 +155,7 @@ impl<'tcx> Deref for TypeErrCtxt<'_, 'tcx> {
 
 pub(super) fn note_and_explain_region<'tcx>(
     tcx: TyCtxt<'tcx>,
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_>,
     prefix: &str,
     region: ty::Region<'tcx>,
     suffix: &str,
@@ -180,7 +180,7 @@ pub(super) fn note_and_explain_region<'tcx>(
 
 fn explain_free_region<'tcx>(
     tcx: TyCtxt<'tcx>,
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_>,
     prefix: &str,
     region: ty::Region<'tcx>,
     suffix: &str,
@@ -262,7 +262,7 @@ fn msg_span_from_named_region<'tcx>(
 }
 
 fn emit_msg_span(
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_>,
     prefix: &str,
     description: String,
     span: Option<Span>,
@@ -278,7 +278,7 @@ fn emit_msg_span(
 }
 
 fn label_msg_span(
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_>,
     prefix: &str,
     description: String,
     span: Option<Span>,
@@ -577,7 +577,11 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     }
 
     /// Adds a note if the types come from similarly named crates
-    fn check_and_note_conflicting_crates(&self, err: &mut Diagnostic, terr: TypeError<'tcx>) {
+    fn check_and_note_conflicting_crates(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        terr: TypeError<'tcx>,
+    ) {
         use hir::def_id::CrateNum;
         use rustc_hir::definitions::DisambiguatedDefPathData;
         use ty::print::Printer;
@@ -651,7 +655,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
             }
         }
 
-        let report_path_match = |err: &mut Diagnostic, did1: DefId, did2: DefId| {
+        let report_path_match = |err: &mut DiagnosticBuilder<'_>, did1: DefId, did2: DefId| {
             // Only report definitions from different crates. If both definitions
             // are from a local module we could have false positives, e.g.
             // let _ = [{struct Foo; Foo}, {struct Foo; Foo}];
@@ -701,7 +705,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
 
     fn note_error_origin(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         cause: &ObligationCause<'tcx>,
         exp_found: Option<ty::error::ExpectedFound<Ty<'tcx>>>,
         terr: TypeError<'tcx>,
@@ -1535,7 +1539,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     )]
     pub fn note_type_err(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         cause: &ObligationCause<'tcx>,
         secondary_span: Option<(Span, Cow<'static, str>)>,
         mut values: Option<ValuePairs<'tcx>>,
@@ -1582,14 +1586,14 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 types_visitor
             }
 
-            fn report(&self, err: &mut Diagnostic) {
+            fn report(&self, err: &mut DiagnosticBuilder<'_>) {
                 self.add_labels_for_types(err, "expected", &self.expected);
                 self.add_labels_for_types(err, "found", &self.found);
             }
 
             fn add_labels_for_types(
                 &self,
-                err: &mut Diagnostic,
+                err: &mut DiagnosticBuilder<'_>,
                 target: &str,
                 types: &FxIndexMap<TyCategory, FxIndexSet<Span>>,
             ) {
@@ -1803,7 +1807,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         |prim: Ty<'tcx>,
                          shadow: Ty<'tcx>,
                          defid: DefId,
-                         diagnostic: &mut Diagnostic| {
+                         diagnostic: &mut DiagnosticBuilder<'_>| {
                             let name = shadow.sort_string(self.tcx);
                             diagnostic.note(format!(
                             "{prim} and {name} have similar names, but are actually distinct types"
@@ -1823,7 +1827,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     let diagnose_adts =
                         |expected_adt: ty::AdtDef<'tcx>,
                          found_adt: ty::AdtDef<'tcx>,
-                         diagnostic: &mut Diagnostic| {
+                         diagnostic: &mut DiagnosticBuilder<'_>| {
                             let found_name = values.found.sort_string(self.tcx);
                             let expected_name = values.expected.sort_string(self.tcx);
 

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/different_lifetimes.rs

@@ -12,7 +12,7 @@ use crate::infer::SubregionOrigin;
 use crate::infer::TyCtxt;
 
 use rustc_errors::AddToDiagnostic;
-use rustc_errors::{Diagnostic, ErrorGuaranteed};
+use rustc_errors::{DiagnosticBuilder, ErrorGuaranteed};
 use rustc_hir::Ty;
 use rustc_middle::ty::Region;
 
@@ -142,7 +142,7 @@ pub fn suggest_adding_lifetime_params<'tcx>(
     sub: Region<'tcx>,
     ty_sup: &'tcx Ty<'_>,
     ty_sub: &'tcx Ty<'_>,
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_>,
 ) {
     let suggestion = AddLifetimeParamsSuggestion { tcx, sub, ty_sup, ty_sub, add_note: false };
     suggestion.add_to_diagnostic(err);

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/static_impl_trait.rs

@@ -9,7 +9,7 @@ use crate::infer::lexical_region_resolve::RegionResolutionError;
 use crate::infer::{SubregionOrigin, TypeTrace};
 use crate::traits::{ObligationCauseCode, UnifyReceiverContext};
 use rustc_data_structures::fx::FxIndexSet;
-use rustc_errors::{AddToDiagnostic, Applicability, Diagnostic, ErrorGuaranteed, MultiSpan};
+use rustc_errors::{AddToDiagnostic, Applicability, DiagnosticBuilder, ErrorGuaranteed, MultiSpan};
 use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit::{walk_ty, Visitor};
 use rustc_hir::{
@@ -261,7 +261,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
 
 pub fn suggest_new_region_bound(
     tcx: TyCtxt<'_>,
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_>,
     fn_returns: Vec<&rustc_hir::Ty<'_>>,
     lifetime_name: String,
     arg: Option<String>,
@@ -488,7 +488,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
     /// `'static` obligation. Suggest relaxing that implicit bound.
     fn find_impl_on_dyn_trait(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         ty: Ty<'_>,
         ctxt: &UnifyReceiverContext<'tcx>,
     ) -> bool {
@@ -521,7 +521,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
 
     fn suggest_constrain_dyn_trait_in_impl(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         found_dids: &FxIndexSet<DefId>,
         ident: Ident,
         self_ty: &hir::Ty<'_>,

compiler/rustc_infer/src/infer/error_reporting/note.rs

@@ -5,7 +5,7 @@ use crate::errors::{
 use crate::fluent_generated as fluent;
 use crate::infer::error_reporting::{note_and_explain_region, TypeErrCtxt};
 use crate::infer::{self, SubregionOrigin};
-use rustc_errors::{AddToDiagnostic, Diagnostic, DiagnosticBuilder};
+use rustc_errors::{AddToDiagnostic, DiagnosticBuilder};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::traits::ObligationCauseCode;
 use rustc_middle::ty::error::TypeError;
@@ -15,7 +15,11 @@ use rustc_span::symbol::kw;
 use super::ObligationCauseAsDiagArg;
 
 impl<'tcx> TypeErrCtxt<'_, 'tcx> {
-    pub(super) fn note_region_origin(&self, err: &mut Diagnostic, origin: &SubregionOrigin<'tcx>) {
+    pub(super) fn note_region_origin(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        origin: &SubregionOrigin<'tcx>,
+    ) {
         match *origin {
             infer::Subtype(ref trace) => RegionOriginNote::WithRequirement {
                 span: trace.cause.span,
@@ -290,7 +294,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         &self,
         trait_item_def_id: DefId,
         impl_item_def_id: LocalDefId,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
     ) {
         // FIXME(compiler-errors): Right now this is only being used for region
         // predicate mismatches. Ideally, we'd use it for *all* predicate mismatches,

compiler/rustc_infer/src/infer/error_reporting/note_and_explain.rs

@@ -1,6 +1,6 @@
 use super::TypeErrCtxt;
 use rustc_errors::Applicability::{MachineApplicable, MaybeIncorrect};
-use rustc_errors::{pluralize, Diagnostic, MultiSpan};
+use rustc_errors::{pluralize, DiagnosticBuilder, MultiSpan};
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_middle::traits::ObligationCauseCode;
@@ -15,7 +15,7 @@ use rustc_span::{def_id::DefId, sym, BytePos, Span, Symbol};
 impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     pub fn note_and_explain_type_err(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         err: TypeError<'tcx>,
         cause: &ObligationCause<'tcx>,
         sp: Span,
@@ -522,7 +522,7 @@ impl<T> Trait<T> for X {
 
     fn suggest_constraint(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         msg: impl Fn() -> String,
         body_owner_def_id: DefId,
         proj_ty: &ty::AliasTy<'tcx>,
@@ -595,7 +595,7 @@ impl<T> Trait<T> for X {
     ///    fn that returns the type.
     fn expected_projection(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         proj_ty: &ty::AliasTy<'tcx>,
         values: ExpectedFound<Ty<'tcx>>,
         body_owner_def_id: DefId,
@@ -705,7 +705,7 @@ fn foo(&self) -> Self::T { String::new() }
     /// a return type. This can occur when dealing with `TryStream` (#71035).
     fn suggest_constraining_opaque_associated_type(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         msg: impl Fn() -> String,
         proj_ty: &ty::AliasTy<'tcx>,
         ty: Ty<'tcx>,
@@ -740,7 +740,7 @@ fn foo(&self) -> Self::T { String::new() }
 
     fn point_at_methods_that_satisfy_associated_type(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         assoc_container_id: DefId,
         current_method_ident: Option<Symbol>,
         proj_ty_item_def_id: DefId,
@@ -798,7 +798,7 @@ fn foo(&self) -> Self::T { String::new() }
 
     fn point_at_associated_type(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         body_owner_def_id: DefId,
         found: Ty<'tcx>,
     ) -> bool {
@@ -879,7 +879,7 @@ fn foo(&self) -> Self::T { String::new() }
     /// type is defined on a supertrait of the one present in the bounds.
     fn constrain_generic_bound_associated_type_structured_suggestion(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         trait_ref: &ty::TraitRef<'tcx>,
         bounds: hir::GenericBounds<'_>,
         assoc: ty::AssocItem,
@@ -916,7 +916,7 @@ fn foo(&self) -> Self::T { String::new() }
     /// associated type to a given type `ty`.
     fn constrain_associated_type_structured_suggestion(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         span: Span,
         assoc: ty::AssocItem,
         assoc_args: &[ty::GenericArg<'tcx>],

compiler/rustc_infer/src/infer/error_reporting/suggest.rs

@@ -1,7 +1,7 @@
 use hir::def::CtorKind;
 use hir::intravisit::{walk_expr, walk_stmt, Visitor};
 use rustc_data_structures::fx::FxIndexSet;
-use rustc_errors::{Applicability, Diagnostic};
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_middle::traits::{
     IfExpressionCause, MatchExpressionArmCause, ObligationCause, ObligationCauseCode,
@@ -76,7 +76,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
 
     pub(super) fn suggest_boxing_for_return_impl_trait(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         return_sp: Span,
         arm_spans: impl Iterator<Item = Span>,
     ) {
@@ -100,7 +100,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         &self,
         cause: &ObligationCause<'tcx>,
         exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
     ) {
         // Heavily inspired by `FnCtxt::suggest_compatible_variants`, with
         // some modifications due to that being in typeck and this being in infer.
@@ -177,7 +177,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         cause: &ObligationCause<'tcx>,
         exp_span: Span,
         exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
     ) {
         debug!(
             "suggest_await_on_expect_found: exp_span={:?}, expected_ty={:?}, found_ty={:?}",
@@ -258,7 +258,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         &self,
         cause: &ObligationCause<'tcx>,
         exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
     ) {
         debug!(
             "suggest_accessing_field_where_appropriate(cause={:?}, exp_found={:?})",
@@ -298,7 +298,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         cause: &ObligationCause<'tcx>,
         span: Span,
         exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
     ) {
         debug!("suggest_function_pointers(cause={:?}, exp_found={:?})", cause, exp_found);
         let ty::error::ExpectedFound { expected, found } = exp_found;
@@ -532,7 +532,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         span: Span,
         hir: hir::Node<'_>,
         exp_found: &ty::error::ExpectedFound<ty::PolyTraitRef<'tcx>>,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
     ) {
         // 0. Extract fn_decl from hir
         let hir::Node::Expr(hir::Expr {
@@ -818,7 +818,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         &self,
         blk: &'tcx hir::Block<'tcx>,
         expected_ty: Ty<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
     ) -> bool {
         let diag = self.consider_returning_binding_diag(blk, expected_ty);
         match diag {

compiler/rustc_lint/src/context/diagnostics.rs

@@ -1,3 +1,6 @@
+#![allow(rustc::diagnostic_outside_of_impl)]
+#![allow(rustc::untranslatable_diagnostic)]
+
 use rustc_ast::util::unicode::TEXT_FLOW_CONTROL_CHARS;
 use rustc_errors::{add_elided_lifetime_in_path_suggestion, DiagnosticBuilder};
 use rustc_errors::{Applicability, SuggestionStyle};

compiler/rustc_lint/src/errors.rs

@@ -1,5 +1,7 @@
 use crate::fluent_generated as fluent;
-use rustc_errors::{codes::*, AddToDiagnostic, Diagnostic, SubdiagnosticMessageOp};
+use rustc_errors::{
+    codes::*, AddToDiagnostic, DiagnosticBuilder, EmissionGuarantee, SubdiagnosticMessageOp,
+};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_session::lint::Level;
 use rustc_span::{Span, Symbol};
@@ -24,7 +26,11 @@ pub enum OverruledAttributeSub {
 }
 
 impl AddToDiagnostic for OverruledAttributeSub {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         match self {
             OverruledAttributeSub::DefaultSource { id } => {
                 diag.note(fluent::lint_default_source);

compiler/rustc_lint/src/levels.rs

@@ -1062,6 +1062,9 @@ impl<'s, P: LintLevelsProvider> LintLevelsBuilder<'s, P> {
                 if self.lint_added_lints {
                     let lint = builtin::UNKNOWN_LINTS;
                     let (level, src) = self.lint_level(builtin::UNKNOWN_LINTS);
+                    // FIXME: make this translatable
+                    #[allow(rustc::diagnostic_outside_of_impl)]
+                    #[allow(rustc::untranslatable_diagnostic)]
                     lint_level(
                         self.sess,
                         lint,

compiler/rustc_lint/src/lints.rs

@@ -5,8 +5,8 @@ use std::num::NonZero;
 use crate::errors::RequestedLevel;
 use crate::fluent_generated as fluent;
 use rustc_errors::{
-    codes::*, AddToDiagnostic, Applicability, DecorateLint, Diagnostic, DiagnosticBuilder,
+    codes::*, AddToDiagnostic, Applicability, DecorateLint, DiagnosticBuilder, DiagnosticMessage,
-    DiagnosticMessage, DiagnosticStyledString, SubdiagnosticMessageOp, SuggestionStyle,
+    DiagnosticStyledString, EmissionGuarantee, SubdiagnosticMessageOp, SuggestionStyle,
 };
 use rustc_hir::def_id::DefId;
 use rustc_macros::{LintDiagnostic, Subdiagnostic};
@@ -267,17 +267,21 @@ pub struct SuggestChangingAssocTypes<'a, 'b> {
     pub ty: &'a rustc_hir::Ty<'b>,
 }
 
-impl AddToDiagnostic for SuggestChangingAssocTypes<'_, '_> {
+impl<'a, 'b> AddToDiagnostic for SuggestChangingAssocTypes<'a, 'b> {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         // Access to associates types should use `<T as Bound>::Assoc`, which does not need a
         // bound. Let's see if this type does that.
 
         // We use a HIR visitor to walk the type.
         use rustc_hir::intravisit::{self, Visitor};
-        struct WalkAssocTypes<'a> {
+        struct WalkAssocTypes<'a, 'b, G: EmissionGuarantee> {
-            err: &'a mut Diagnostic,
+            err: &'a mut DiagnosticBuilder<'b, G>,
         }
-        impl Visitor<'_> for WalkAssocTypes<'_> {
+        impl<'a, 'b, G: EmissionGuarantee> Visitor<'_> for WalkAssocTypes<'a, 'b, G> {
             fn visit_qpath(
                 &mut self,
                 qpath: &rustc_hir::QPath<'_>,
@@ -320,7 +324,11 @@ pub struct BuiltinTypeAliasGenericBoundsSuggestion {
 }
 
 impl AddToDiagnostic for BuiltinTypeAliasGenericBoundsSuggestion {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         diag.multipart_suggestion(
             fluent::lint_suggestion,
             self.suggestions,
@@ -437,7 +445,11 @@ pub struct BuiltinUnpermittedTypeInitSub {
 }
 
 impl AddToDiagnostic for BuiltinUnpermittedTypeInitSub {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         let mut err = self.err;
         loop {
             if let Some(span) = err.span {
@@ -488,7 +500,11 @@ pub struct BuiltinClashingExternSub<'a> {
 }
 
 impl AddToDiagnostic for BuiltinClashingExternSub<'_> {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         let mut expected_str = DiagnosticStyledString::new();
         expected_str.push(self.expected.fn_sig(self.tcx).to_string(), false);
         let mut found_str = DiagnosticStyledString::new();
@@ -766,7 +782,11 @@ pub struct HiddenUnicodeCodepointsDiagLabels {
 }
 
 impl AddToDiagnostic for HiddenUnicodeCodepointsDiagLabels {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         for (c, span) in self.spans {
             diag.span_label(span, format!("{c:?}"));
         }
@@ -780,7 +800,11 @@ pub enum HiddenUnicodeCodepointsDiagSub {
 
 // Used because of multiple multipart_suggestion and note
 impl AddToDiagnostic for HiddenUnicodeCodepointsDiagSub {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         match self {
             HiddenUnicodeCodepointsDiagSub::Escape { spans } => {
                 diag.multipart_suggestion_with_style(
@@ -928,7 +952,11 @@ pub struct NonBindingLetSub {
 }
 
 impl AddToDiagnostic for NonBindingLetSub {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         let can_suggest_binding = self.drop_fn_start_end.is_some() || !self.is_assign_desugar;
 
         if can_suggest_binding {
@@ -1208,7 +1236,11 @@ pub enum NonSnakeCaseDiagSub {
 }
 
 impl AddToDiagnostic for NonSnakeCaseDiagSub {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         match self {
             NonSnakeCaseDiagSub::Label { span } => {
                 diag.span_label(span, fluent::lint_label);
@@ -1401,7 +1433,11 @@ pub enum OverflowingBinHexSign {
 }
 
 impl AddToDiagnostic for OverflowingBinHexSign {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         match self {
             OverflowingBinHexSign::Positive => {
                 diag.note(fluent::lint_positive_note);

compiler/rustc_macros/src/diagnostics/subdiagnostic.rs

@@ -87,9 +87,13 @@ impl SubdiagnosticDeriveBuilder {
         let f = &self.f;
         let ret = structure.gen_impl(quote! {
             gen impl rustc_errors::AddToDiagnostic for @Self {
-                fn add_to_diagnostic_with<__F>(self, #diag: &mut rustc_errors::Diagnostic, #f: __F)
+                fn add_to_diagnostic_with<__G, __F>(
-                where
+                    self,
-                    __F: rustc_errors::SubdiagnosticMessageOp,
+                    #diag: &mut rustc_errors::DiagnosticBuilder<'_, __G>,
+                    #f: __F
+                ) where
+                    __G: rustc_errors::EmissionGuarantee,
+                    __F: rustc_errors::SubdiagnosticMessageOp<__G>,
                 {
                     #implementation
                 }

compiler/rustc_metadata/src/errors.rs

@@ -505,6 +505,8 @@ impl<G: EmissionGuarantee> IntoDiagnostic<'_, G> for MultipleCandidates {
         diag.code(E0464);
         diag.span(self.span);
         for (i, candidate) in self.candidates.iter().enumerate() {
+            // FIXME: make this translatable
+            #[allow(rustc::untranslatable_diagnostic)]
             diag.note(format!("candidate #{}: {}", i + 1, candidate.display()));
         }
         diag
@@ -601,6 +603,8 @@ impl<G: EmissionGuarantee> IntoDiagnostic<'_, G> for InvalidMetadataFiles {
         diag.code(E0786);
         diag.span(self.span);
         for crate_rejection in self.crate_rejections {
+            // FIXME: make this translatable
+            #[allow(rustc::untranslatable_diagnostic)]
             diag.note(crate_rejection);
         }
         diag

compiler/rustc_middle/src/lint.rs

@@ -2,7 +2,7 @@ use std::cmp;
 
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::sorted_map::SortedMap;
-use rustc_errors::{Diagnostic, DiagnosticBuilder, DiagnosticMessage, MultiSpan};
+use rustc_errors::{DiagnosticBuilder, DiagnosticMessage, MultiSpan};
 use rustc_hir::{HirId, ItemLocalId};
 use rustc_session::lint::{
     builtin::{self, FORBIDDEN_LINT_GROUPS},
@@ -204,7 +204,7 @@ pub fn explain_lint_level_source(
     lint: &'static Lint,
     level: Level,
     src: LintLevelSource,
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_, ()>,
 ) {
     let name = lint.name_lower();
     if let Level::Allow = level {
@@ -359,7 +359,7 @@ pub fn lint_level(
         // Lint diagnostics that are covered by the expect level will not be emitted outside
         // the compiler. It is therefore not necessary to add any information for the user.
         // This will therefore directly call the decorate function which will in turn emit
-        // the `Diagnostic`.
+        // the diagnostic.
         if let Level::Expect(_) = level {
             decorate(&mut err);
             err.emit();
@@ -401,7 +401,7 @@ pub fn lint_level(
 
         // Finally, run `decorate`.
         decorate(&mut err);
-        explain_lint_level_source(lint, level, src, &mut *err);
+        explain_lint_level_source(lint, level, src, &mut err);
         err.emit()
     }
     lint_level_impl(sess, lint, level, src, span, msg, Box::new(decorate))

compiler/rustc_middle/src/middle/stability.rs

@@ -9,7 +9,7 @@ use rustc_attr::{
     self as attr, ConstStability, DefaultBodyStability, DeprecatedSince, Deprecation, Stability,
 };
 use rustc_data_structures::unord::UnordMap;
-use rustc_errors::{Applicability, Diagnostic};
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_feature::GateIssue;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId, LocalDefIdMap};
@@ -125,7 +125,7 @@ pub fn report_unstable(
 }
 
 pub fn deprecation_suggestion(
-    diag: &mut Diagnostic,
+    diag: &mut DiagnosticBuilder<'_, ()>,
     kind: &str,
     suggestion: Option<Symbol>,
     span: Span,

compiler/rustc_middle/src/traits/mod.rs

@@ -16,7 +16,7 @@ use crate::ty::GenericArgsRef;
 use crate::ty::{self, AdtKind, Ty};
 
 use rustc_data_structures::sync::Lrc;
-use rustc_errors::{Applicability, Diagnostic};
+use rustc_errors::{Applicability, DiagnosticBuilder, EmissionGuarantee};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_span::def_id::{LocalDefId, CRATE_DEF_ID};
@@ -908,7 +908,7 @@ pub enum ObjectSafetyViolationSolution {
 }
 
 impl ObjectSafetyViolationSolution {
-    pub fn add_to(self, err: &mut Diagnostic) {
+    pub fn add_to<G: EmissionGuarantee>(self, err: &mut DiagnosticBuilder<'_, G>) {
         match self {
             ObjectSafetyViolationSolution::None => {}
             ObjectSafetyViolationSolution::AddSelfOrMakeSized {

compiler/rustc_middle/src/ty/diagnostics.rs

@@ -11,7 +11,7 @@ use crate::ty::{
 };
 
 use rustc_data_structures::fx::FxHashMap;
-use rustc_errors::{Applicability, Diagnostic, DiagnosticArgValue, IntoDiagnosticArg};
+use rustc_errors::{Applicability, DiagnosticArgValue, DiagnosticBuilder, IntoDiagnosticArg};
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::DefId;
@@ -111,7 +111,7 @@ where
 pub fn suggest_arbitrary_trait_bound<'tcx>(
     tcx: TyCtxt<'tcx>,
     generics: &hir::Generics<'_>,
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_>,
     trait_pred: PolyTraitPredicate<'tcx>,
     associated_ty: Option<(&'static str, Ty<'tcx>)>,
 ) -> bool {
@@ -216,7 +216,7 @@ fn suggest_changing_unsized_bound(
 pub fn suggest_constraining_type_param(
     tcx: TyCtxt<'_>,
     generics: &hir::Generics<'_>,
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_>,
     param_name: &str,
     constraint: &str,
     def_id: Option<DefId>,
@@ -235,7 +235,7 @@ pub fn suggest_constraining_type_param(
 pub fn suggest_constraining_type_params<'a>(
     tcx: TyCtxt<'_>,
     generics: &hir::Generics<'_>,
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_>,
     param_names_and_constraints: impl Iterator<Item = (&'a str, &'a str, Option<DefId>)>,
     span_to_replace: Option<Span>,
 ) -> bool {

compiler/rustc_mir_build/src/errors.rs

@@ -1,7 +1,7 @@
 use crate::fluent_generated as fluent;
 use rustc_errors::DiagnosticArgValue;
 use rustc_errors::{
-    codes::*, AddToDiagnostic, Applicability, DiagCtxt, Diagnostic, DiagnosticBuilder,
+    codes::*, AddToDiagnostic, Applicability, DiagCtxt, DiagnosticBuilder, EmissionGuarantee,
     IntoDiagnostic, Level, MultiSpan, SubdiagnosticMessageOp,
 };
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
@@ -420,7 +420,11 @@ pub struct UnsafeNotInheritedLintNote {
 }
 
 impl AddToDiagnostic for UnsafeNotInheritedLintNote {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         diag.span_note(self.signature_span, fluent::mir_build_unsafe_fn_safe_body);
         let body_start = self.body_span.shrink_to_lo();
         let body_end = self.body_span.shrink_to_hi();
@@ -863,7 +867,11 @@ pub struct Variant {
 }
 
 impl<'tcx> AddToDiagnostic for AdtDefinedHere<'tcx> {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         diag.arg("ty", self.ty);
         let mut spans = MultiSpan::from(self.adt_def_span);
 

compiler/rustc_mir_transform/src/errors.rs

@@ -87,6 +87,9 @@ pub(crate) struct RequiresUnsafeDetail {
 }
 
 impl RequiresUnsafeDetail {
+    // FIXME: make this translatable
+    #[allow(rustc::diagnostic_outside_of_impl)]
+    #[allow(rustc::untranslatable_diagnostic)]
     fn add_subdiagnostics<G: EmissionGuarantee>(&self, diag: &mut DiagnosticBuilder<'_, G>) {
         use UnsafetyViolationDetails::*;
         match self.violation {

compiler/rustc_monomorphize/src/errors.rs

@@ -56,6 +56,7 @@ impl<G: EmissionGuarantee> IntoDiagnostic<'_, G> for UnusedGenericParamsHint {
             // FIXME: I can figure out how to do a label with a fluent string with a fixed message,
             // or a label with a dynamic value in a hard-coded string, but I haven't figured out
             // how to combine the two. 😢
+            #[allow(rustc::untranslatable_diagnostic)]
             diag.span_label(span, format!("generic parameter `{name}` is unused"));
         }
         diag

compiler/rustc_parse/src/errors.rs

@@ -3,7 +3,7 @@ use std::borrow::Cow;
 use rustc_ast::token::Token;
 use rustc_ast::{Path, Visibility};
 use rustc_errors::{
-    codes::*, AddToDiagnostic, Applicability, DiagCtxt, Diagnostic, DiagnosticBuilder,
+    codes::*, AddToDiagnostic, Applicability, DiagCtxt, DiagnosticBuilder, EmissionGuarantee,
     IntoDiagnostic, Level, SubdiagnosticMessageOp,
 };
 use rustc_macros::{Diagnostic, Subdiagnostic};
@@ -1475,7 +1475,11 @@ pub(crate) struct FnTraitMissingParen {
 }
 
 impl AddToDiagnostic for FnTraitMissingParen {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _: F,
+    ) {
         diag.span_label(self.span, crate::fluent_generated::parse_fn_trait_missing_paren);
         let applicability = if self.machine_applicable {
             Applicability::MachineApplicable

compiler/rustc_parse/src/lexer/diagnostics.rs

@@ -1,6 +1,6 @@
 use super::UnmatchedDelim;
 use rustc_ast::token::Delimiter;
-use rustc_errors::Diagnostic;
+use rustc_errors::DiagnosticBuilder;
 use rustc_span::source_map::SourceMap;
 use rustc_span::Span;
 
@@ -31,7 +31,7 @@ pub fn same_indentation_level(sm: &SourceMap, open_sp: Span, close_sp: Span) ->
 // When we get a `)` or `]` for `{`, we should emit help message here
 // it's more friendly compared to report `unmatched error` in later phase
 pub fn report_missing_open_delim(
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_>,
     unmatched_delims: &[UnmatchedDelim],
 ) -> bool {
     let mut reported_missing_open = false;
@@ -55,7 +55,7 @@ pub fn report_missing_open_delim(
 }
 
 pub fn report_suspicious_mismatch_block(
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_>,
     diag_info: &TokenTreeDiagInfo,
     sm: &SourceMap,
     delim: Delimiter,

compiler/rustc_parse/src/parser/attr.rs

@@ -8,7 +8,7 @@ use super::{AttrWrapper, Capturing, FnParseMode, ForceCollect, Parser, PathStyle
 use rustc_ast as ast;
 use rustc_ast::attr;
 use rustc_ast::token::{self, Delimiter, Nonterminal};
-use rustc_errors::{codes::*, Diagnostic, PResult};
+use rustc_errors::{codes::*, DiagnosticBuilder, PResult};
 use rustc_span::{sym, BytePos, Span};
 use thin_vec::ThinVec;
 use tracing::debug;
@@ -141,7 +141,7 @@ impl<'a> Parser<'a> {
 
     fn annotate_following_item_if_applicable(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         span: Span,
         attr_type: OuterAttributeType,
     ) -> Option<Span> {

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -34,8 +34,8 @@ use rustc_ast::{
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::{
-    pluralize, AddToDiagnostic, Applicability, DiagCtxt, Diagnostic, DiagnosticBuilder,
+    pluralize, AddToDiagnostic, Applicability, DiagCtxt, DiagnosticBuilder, ErrorGuaranteed,
-    ErrorGuaranteed, FatalError, PErr, PResult,
+    FatalError, PErr, PResult,
 };
 use rustc_session::errors::ExprParenthesesNeeded;
 use rustc_span::source_map::Spanned;
@@ -208,11 +208,11 @@ struct MultiSugg {
 }
 
 impl MultiSugg {
-    fn emit(self, err: &mut Diagnostic) {
+    fn emit(self, err: &mut DiagnosticBuilder<'_>) {
         err.multipart_suggestion(self.msg, self.patches, self.applicability);
     }
 
-    fn emit_verbose(self, err: &mut Diagnostic) {
+    fn emit_verbose(self, err: &mut DiagnosticBuilder<'_>) {
         err.multipart_suggestion_verbose(self.msg, self.patches, self.applicability);
     }
 }
@@ -846,7 +846,7 @@ impl<'a> Parser<'a> {
         err.emit();
     }
 
-    fn check_too_many_raw_str_terminators(&mut self, err: &mut Diagnostic) -> bool {
+    fn check_too_many_raw_str_terminators(&mut self, err: &mut DiagnosticBuilder<'_>) -> bool {
         let sm = self.sess.source_map();
         match (&self.prev_token.kind, &self.token.kind) {
             (
@@ -2179,7 +2179,7 @@ impl<'a> Parser<'a> {
 
     pub(super) fn parameter_without_type(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         pat: P<ast::Pat>,
         require_name: bool,
         first_param: bool,

compiler/rustc_parse/src/parser/expr.rs

@@ -25,9 +25,7 @@ use rustc_ast::{Arm, BlockCheckMode, Expr, ExprKind, Label, Movability, RangeLim
 use rustc_ast::{ClosureBinder, MetaItemLit, StmtKind};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::stack::ensure_sufficient_stack;
-use rustc_errors::{
+use rustc_errors::{AddToDiagnostic, Applicability, DiagnosticBuilder, PResult, StashKey};
-    AddToDiagnostic, Applicability, Diagnostic, DiagnosticBuilder, PResult, StashKey,
-};
 use rustc_lexer::unescape::unescape_char;
 use rustc_macros::Subdiagnostic;
 use rustc_session::errors::{report_lit_error, ExprParenthesesNeeded};
@@ -865,7 +863,7 @@ impl<'a> Parser<'a> {
             );
             let mut err = self.dcx().struct_span_err(span, msg);
 
-            let suggest_parens = |err: &mut Diagnostic| {
+            let suggest_parens = |err: &mut DiagnosticBuilder<'_>| {
                 let suggestions = vec![
                     (span.shrink_to_lo(), "(".to_string()),
                     (span.shrink_to_hi(), ")".to_string()),
@@ -3437,7 +3435,7 @@ impl<'a> Parser<'a> {
         let mut recover_async = false;
         let in_if_guard = self.restrictions.contains(Restrictions::IN_IF_GUARD);
 
-        let mut async_block_err = |e: &mut Diagnostic, span: Span| {
+        let mut async_block_err = |e: &mut DiagnosticBuilder<'_>, span: Span| {
             recover_async = true;
             errors::AsyncBlockIn2015 { span }.add_to_diagnostic(e);
             errors::HelpUseLatestEdition::new().add_to_diagnostic(e);

compiler/rustc_passes/src/check_const.rs

@@ -156,9 +156,12 @@ impl<'tcx> CheckConstVisitor<'tcx> {
                 // is a pretty narrow case, however.
                 if tcx.sess.is_nightly_build() {
                     for gate in missing_secondary {
-                        let note =
+                        // FIXME: make this translatable
-                            format!("add `#![feature({gate})]` to the crate attributes to enable",);
+                        #[allow(rustc::diagnostic_outside_of_impl)]
-                        err.help(note);
+                        #[allow(rustc::untranslatable_diagnostic)]
+                        err.help(format!(
+                            "add `#![feature({gate})]` to the crate attributes to enable"
+                        ));
                     }
                 }
 

compiler/rustc_passes/src/errors.rs

@@ -6,9 +6,8 @@ use std::{
 use crate::fluent_generated as fluent;
 use rustc_ast::Label;
 use rustc_errors::{
-    codes::*, AddToDiagnostic, Applicability, DiagCtxt, Diagnostic, DiagnosticBuilder,
+    codes::*, AddToDiagnostic, Applicability, DiagCtxt, DiagnosticBuilder, DiagnosticSymbolList,
-    DiagnosticSymbolList, EmissionGuarantee, IntoDiagnostic, Level, MultiSpan,
+    EmissionGuarantee, IntoDiagnostic, Level, MultiSpan, SubdiagnosticMessageOp,
-    SubdiagnosticMessageOp,
 };
 use rustc_hir::{self as hir, ExprKind, Target};
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
@@ -1754,7 +1753,11 @@ pub struct UnusedVariableStringInterp {
 }
 
 impl AddToDiagnostic for UnusedVariableStringInterp {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         diag.span_label(self.lit, crate::fluent_generated::passes_maybe_string_interpolation);
         diag.multipart_suggestion(
             crate::fluent_generated::passes_string_interpolation_only_works,

compiler/rustc_pattern_analysis/src/errors.rs

@@ -1,4 +1,4 @@
-use rustc_errors::{AddToDiagnostic, Diagnostic, SubdiagnosticMessageOp};
+use rustc_errors::{AddToDiagnostic, DiagnosticBuilder, EmissionGuarantee, SubdiagnosticMessageOp};
 use rustc_macros::{LintDiagnostic, Subdiagnostic};
 use rustc_middle::thir::Pat;
 use rustc_middle::ty::Ty;
@@ -62,7 +62,11 @@ pub struct Overlap<'tcx> {
 }
 
 impl<'tcx> AddToDiagnostic for Overlap<'tcx> {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _: F,
+    ) {
         let Overlap { span, range } = self;
 
         // FIXME(mejrs) unfortunately `#[derive(LintDiagnostic)]`

compiler/rustc_resolve/src/diagnostics.rs

@@ -7,7 +7,7 @@ use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::{
     codes::*, pluralize, report_ambiguity_error, struct_span_code_err, Applicability, DiagCtxt,
-    Diagnostic, DiagnosticBuilder, ErrorGuaranteed, MultiSpan, SuggestionStyle,
+    DiagnosticBuilder, ErrorGuaranteed, MultiSpan, SuggestionStyle,
 };
 use rustc_feature::BUILTIN_ATTRIBUTES;
 use rustc_hir::def::Namespace::{self, *};
@@ -360,7 +360,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
     /// ```
     fn add_suggestion_for_rename_of_use(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         name: Symbol,
         import: Import<'_>,
         binding_span: Span,
@@ -436,7 +436,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
     /// as characters expected by span manipulations won't be present.
     fn add_suggestion_for_duplicate_nested_use(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         import: Import<'_>,
         binding_span: Span,
     ) {
@@ -1399,7 +1399,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
 
     pub(crate) fn unresolved_macro_suggestions(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         macro_kind: MacroKind,
         parent_scope: &ParentScope<'a>,
         ident: Ident,
@@ -1515,7 +1515,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
 
     pub(crate) fn add_typo_suggestion(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         suggestion: Option<TypoSuggestion>,
         span: Span,
     ) -> bool {
@@ -2461,7 +2461,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
     /// Finds a cfg-ed out item inside `module` with the matching name.
     pub(crate) fn find_cfg_stripped(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         segment: &Symbol,
         module: DefId,
     ) {
@@ -2670,7 +2670,7 @@ pub(crate) enum DiagnosticMode {
 
 pub(crate) fn import_candidates(
     tcx: TyCtxt<'_>,
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_>,
     // This is `None` if all placement locations are inside expansions
     use_placement_span: Option<Span>,
     candidates: &[ImportSuggestion],
@@ -2696,7 +2696,7 @@ pub(crate) fn import_candidates(
 /// found and suggested, returns `true`, otherwise returns `false`.
 fn show_candidates(
     tcx: TyCtxt<'_>,
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_>,
     // This is `None` if all placement locations are inside expansions
     use_placement_span: Option<Span>,
     candidates: &[ImportSuggestion],

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -1,3 +1,5 @@
+// ignore-tidy-filelength
+
 use crate::diagnostics::{ImportSuggestion, LabelSuggestion, TypoSuggestion};
 use crate::late::{AliasPossibility, LateResolutionVisitor, RibKind};
 use crate::late::{LifetimeBinderKind, LifetimeRes, LifetimeRibKind, LifetimeUseSet};
@@ -16,8 +18,8 @@ use rustc_ast::{
 use rustc_ast_pretty::pprust::where_bound_predicate_to_string;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::{
-    codes::*, pluralize, struct_span_code_err, Applicability, Diagnostic, DiagnosticBuilder,
+    codes::*, pluralize, struct_span_code_err, Applicability, DiagnosticBuilder, ErrorGuaranteed,
-    ErrorGuaranteed, MultiSpan, SuggestionStyle,
+    MultiSpan, SuggestionStyle,
 };
 use rustc_hir as hir;
 use rustc_hir::def::{self, CtorKind, CtorOf, DefKind};
@@ -496,7 +498,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
 
     fn detect_assoc_type_constraint_meant_as_path(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         base_error: &BaseError,
     ) {
         let Some(ty) = self.diagnostic_metadata.current_type_path else {
@@ -537,7 +539,12 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
         }
     }
 
-    fn suggest_self_or_self_ref(&mut self, err: &mut Diagnostic, path: &[Segment], span: Span) {
+    fn suggest_self_or_self_ref(
+        &mut self,
+        err: &mut DiagnosticBuilder<'_>,
+        path: &[Segment],
+        span: Span,
+    ) {
         if !self.self_type_is_available() {
             return;
         }
@@ -582,7 +589,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
 
     fn try_lookup_name_relaxed(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         source: PathSource<'_>,
         path: &[Segment],
         following_seg: Option<&Segment>,
@@ -786,7 +793,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
 
     fn suggest_trait_and_bounds(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         source: PathSource<'_>,
         res: Option<Res>,
         span: Span,
@@ -863,7 +870,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
 
     fn suggest_typo(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         source: PathSource<'_>,
         path: &[Segment],
         following_seg: Option<&Segment>,
@@ -903,7 +910,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
 
     fn suggest_shadowed(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         source: PathSource<'_>,
         path: &[Segment],
         following_seg: Option<&Segment>,
@@ -936,7 +943,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
 
     fn err_code_special_cases(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         source: PathSource<'_>,
         path: &[Segment],
         span: Span,
@@ -981,7 +988,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
     /// Emit special messages for unresolved `Self` and `self`.
     fn suggest_self_ty(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         source: PathSource<'_>,
         path: &[Segment],
         span: Span,
@@ -1008,7 +1015,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
 
     fn suggest_self_value(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         source: PathSource<'_>,
         path: &[Segment],
         span: Span,
@@ -1090,7 +1097,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
 
     fn suggest_at_operator_in_slice_pat_with_range(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         path: &[Segment],
     ) {
         let Some(pat) = self.diagnostic_metadata.current_pat else { return };
@@ -1129,7 +1136,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
 
     fn suggest_swapping_misplaced_self_ty_and_trait(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         source: PathSource<'_>,
         res: Option<Res>,
         span: Span,
@@ -1155,7 +1162,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
         }
     }
 
-    fn suggest_bare_struct_literal(&mut self, err: &mut Diagnostic) {
+    fn suggest_bare_struct_literal(&mut self, err: &mut DiagnosticBuilder<'_>) {
         if let Some(span) = self.diagnostic_metadata.current_block_could_be_bare_struct_literal {
             err.multipart_suggestion(
                 "you might have meant to write a `struct` literal",
@@ -1170,7 +1177,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
 
     fn suggest_changing_type_to_const_param(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         res: Option<Res>,
         source: PathSource<'_>,
         span: Span,
@@ -1222,7 +1229,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
 
     fn suggest_pattern_match_with_let(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         source: PathSource<'_>,
         span: Span,
     ) -> bool {
@@ -1277,7 +1284,11 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
     }
 
     /// Given `where <T as Bar>::Baz: String`, suggest `where T: Bar<Baz = String>`.
-    fn restrict_assoc_type_in_where_clause(&mut self, span: Span, err: &mut Diagnostic) -> bool {
+    fn restrict_assoc_type_in_where_clause(
+        &mut self,
+        span: Span,
+        err: &mut DiagnosticBuilder<'_>,
+    ) -> bool {
         // Detect that we are actually in a `where` predicate.
         let (bounded_ty, bounds, where_span) =
             if let Some(ast::WherePredicate::BoundPredicate(ast::WhereBoundPredicate {
@@ -1410,7 +1421,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
     /// Returns `true` if able to provide context-dependent help.
     fn smart_resolve_context_dependent_help(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         span: Span,
         source: PathSource<'_>,
         path: &[Segment],
@@ -1421,7 +1432,8 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
         let ns = source.namespace();
         let is_expected = &|res| source.is_expected(res);
 
-        let path_sep = |this: &mut Self, err: &mut Diagnostic, expr: &Expr, kind: DefKind| {
+        let path_sep =
+            |this: &mut Self, err: &mut DiagnosticBuilder<'_>, expr: &Expr, kind: DefKind| {
                 const MESSAGE: &str = "use the path separator to refer to an item";
 
                 let (lhs_span, rhs_span) = match &expr.kind {
@@ -1442,7 +1454,8 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
                     true
                 } else if kind == DefKind::Struct
                     && let Some(lhs_source_span) = lhs_span.find_ancestor_inside(expr.span)
-                && let Ok(snippet) = this.r.tcx.sess.source_map().span_to_snippet(lhs_source_span)
+                    && let Ok(snippet) =
+                        this.r.tcx.sess.source_map().span_to_snippet(lhs_source_span)
                 {
                     // The LHS is a type that originates from a macro call.
                     // We have to add angle brackets around it.
@@ -1464,7 +1477,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
                 }
             };
 
-        let find_span = |source: &PathSource<'_>, err: &mut Diagnostic| {
+        let find_span = |source: &PathSource<'_>, err: &mut DiagnosticBuilder<'_>| {
             match source {
                 PathSource::Expr(Some(Expr { span, kind: ExprKind::Call(_, _), .. }))
                 | PathSource::TupleStruct(span, _) => {
@@ -1820,7 +1833,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
     fn suggest_alternative_construction_methods(
         &mut self,
         def_id: DefId,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         path_span: Span,
         call_span: Span,
         args: &[P<Expr>],
@@ -2250,7 +2263,11 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
 
     // try to give a suggestion for this pattern: `name = blah`, which is common in other languages
     // suggest `let name = blah` to introduce a new binding
-    fn let_binding_suggestion(&mut self, err: &mut Diagnostic, ident_span: Span) -> bool {
+    fn let_binding_suggestion(
+        &mut self,
+        err: &mut DiagnosticBuilder<'_>,
+        ident_span: Span,
+    ) -> bool {
         if let Some(Expr { kind: ExprKind::Assign(lhs, ..), .. }) =
             self.diagnostic_metadata.in_assignment
             && let ast::ExprKind::Path(None, ref path) = lhs.kind
@@ -2351,7 +2368,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
     /// Adds a suggestion for using an enum's variant when an enum is used instead.
     fn suggest_using_enum_variant(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         source: PathSource<'_>,
         def_id: DefId,
         span: Span,
@@ -2727,9 +2744,9 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
 
     fn suggest_introducing_lifetime(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         name: Option<&str>,
-        suggest: impl Fn(&mut Diagnostic, bool, Span, Cow<'static, str>, String) -> bool,
+        suggest: impl Fn(&mut DiagnosticBuilder<'_>, bool, Span, Cow<'static, str>, String) -> bool,
     ) {
         let mut suggest_note = true;
         for rib in self.lifetime_ribs.iter().rev() {
@@ -2887,7 +2904,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
 
     fn add_missing_lifetime_specifiers_label(
         &mut self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         lifetime_refs: Vec<MissingLifetime>,
         function_param_lifetimes: Option<(Vec<MissingLifetime>, Vec<ElisionFnParameter>)>,
     ) {

compiler/rustc_session/src/config.rs

@@ -2468,6 +2468,9 @@ pub fn parse_externs(
             ));
             let adjusted_name = name.replace('-', "_");
             if is_ascii_ident(&adjusted_name) {
+                // FIXME: make this translatable
+                #[allow(rustc::diagnostic_outside_of_impl)]
+                #[allow(rustc::untranslatable_diagnostic)]
                 error.help(format!(
                     "consider replacing the dashes with underscores: `{adjusted_name}`"
                 ));

compiler/rustc_session/src/parse.rs

@@ -15,7 +15,8 @@ use rustc_data_structures::fx::{FxHashMap, FxIndexMap, FxIndexSet};
 use rustc_data_structures::sync::{AppendOnlyVec, Lock, Lrc};
 use rustc_errors::{emitter::SilentEmitter, DiagCtxt};
 use rustc_errors::{
-    fallback_fluent_bundle, Diagnostic, DiagnosticBuilder, DiagnosticMessage, MultiSpan, StashKey,
+    fallback_fluent_bundle, DiagnosticBuilder, DiagnosticMessage, EmissionGuarantee, MultiSpan,
+    StashKey,
 };
 use rustc_feature::{find_feature_issue, GateIssue, UnstableFeatures};
 use rustc_span::edition::Edition;
@@ -156,7 +157,11 @@ pub fn feature_warn_issue(
 }
 
 /// Adds the diagnostics for a feature to an existing error.
-pub fn add_feature_diagnostics(err: &mut Diagnostic, sess: &Session, feature: Symbol) {
+pub fn add_feature_diagnostics<G: EmissionGuarantee>(
+    err: &mut DiagnosticBuilder<'_, G>,
+    sess: &Session,
+    feature: Symbol,
+) {
     add_feature_diagnostics_for_issue(err, sess, feature, GateIssue::Language, false);
 }
 
@@ -165,8 +170,8 @@ pub fn add_feature_diagnostics(err: &mut Diagnostic, sess: &Session, feature: Sy
 /// This variant allows you to control whether it is a library or language feature.
 /// Almost always, you want to use this for a language feature. If so, prefer
 /// `add_feature_diagnostics`.
-pub fn add_feature_diagnostics_for_issue(
+pub fn add_feature_diagnostics_for_issue<G: EmissionGuarantee>(
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_, G>,
     sess: &Session,
     feature: Symbol,
     issue: GateIssue,

compiler/rustc_trait_selection/src/errors.rs

@@ -1,7 +1,7 @@
 use crate::fluent_generated as fluent;
 use rustc_errors::{
-    codes::*, AddToDiagnostic, Applicability, DiagCtxt, Diagnostic, DiagnosticBuilder,
+    codes::*, AddToDiagnostic, Applicability, DiagCtxt, DiagnosticBuilder, EmissionGuarantee,
-    EmissionGuarantee, IntoDiagnostic, Level, SubdiagnosticMessageOp,
+    IntoDiagnostic, Level, SubdiagnosticMessageOp,
 };
 use rustc_macros::Diagnostic;
 use rustc_middle::ty::{self, ClosureKind, PolyTraitRef, Ty};
@@ -102,7 +102,11 @@ pub enum AdjustSignatureBorrow {
 }
 
 impl AddToDiagnostic for AdjustSignatureBorrow {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _f: F,
+    ) {
         match self {
             AdjustSignatureBorrow::Borrow { to_borrow } => {
                 diag.arg("len", to_borrow.len());

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -18,7 +18,7 @@ use crate::traits::{
     Obligation, ObligationCause, PredicateObligation, PredicateObligations, SelectionContext,
 };
 use rustc_data_structures::fx::FxIndexSet;
-use rustc_errors::Diagnostic;
+use rustc_errors::{DiagnosticBuilder, EmissionGuarantee};
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
 use rustc_infer::infer::{DefineOpaqueTypes, InferCtxt, TyCtxtInferExt};
@@ -58,7 +58,7 @@ pub struct OverlapResult<'tcx> {
     pub involves_placeholder: bool,
 }
 
-pub fn add_placeholder_note(err: &mut Diagnostic) {
+pub fn add_placeholder_note<G: EmissionGuarantee>(err: &mut DiagnosticBuilder<'_, G>) {
     err.note(
         "this behavior recently changed as a result of a bug fix; \
          see rust-lang/rust#56105 for details",

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -13,7 +13,7 @@ use hir::def::CtorOf;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_errors::{
-    codes::*, pluralize, struct_span_code_err, Applicability, Diagnostic, DiagnosticBuilder,
+    codes::*, pluralize, struct_span_code_err, Applicability, DiagnosticBuilder, EmissionGuarantee,
     MultiSpan, Style, SuggestionStyle,
 };
 use rustc_hir as hir;
@@ -116,12 +116,12 @@ fn predicate_constraint(generics: &hir::Generics<'_>, pred: ty::Predicate<'_>) -
 /// Type parameter needs more bounds. The trivial case is `T` `where T: Bound`, but
 /// it can also be an `impl Trait` param that needs to be decomposed to a type
 /// param for cleaner code.
-pub fn suggest_restriction<'tcx>(
+pub fn suggest_restriction<'tcx, G: EmissionGuarantee>(
     tcx: TyCtxt<'tcx>,
     item_id: LocalDefId,
     hir_generics: &hir::Generics<'tcx>,
     msg: &str,
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_, G>,
     fn_sig: Option<&hir::FnSig<'_>>,
     projection: Option<&ty::AliasTy<'_>>,
     trait_pred: ty::PolyTraitPredicate<'tcx>,
@@ -240,7 +240,7 @@ pub fn suggest_restriction<'tcx>(
 impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     fn suggest_restricting_param_bound(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
         associated_ty: Option<(&'static str, Ty<'tcx>)>,
         mut body_id: LocalDefId,
@@ -450,7 +450,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     fn suggest_dereferences(
         &self,
         obligation: &PredicateObligation<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
     ) -> bool {
         let mut code = obligation.cause.code();
@@ -743,10 +743,11 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     fn get_closure_name(
         &self,
         def_id: DefId,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         msg: Cow<'static, str>,
     ) -> Option<Symbol> {
-        let get_name = |err: &mut Diagnostic, kind: &hir::PatKind<'_>| -> Option<Symbol> {
+        let get_name =
+            |err: &mut DiagnosticBuilder<'_>, kind: &hir::PatKind<'_>| -> Option<Symbol> {
                 // Get the local name of this closure. This can be inaccurate because
                 // of the possibility of reassignment, but this should be good enough.
                 match &kind {
@@ -778,7 +779,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     fn suggest_fn_call(
         &self,
         obligation: &PredicateObligation<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
     ) -> bool {
         // It doesn't make sense to make this suggestion outside of typeck...
@@ -894,7 +895,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     fn check_for_binding_assigned_block_without_tail_expression(
         &self,
         obligation: &PredicateObligation<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
     ) {
         let mut span = obligation.cause.span;
@@ -971,7 +972,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     fn suggest_add_clone_to_arg(
         &self,
         obligation: &PredicateObligation<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
     ) -> bool {
         let self_ty = self.resolve_vars_if_possible(trait_pred.self_ty());
@@ -1156,7 +1157,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     fn suggest_add_reference_to_arg(
         &self,
         obligation: &PredicateObligation<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         poly_trait_pred: ty::PolyTraitPredicate<'tcx>,
         has_custom_message: bool,
     ) -> bool {
@@ -1374,7 +1375,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     // Suggest borrowing the type
     fn suggest_borrowing_for_object_cast(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         obligation: &PredicateObligation<'tcx>,
         self_ty: Ty<'tcx>,
         target_ty: Ty<'tcx>,
@@ -1410,7 +1411,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     fn suggest_remove_reference(
         &self,
         obligation: &PredicateObligation<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
     ) -> bool {
         let mut span = obligation.cause.span;
@@ -1529,7 +1530,11 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         false
     }
 
-    fn suggest_remove_await(&self, obligation: &PredicateObligation<'tcx>, err: &mut Diagnostic) {
+    fn suggest_remove_await(
+        &self,
+        obligation: &PredicateObligation<'tcx>,
+        err: &mut DiagnosticBuilder<'_>,
+    ) {
         let hir = self.tcx.hir();
         if let ObligationCauseCode::AwaitableExpr(hir_id) = obligation.cause.code().peel_derives()
             && let hir::Node::Expr(expr) = self.tcx.hir_node(*hir_id)
@@ -1599,7 +1604,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     fn suggest_change_mut(
         &self,
         obligation: &PredicateObligation<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
     ) {
         let points_at_arg = matches!(
@@ -1677,7 +1682,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     fn suggest_semicolon_removal(
         &self,
         obligation: &PredicateObligation<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         span: Span,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
     ) -> bool {
@@ -1731,7 +1736,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     /// emitted.
     fn suggest_impl_trait(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         obligation: &PredicateObligation<'tcx>,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
     ) -> bool {
@@ -1913,7 +1918,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
 
     fn note_conflicting_fn_args(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         cause: &ObligationCauseCode<'tcx>,
         expected: Ty<'tcx>,
         found: Ty<'tcx>,
@@ -2120,7 +2125,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
 
     fn suggest_fully_qualified_path(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         item_def_id: DefId,
         span: Span,
         trait_ref: DefId,
@@ -2185,9 +2190,9 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     ///
     /// Returns `true` if an async-await specific note was added to the diagnostic.
     #[instrument(level = "debug", skip_all, fields(?obligation.predicate, ?obligation.cause.span))]
-    fn maybe_note_obligation_cause_for_async_await(
+    fn maybe_note_obligation_cause_for_async_await<G: EmissionGuarantee>(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_, G>,
         obligation: &PredicateObligation<'tcx>,
     ) -> bool {
         let hir = self.tcx.hir();
@@ -2421,9 +2426,9 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     /// Unconditionally adds the diagnostic note described in
     /// `maybe_note_obligation_cause_for_async_await`'s documentation comment.
     #[instrument(level = "debug", skip_all)]
-    fn note_obligation_cause_for_async_await(
+    fn note_obligation_cause_for_async_await<G: EmissionGuarantee>(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_, G>,
         interior_or_upvar_span: CoroutineInteriorOrUpvar,
         is_async: bool,
         outer_coroutine: Option<DefId>,
@@ -2656,10 +2661,10 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         );
     }
 
-    fn note_obligation_cause_code<T>(
+    fn note_obligation_cause_code<G: EmissionGuarantee, T>(
         &self,
         body_id: LocalDefId,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_, G>,
         predicate: T,
         param_env: ty::ParamEnv<'tcx>,
         cause_code: &ObligationCauseCode<'tcx>,
@@ -3507,7 +3512,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     )]
     fn suggest_await_before_try(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         obligation: &PredicateObligation<'tcx>,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
         span: Span,
@@ -3565,7 +3570,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     fn suggest_floating_point_literal(
         &self,
         obligation: &PredicateObligation<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         trait_ref: &ty::PolyTraitRef<'tcx>,
     ) {
         let rhs_span = match obligation.cause.code() {
@@ -3589,7 +3594,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     fn suggest_derive(
         &self,
         obligation: &PredicateObligation<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
     ) {
         let Some(diagnostic_name) = self.tcx.get_diagnostic_name(trait_pred.def_id()) else {
@@ -3655,7 +3660,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     fn suggest_dereferencing_index(
         &self,
         obligation: &PredicateObligation<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         trait_pred: ty::PolyTraitPredicate<'tcx>,
     ) {
         if let ObligationCauseCode::ImplDerivedObligation(_) = obligation.cause.code()
@@ -3675,10 +3680,10 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         }
     }
 
-    fn note_function_argument_obligation(
+    fn note_function_argument_obligation<G: EmissionGuarantee>(
         &self,
         body_id: LocalDefId,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_, G>,
         arg_hir_id: HirId,
         parent_code: &ObligationCauseCode<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
@@ -3860,11 +3865,11 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         }
     }
 
-    fn suggest_option_method_if_applicable(
+    fn suggest_option_method_if_applicable<G: EmissionGuarantee>(
         &self,
         failed_pred: ty::Predicate<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_, G>,
         expr: &hir::Expr<'_>,
     ) {
         let tcx = self.tcx;
@@ -3934,7 +3939,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         }
     }
 
-    fn look_for_iterator_item_mistakes(
+    fn look_for_iterator_item_mistakes<G: EmissionGuarantee>(
         &self,
         assocs_in_this_method: &[Option<(Span, (DefId, Ty<'tcx>))>],
         typeck_results: &TypeckResults<'tcx>,
@@ -3942,7 +3947,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         param_env: ty::ParamEnv<'tcx>,
         path_segment: &hir::PathSegment<'_>,
         args: &[hir::Expr<'_>],
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_, G>,
     ) {
         let tcx = self.tcx;
         // Special case for iterator chains, we look at potential failures of `Iterator::Item`
@@ -4037,13 +4042,13 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         }
     }
 
-    fn point_at_chain(
+    fn point_at_chain<G: EmissionGuarantee>(
         &self,
         expr: &hir::Expr<'_>,
         typeck_results: &TypeckResults<'tcx>,
         type_diffs: Vec<TypeError<'tcx>>,
         param_env: ty::ParamEnv<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_, G>,
     ) {
         let mut primary_spans = vec![];
         let mut span_labels = vec![];
@@ -4279,7 +4284,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     /// the array into a slice.
     fn suggest_convert_to_slice(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         obligation: &PredicateObligation<'tcx>,
         trait_ref: ty::PolyTraitRef<'tcx>,
         candidate_impls: &[ImplCandidate<'tcx>],
@@ -4351,7 +4356,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
 
     fn explain_hrtb_projection(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         pred: ty::PolyTraitPredicate<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
         cause: &ObligationCause<'tcx>,
@@ -4417,7 +4422,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
 
     fn suggest_desugaring_async_fn_in_trait(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         trait_ref: ty::PolyTraitRef<'tcx>,
     ) {
         // Don't suggest if RTN is active -- we should prefer a where-clause bound instead.
@@ -4508,7 +4513,7 @@ fn hint_missing_borrow<'tcx>(
     found: Ty<'tcx>,
     expected: Ty<'tcx>,
     found_node: Node<'_>,
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_>,
 ) {
     if matches!(found_node, Node::TraitItem(..)) {
         return;
@@ -4867,9 +4872,9 @@ pub fn suggest_desugaring_async_fn_to_impl_future_in_trait<'tcx>(
 
 /// On `impl` evaluation cycles, look for `Self::AssocTy` restrictions in `where` clauses, explain
 /// they are not allowed and if possible suggest alternatives.
-fn point_at_assoc_type_restriction(
+fn point_at_assoc_type_restriction<G: EmissionGuarantee>(
     tcx: TyCtxt<'_>,
-    err: &mut Diagnostic,
+    err: &mut DiagnosticBuilder<'_, G>,
     self_ty_str: &str,
     trait_name: &str,
     predicate: ty::Predicate<'_>,

compiler/rustc_trait_selection/src/traits/error_reporting/type_err_ctxt_ext.rs

@@ -21,8 +21,8 @@ use crate::traits::{
 };
 use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
 use rustc_errors::{
-    codes::*, pluralize, struct_span_code_err, Applicability, Diagnostic, DiagnosticBuilder,
+    codes::*, pluralize, struct_span_code_err, Applicability, DiagnosticBuilder, ErrorGuaranteed,
-    ErrorGuaranteed, MultiSpan, StashKey, StringPart,
+    MultiSpan, StashKey, StringPart,
 };
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Namespace, Res};
@@ -185,7 +185,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         predicate: &T,
         span: Span,
         suggest_increasing_limit: bool,
-        mutate: impl FnOnce(&mut Diagnostic),
+        mutate: impl FnOnce(&mut DiagnosticBuilder<'_>),
     ) -> !
     where
         T: fmt::Display + TypeFoldable<TyCtxt<'tcx>> + Print<'tcx, FmtPrinter<'tcx, 'tcx>>,
@@ -272,7 +272,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         );
     }
 
-    fn suggest_new_overflow_limit(&self, err: &mut Diagnostic) {
+    fn suggest_new_overflow_limit(&self, err: &mut DiagnosticBuilder<'_>) {
         let suggested_limit = match self.tcx.recursion_limit() {
             Limit(0) => Limit(2),
             limit => limit * 2,
@@ -1020,7 +1020,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         &self,
         obligation: &PredicateObligation<'tcx>,
         trait_ref: ty::TraitRef<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
     ) -> bool {
         let span = obligation.cause.span;
         struct V<'v> {
@@ -1810,7 +1810,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         impl_candidates: &[ImplCandidate<'tcx>],
         trait_ref: ty::PolyTraitRef<'tcx>,
         body_def_id: LocalDefId,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         other: bool,
         param_env: ty::ParamEnv<'tcx>,
     ) -> bool {
@@ -1897,7 +1897,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         }
 
         let other = if other { "other " } else { "" };
-        let report = |candidates: Vec<TraitRef<'tcx>>, err: &mut Diagnostic| {
+        let report = |candidates: Vec<TraitRef<'tcx>>, err: &mut DiagnosticBuilder<'_>| {
             if candidates.is_empty() {
                 return false;
             }
@@ -2032,7 +2032,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         obligation: &PredicateObligation<'tcx>,
         trait_predicate: ty::Binder<'tcx, ty::TraitPredicate<'tcx>>,
         body_def_id: LocalDefId,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
     ) {
         // This is *almost* equivalent to
         // `obligation.cause.code().peel_derives()`, but it gives us the
@@ -2103,7 +2103,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     /// a probable version mismatch is added to `err`
     fn note_version_mismatch(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         trait_ref: &ty::PolyTraitRef<'tcx>,
     ) -> bool {
         let get_trait_impls = |trait_def_id| {
@@ -2572,7 +2572,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
 
     fn annotate_source_of_ambiguity(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         ambiguities: &[ambiguity::Ambiguity],
         predicate: ty::Predicate<'tcx>,
     ) {
@@ -2715,7 +2715,11 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         })
     }
 
-    fn note_obligation_cause(&self, err: &mut Diagnostic, obligation: &PredicateObligation<'tcx>) {
+    fn note_obligation_cause(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        obligation: &PredicateObligation<'tcx>,
+    ) {
         // First, attempt to add note to this error with an async-await-specific
         // message, and fall back to regular note otherwise.
         if !self.maybe_note_obligation_cause_for_async_await(err, obligation) {
@@ -2744,7 +2748,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     #[instrument(level = "debug", skip_all)]
     fn suggest_unsized_bound_if_applicable(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         obligation: &PredicateObligation<'tcx>,
     ) {
         let ty::PredicateKind::Clause(ty::ClauseKind::Trait(pred)) =
@@ -2770,7 +2774,12 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     }
 
     #[instrument(level = "debug", skip_all)]
-    fn maybe_suggest_unsized_generics(&self, err: &mut Diagnostic, span: Span, node: Node<'tcx>) {
+    fn maybe_suggest_unsized_generics(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        span: Span,
+        node: Node<'tcx>,
+    ) {
         let Some(generics) = node.generics() else {
             return;
         };
@@ -2822,7 +2831,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
 
     fn maybe_indirection_for_unsized(
         &self,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         item: &Item<'tcx>,
         param: &GenericParam<'tcx>,
     ) -> bool {
@@ -3016,7 +3025,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     fn add_tuple_trait_message(
         &self,
         obligation_cause_code: &ObligationCauseCode<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
     ) {
         match obligation_cause_code {
             ObligationCauseCode::RustCall => {
@@ -3041,7 +3050,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         obligation: &PredicateObligation<'tcx>,
         trait_ref: ty::PolyTraitRef<'tcx>,
         trait_predicate: &ty::PolyTraitPredicate<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         span: Span,
         is_fn_trait: bool,
         suggested: bool,
@@ -3122,7 +3131,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     fn add_help_message_for_fn_trait(
         &self,
         trait_ref: ty::PolyTraitRef<'tcx>,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_>,
         implemented_kind: ty::ClosureKind,
         params: ty::Binder<'tcx, Ty<'tcx>>,
     ) {
@@ -3178,7 +3187,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
     fn maybe_add_note_for_unsatisfied_const(
         &self,
         _trait_predicate: &ty::PolyTraitPredicate<'tcx>,
-        _err: &mut Diagnostic,
+        _err: &mut DiagnosticBuilder<'_>,
         _span: Span,
     ) -> UnsatisfiedConst {
         let unsatisfied_const = UnsatisfiedConst(false);

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -29,7 +29,7 @@ use crate::traits::ProjectionCacheKey;
 use crate::traits::Unimplemented;
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap, FxIndexSet};
 use rustc_data_structures::stack::ensure_sufficient_stack;
-use rustc_errors::Diagnostic;
+use rustc_errors::{DiagnosticBuilder, EmissionGuarantee};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_infer::infer::BoundRegionConversionTime;
@@ -70,7 +70,10 @@ pub enum IntercrateAmbiguityCause<'tcx> {
 impl<'tcx> IntercrateAmbiguityCause<'tcx> {
     /// Emits notes when the overlap is caused by complex intercrate ambiguities.
     /// See #23980 for details.
-    pub fn add_intercrate_ambiguity_hint(&self, err: &mut Diagnostic) {
+    pub fn add_intercrate_ambiguity_hint<G: EmissionGuarantee>(
+        &self,
+        err: &mut DiagnosticBuilder<'_, G>,
+    ) {
         err.note(self.intercrate_ambiguity_hint());
     }
 

compiler/rustc_trait_selection/src/traits/specialize/mod.rs

@@ -20,7 +20,7 @@ use crate::traits::{
     self, coherence, FutureCompatOverlapErrorKind, ObligationCause, ObligationCtxt,
 };
 use rustc_data_structures::fx::FxIndexSet;
-use rustc_errors::{codes::*, DelayDm, Diagnostic};
+use rustc_errors::{codes::*, DelayDm, DiagnosticBuilder, EmissionGuarantee};
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::ty::{self, ImplSubject, Ty, TyCtxt, TypeVisitableExt};
 use rustc_middle::ty::{GenericArgs, GenericArgsRef};
@@ -395,11 +395,11 @@ fn report_conflicting_impls<'tcx>(
     // Work to be done after we've built the DiagnosticBuilder. We have to define it
     // now because the lint emit methods don't return back the DiagnosticBuilder
     // that's passed in.
-    fn decorate<'tcx>(
+    fn decorate<'tcx, G: EmissionGuarantee>(
         tcx: TyCtxt<'tcx>,
         overlap: &OverlapError<'tcx>,
         impl_span: Span,
-        err: &mut Diagnostic,
+        err: &mut DiagnosticBuilder<'_, G>,
     ) {
         if (overlap.trait_ref, overlap.self_ty).references_error() {
             err.downgrade_to_delayed_bug();

compiler/rustc_trait_selection/src/traits/util.rs

@@ -3,7 +3,7 @@ use std::collections::BTreeMap;
 use super::NormalizeExt;
 use super::{ObligationCause, PredicateObligation, SelectionContext};
 use rustc_data_structures::fx::FxHashSet;
-use rustc_errors::Diagnostic;
+use rustc_errors::DiagnosticBuilder;
 use rustc_hir::def_id::DefId;
 use rustc_infer::infer::{InferCtxt, InferOk};
 use rustc_middle::ty::GenericArgsRef;
@@ -46,7 +46,7 @@ impl<'tcx> TraitAliasExpansionInfo<'tcx> {
     /// trait aliases.
     pub fn label_with_exp_info(
         &self,
-        diag: &mut Diagnostic,
+        diag: &mut DiagnosticBuilder<'_>,
         top_label: &'static str,
         use_desc: &str,
     ) {

src/librustdoc/passes/collect_intra_doc_links.rs

@@ -8,7 +8,7 @@ use rustc_data_structures::{
     fx::{FxHashMap, FxHashSet},
     intern::Interned,
 };
-use rustc_errors::{Applicability, Diagnostic, DiagnosticMessage};
+use rustc_errors::{Applicability, DiagnosticBuilder, DiagnosticMessage};
 use rustc_hir::def::Namespace::*;
 use rustc_hir::def::{DefKind, Namespace, PerNS};
 use rustc_hir::def_id::{DefId, CRATE_DEF_ID};
@@ -1173,7 +1173,8 @@ impl LinkCollector<'_, '_> {
     ) {
         // The resolved item did not match the disambiguator; give a better error than 'not found'
         let msg = format!("incompatible link kind for `{path_str}`");
-        let callback = |diag: &mut Diagnostic, sp: Option<rustc_span::Span>, link_range| {
+        let callback =
+            |diag: &mut DiagnosticBuilder<'_, ()>, sp: Option<rustc_span::Span>, link_range| {
                 let note = format!(
                     "this link resolved to {} {}, which is not {} {}",
                     resolved.article(),
@@ -1676,7 +1677,7 @@ fn report_diagnostic(
     lint: &'static Lint,
     msg: impl Into<DiagnosticMessage> + Display,
     DiagnosticInfo { item, ori_link: _, dox, link_range }: &DiagnosticInfo<'_>,
-    decorate: impl FnOnce(&mut Diagnostic, Option<rustc_span::Span>, MarkdownLinkRange),
+    decorate: impl FnOnce(&mut DiagnosticBuilder<'_, ()>, Option<rustc_span::Span>, MarkdownLinkRange),
 ) {
     let Some(hir_id) = DocContext::as_local_hir_id(tcx, item.item_id) else {
         // If non-local, no need to check anything.
@@ -2124,7 +2125,7 @@ fn ambiguity_error(
 /// disambiguator.
 fn suggest_disambiguator(
     res: Res,
-    diag: &mut Diagnostic,
+    diag: &mut DiagnosticBuilder<'_, ()>,
     path_str: &str,
     link_range: MarkdownLinkRange,
     sp: Option<rustc_span::Span>,

src/tools/clippy/clippy_lints/src/casts/cast_possible_truncation.rs

@@ -4,7 +4,7 @@ use clippy_utils::expr_or_init;
 use clippy_utils::source::snippet;
 use clippy_utils::sugg::Sugg;
 use clippy_utils::ty::{get_discriminant_value, is_isize_or_usize};
-use rustc_errors::{Applicability, Diagnostic, SuggestionStyle};
+use rustc_errors::{Applicability, DiagnosticBuilder, SuggestionStyle};
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::{BinOpKind, Expr, ExprKind};
 use rustc_lint::LateContext;
@@ -177,7 +177,7 @@ fn offer_suggestion(
     expr: &Expr<'_>,
     cast_expr: &Expr<'_>,
     cast_to_span: Span,
-    diag: &mut Diagnostic,
+    diag: &mut DiagnosticBuilder<'_, ()>,
 ) {
     let cast_to_snip = snippet(cx, cast_to_span, "..");
     let suggestion = if cast_to_snip == "_" {

src/tools/clippy/clippy_lints/src/functions/result.rs

@@ -1,4 +1,4 @@
-use rustc_errors::Diagnostic;
+use rustc_errors::DiagnosticBuilder;
 use rustc_hir as hir;
 use rustc_lint::{LateContext, LintContext};
 use rustc_middle::lint::in_external_macro;
@@ -135,7 +135,7 @@ fn check_result_large_err<'tcx>(cx: &LateContext<'tcx>, err_ty: Ty<'tcx>, hir_ty
                 RESULT_LARGE_ERR,
                 hir_ty_span,
                 "the `Err`-variant returned from this function is very large",
-                |diag: &mut Diagnostic| {
+                |diag: &mut DiagnosticBuilder<'_, ()>| {
                     diag.span_label(hir_ty_span, format!("the `Err`-variant is at least {ty_size} bytes"));
                     diag.help(format!("try reducing the size of `{err_ty}`, for example by boxing large elements or replacing it with `Box<{err_ty}>`"));
                 },

src/tools/clippy/clippy_lints/src/if_let_mutex.rs

@@ -1,7 +1,7 @@
 use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::ty::is_type_diagnostic_item;
 use clippy_utils::{higher, SpanlessEq};
-use rustc_errors::Diagnostic;
+use rustc_errors::DiagnosticBuilder;
 use rustc_hir::intravisit::{self as visit, Visitor};
 use rustc_hir::{Expr, ExprKind};
 use rustc_lint::{LateContext, LateLintPass};
@@ -59,7 +59,7 @@ impl<'tcx> LateLintPass<'tcx> for IfLetMutex {
                 arm_visit.visit_expr(if_else);
 
                 if let Some(arm_mutex) = arm_visit.found_mutex_if_same_as(op_mutex) {
-                    let diag = |diag: &mut Diagnostic| {
+                    let diag = |diag: &mut DiagnosticBuilder<'_, ()>| {
                         diag.span_label(
                             op_mutex.span,
                             "this Mutex will remain locked for the entire `if let`-block...",

src/tools/clippy/clippy_lints/src/implicit_hasher.rs

@@ -1,7 +1,7 @@
 use std::borrow::Cow;
 use std::collections::BTreeMap;
 
-use rustc_errors::Diagnostic;
+use rustc_errors::DiagnosticBuilder;
 use rustc_hir as hir;
 use rustc_hir::intravisit::{walk_body, walk_expr, walk_inf, walk_ty, Visitor};
 use rustc_hir::{Body, Expr, ExprKind, GenericArg, Item, ItemKind, QPath, TyKind};
@@ -65,7 +65,7 @@ impl<'tcx> LateLintPass<'tcx> for ImplicitHasher {
 
         fn suggestion(
             cx: &LateContext<'_>,
-            diag: &mut Diagnostic,
+            diag: &mut DiagnosticBuilder<'_, ()>,
             generics_span: Span,
             generics_suggestion_span: Span,
             target: &ImplicitHasherType<'_>,

src/tools/clippy/clippy_lints/src/manual_clamp.rs

@@ -9,7 +9,7 @@ use clippy_utils::{
     peel_blocks_with_stmt, MaybePath,
 };
 use itertools::Itertools;
-use rustc_errors::{Applicability, Diagnostic};
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir::def::Res;
 use rustc_hir::{Arm, BinOpKind, Block, Expr, ExprKind, HirId, PatKind, PathSegment, PrimTy, QPath, StmtKind};
 use rustc_lint::{LateContext, LateLintPass};
@@ -163,7 +163,7 @@ fn emit_suggestion<'tcx>(cx: &LateContext<'tcx>, suggestion: &ClampSuggestion<'t
     };
     let suggestion = format!("{assignment}{input}.clamp({min}, {max}){semicolon}");
     let msg = "clamp-like pattern without using clamp function";
-    let lint_builder = |d: &mut Diagnostic| {
+    let lint_builder = |d: &mut DiagnosticBuilder<'_, ()>| {
         d.span_suggestion(*span, "replace with clamp", suggestion, Applicability::MaybeIncorrect);
         if *is_float {
             d.note("clamp will panic if max < min, min.is_nan(), or max.is_nan()")

src/tools/clippy/clippy_lints/src/matches/significant_drop_in_scrutinee.rs

@@ -2,7 +2,7 @@ use crate::FxHashSet;
 use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::source::{indent_of, snippet};
 use clippy_utils::{get_attr, is_lint_allowed};
-use rustc_errors::{Applicability, Diagnostic};
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir::intravisit::{walk_expr, Visitor};
 use rustc_hir::{Arm, Expr, ExprKind, MatchSource};
 use rustc_lint::{LateContext, LintContext};
@@ -37,7 +37,7 @@ pub(super) fn check<'tcx>(
     }
 }
 
-fn set_diagnostic<'tcx>(diag: &mut Diagnostic, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>, found: FoundSigDrop) {
+fn set_diagnostic<'tcx>(diag: &mut DiagnosticBuilder<'_, ()>, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>, found: FoundSigDrop) {
     if found.lint_suggestion == LintSuggestion::MoveAndClone {
         // If our suggestion is to move and clone, then we want to leave it to the user to
         // decide how to address this lint, since it may be that cloning is inappropriate.

src/tools/clippy/clippy_lints/src/methods/suspicious_command_arg_space.rs

@@ -1,6 +1,6 @@
 use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::ty::is_type_diagnostic_item;
-use rustc_errors::{Applicability, Diagnostic};
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_lint::LateContext;
 use rustc_span::{sym, Span};
 use {rustc_ast as ast, rustc_hir as hir};
@@ -22,7 +22,7 @@ pub(super) fn check<'tcx>(cx: &LateContext<'tcx>, recv: &'tcx hir::Expr<'_>, arg
             SUSPICIOUS_COMMAND_ARG_SPACE,
             arg.span,
             "single argument that looks like it should be multiple arguments",
-            |diag: &mut Diagnostic| {
+            |diag: &mut DiagnosticBuilder<'_, ()>| {
                 diag.multipart_suggestion_verbose(
                     "consider splitting the argument",
                     vec![(span, "args".to_string()), (arg.span, format!("[{arg1:?}, {arg2:?}]"))],

src/tools/clippy/clippy_lints/src/missing_asserts_for_indexing.rs

@@ -9,7 +9,7 @@ use clippy_utils::{eq_expr_value, hash_expr, higher};
 use rustc_ast::{LitKind, RangeLimits};
 use rustc_data_structures::packed::Pu128;
 use rustc_data_structures::unhash::UnhashMap;
-use rustc_errors::{Applicability, Diagnostic};
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir::{BinOp, Block, Body, Expr, ExprKind, UnOp};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_session::declare_lint_pass;
@@ -67,7 +67,7 @@ declare_lint_pass!(MissingAssertsForIndexing => [MISSING_ASSERTS_FOR_INDEXING]);
 
 fn report_lint<F>(cx: &LateContext<'_>, full_span: Span, msg: &str, indexes: &[Span], f: F)
 where
-    F: FnOnce(&mut Diagnostic),
+    F: FnOnce(&mut DiagnosticBuilder<'_, ()>),
 {
     span_lint_and_then(cx, MISSING_ASSERTS_FOR_INDEXING, full_span, msg, |diag| {
         f(diag);

src/tools/clippy/clippy_lints/src/needless_pass_by_value.rs

@@ -6,7 +6,7 @@ use clippy_utils::ty::{
     implements_trait, implements_trait_with_env_from_iter, is_copy, is_type_diagnostic_item, is_type_lang_item,
 };
 use rustc_ast::ast::Attribute;
-use rustc_errors::{Applicability, Diagnostic};
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_hir::intravisit::FnKind;
 use rustc_hir::{
     BindingAnnotation, Body, FnDecl, GenericArg, HirId, HirIdSet, Impl, ItemKind, LangItem, Mutability, Node, PatKind,
@@ -196,7 +196,7 @@ impl<'tcx> LateLintPass<'tcx> for NeedlessPassByValue {
                 && !moved_vars.contains(&canonical_id)
             {
                 // Dereference suggestion
-                let sugg = |diag: &mut Diagnostic| {
+                let sugg = |diag: &mut DiagnosticBuilder<'_, ()>| {
                     if let ty::Adt(def, ..) = ty.kind() {
                         if let Some(span) = cx.tcx.hir().span_if_local(def.did()) {
                             if type_allowed_to_implement_copy(

src/tools/clippy/clippy_utils/src/diagnostics.rs

@@ -8,13 +8,13 @@
 //! Thank you!
 //! ~The `INTERNAL_METADATA_COLLECTOR` lint
 
-use rustc_errors::{Applicability, Diagnostic, MultiSpan};
+use rustc_errors::{Applicability, DiagnosticBuilder, MultiSpan};
 use rustc_hir::HirId;
 use rustc_lint::{LateContext, Lint, LintContext};
 use rustc_span::Span;
 use std::env;
 
-fn docs_link(diag: &mut Diagnostic, lint: &'static Lint) {
+fn docs_link(diag: &mut DiagnosticBuilder<'_, ()>, lint: &'static Lint) {
     if env::var("CLIPPY_DISABLE_DOCS_LINKS").is_err() {
         if let Some(lint) = lint.name_lower().strip_prefix("clippy::") {
             diag.help(format!(
@@ -143,7 +143,7 @@ pub fn span_lint_and_then<C, S, F>(cx: &C, lint: &'static Lint, sp: S, msg: &str
 where
     C: LintContext,
     S: Into<MultiSpan>,
-    F: FnOnce(&mut Diagnostic),
+    F: FnOnce(&mut DiagnosticBuilder<'_, ()>),
 {
     #[expect(clippy::disallowed_methods)]
     cx.span_lint(lint, sp, msg.to_string(), |diag| {
@@ -165,7 +165,7 @@ pub fn span_lint_hir_and_then(
     hir_id: HirId,
     sp: impl Into<MultiSpan>,
     msg: &str,
-    f: impl FnOnce(&mut Diagnostic),
+    f: impl FnOnce(&mut DiagnosticBuilder<'_, ()>),
 ) {
     #[expect(clippy::disallowed_methods)]
     cx.tcx.node_span_lint(lint, hir_id, sp, msg.to_string(), |diag| {
@@ -214,7 +214,7 @@ pub fn span_lint_and_sugg<T: LintContext>(
 /// appear once per
 /// replacement. In human-readable format though, it only appears once before
 /// the whole suggestion.
-pub fn multispan_sugg<I>(diag: &mut Diagnostic, help_msg: &str, sugg: I)
+pub fn multispan_sugg<I>(diag: &mut DiagnosticBuilder<'_, ()>, help_msg: &str, sugg: I)
 where
     I: IntoIterator<Item = (Span, String)>,
 {
@@ -227,7 +227,7 @@ where
 /// multiple spans. This is tracked in issue [rustfix#141](https://github.com/rust-lang/rustfix/issues/141).
 /// Suggestions with multiple spans will be silently ignored.
 pub fn multispan_sugg_with_applicability<I>(
-    diag: &mut Diagnostic,
+    diag: &mut DiagnosticBuilder<'_, ()>,
     help_msg: &str,
     applicability: Applicability,
     sugg: I,

src/tools/clippy/clippy_utils/src/sugg.rs

@@ -683,7 +683,7 @@ fn indentation<T: LintContext>(cx: &T, span: Span) -> Option<String> {
         })
 }
 
-/// Convenience extension trait for `Diagnostic`.
+/// Convenience extension trait for `DiagnosticBuilder`.
 pub trait DiagnosticExt<T: LintContext> {
     /// Suggests to add an attribute to an item.
     ///
@@ -731,7 +731,7 @@ pub trait DiagnosticExt<T: LintContext> {
     fn suggest_remove_item(&mut self, cx: &T, item: Span, msg: &str, applicability: Applicability);
 }
 
-impl<T: LintContext> DiagnosticExt<T> for rustc_errors::Diagnostic {
+impl<T: LintContext> DiagnosticExt<T> for rustc_errors::DiagnosticBuilder<'_, ()> {
     fn suggest_item_with_attr<D: Display + ?Sized>(
         &mut self,
         cx: &T,

tests/ui-fulldeps/internal-lints/diagnostics.rs

@@ -55,8 +55,11 @@ impl<'a, G: EmissionGuarantee> IntoDiagnostic<'a, G> for TranslatableInIntoDiagn
 pub struct UntranslatableInAddToDiagnostic;
 
 impl AddToDiagnostic for UntranslatableInAddToDiagnostic {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F)
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
-    {
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        f: F,
+    ) {
         diag.note("untranslatable diagnostic");
         //~^ ERROR diagnostics should be created using translatable messages
     }
@@ -65,7 +68,11 @@ impl AddToDiagnostic for UntranslatableInAddToDiagnostic {
 pub struct TranslatableInAddToDiagnostic;
 
 impl AddToDiagnostic for TranslatableInAddToDiagnostic {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        f: F,
+    ) {
         diag.note(crate::fluent_generated::no_crate_note);
     }
 }