Auto merge of #112116 - compiler-errors:misc-hir-typeck-mismatch-tweaks, r=WaffleLapkin

Misc HIR typeck type mismatch tweaks

These are all intended to improve #112104, but I couldn't get it to actually suggest adding `as_ref` to the LHS of the equality expr without some hacks that I may play around with some more.

Each commit's title should explain what it's doing except for perhaps the last one, which addresses the bogus suggestion on #112104 itself.

compiler/rustc_hir_typeck/messages.ftl

@@ -25,6 +25,8 @@ hir_typeck_const_select_must_be_fn = this argument must be a function item
 
 hir_typeck_convert_to_str = try converting the passed type into a `&str`
 
+hir_typeck_convert_using_method = try using `{$sugg}` to convert `{$found}` to `{$expected}`
+
 hir_typeck_ctor_is_private = tuple struct constructor `{$def}` is private
 
 hir_typeck_expected_default_return_type = expected `()` because of default return type

compiler/rustc_hir_typeck/src/demand.rs

@@ -3,7 +3,7 @@ use rustc_ast::util::parser::PREC_POSTFIX;
 use rustc_errors::MultiSpan;
 use rustc_errors::{Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed};
 use rustc_hir as hir;
-use rustc_hir::def::CtorKind;
+use rustc_hir::def::{CtorKind, Res};
 use rustc_hir::intravisit::Visitor;
 use rustc_hir::lang_items::LangItem;
 use rustc_hir::{is_range_literal, Node};
@@ -91,6 +91,56 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self.note_wrong_return_ty_due_to_generic_arg(err, expr, expr_ty);
     }
 
+    /// Really hacky heuristic to remap an `assert_eq!` error to the user
+    /// expressions provided to the macro.
+    fn adjust_expr_for_assert_eq_macro(
+        &self,
+        found_expr: &mut &'tcx hir::Expr<'tcx>,
+        expected_expr: &mut Option<&'tcx hir::Expr<'tcx>>,
+    ) {
+        let Some(expected_expr) = expected_expr else { return; };
+
+        if !found_expr.span.eq_ctxt(expected_expr.span) {
+            return;
+        }
+
+        if !found_expr
+            .span
+            .ctxt()
+            .outer_expn_data()
+            .macro_def_id
+            .is_some_and(|def_id| self.tcx.is_diagnostic_item(sym::assert_eq_macro, def_id))
+        {
+            return;
+        }
+
+        let hir::ExprKind::Unary(
+            hir::UnOp::Deref,
+            hir::Expr { kind: hir::ExprKind::Path(found_path), .. },
+        ) = found_expr.kind else { return; };
+        let hir::ExprKind::Unary(
+            hir::UnOp::Deref,
+            hir::Expr { kind: hir::ExprKind::Path(expected_path), .. },
+        ) = expected_expr.kind else { return; };
+
+        for (path, name, idx, var) in [
+            (expected_path, "left_val", 0, expected_expr),
+            (found_path, "right_val", 1, found_expr),
+        ] {
+            if let hir::QPath::Resolved(_, path) = path
+                && let [segment] = path.segments
+                && segment.ident.name.as_str() == name
+                && let Res::Local(hir_id) = path.res
+                && let Some((_, hir::Node::Expr(match_expr))) = self.tcx.hir().parent_iter(hir_id).nth(2)
+                && let hir::ExprKind::Match(scrutinee, _, _) = match_expr.kind
+                && let hir::ExprKind::Tup(exprs) = scrutinee.kind
+                && let hir::ExprKind::AddrOf(_, _, macro_arg) = exprs[idx].kind
+            {
+                *var = macro_arg;
+            }
+        }
+    }
+
     /// Requires that the two types unify, and prints an error message if
     /// they don't.
     pub fn demand_suptype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
@@ -156,7 +206,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     pub fn demand_coerce(
         &self,
-        expr: &hir::Expr<'tcx>,
+        expr: &'tcx hir::Expr<'tcx>,
         checked_ty: Ty<'tcx>,
         expected: Ty<'tcx>,
         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
@@ -177,10 +227,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     #[instrument(level = "debug", skip(self, expr, expected_ty_expr, allow_two_phase))]
     pub fn demand_coerce_diag(
         &self,
-        expr: &hir::Expr<'tcx>,
+        mut expr: &'tcx hir::Expr<'tcx>,
         checked_ty: Ty<'tcx>,
         expected: Ty<'tcx>,
-        expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
+        mut expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
         allow_two_phase: AllowTwoPhase,
     ) -> (Ty<'tcx>, Option<DiagnosticBuilder<'tcx, ErrorGuaranteed>>) {
         let expected = self.resolve_vars_with_obligations(expected);
@@ -190,6 +240,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             Err(e) => e,
         };
 
+        self.adjust_expr_for_assert_eq_macro(&mut expr, &mut expected_ty_expr);
+
         self.set_tainted_by_errors(self.tcx.sess.delay_span_bug(
             expr.span,
             "`TypeError` when attempting coercion but no error emitted",

compiler/rustc_hir_typeck/src/errors.rs

@@ -327,3 +327,19 @@ pub struct CtorIsPrivate {
     pub span: Span,
     pub def: String,
 }
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(
+    hir_typeck_convert_using_method,
+    applicability = "machine-applicable",
+    style = "verbose"
+)]
+pub struct SuggestConvertViaMethod<'tcx> {
+    #[suggestion_part(code = "{sugg}")]
+    pub span: Span,
+    #[suggestion_part(code = "")]
+    pub borrow_removal_span: Option<Span>,
+    pub sugg: &'static str,
+    pub expected: Ty<'tcx>,
+    pub found: Ty<'tcx>,
+}

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -1404,7 +1404,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // type of the place it is referencing, and not some
             // supertype thereof.
             let init_ty = self.check_expr_with_needs(init, Needs::maybe_mut_place(m));
-            self.demand_eqtype(init.span, local_ty, init_ty);
+            if let Some(mut diag) = self.demand_eqtype_diag(init.span, local_ty, init_ty) {
+                self.emit_type_mismatch_suggestions(
+                    &mut diag,
+                    init.peel_drop_temps(),
+                    init_ty,
+                    local_ty,
+                    None,
+                    None,
+                );
+                diag.emit();
+            }
             init_ty
         } else {
             self.check_expr_coercible_to_type(init, local_ty, None)

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -1,6 +1,8 @@
 use super::FnCtxt;
 
-use crate::errors::{AddReturnTypeSuggestion, ExpectedReturnTypeLabel, SuggestBoxing};
+use crate::errors::{
+    AddReturnTypeSuggestion, ExpectedReturnTypeLabel, SuggestBoxing, SuggestConvertViaMethod,
+};
 use crate::fluent_generated as fluent;
 use crate::method::probe::{IsSuggestion, Mode, ProbeScope};
 use rustc_ast::util::parser::{ExprPrecedence, PREC_POSTFIX};
@@ -275,6 +277,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expected_ty_expr: Option<&'tcx hir::Expr<'tcx>>,
     ) -> bool {
         let expr = expr.peel_blocks();
+        let methods = self.get_conversion_methods(expr.span, expected, found, expr.hir_id);
+
         if let Some((suggestion, msg, applicability, verbose, annotation)) =
             self.suggest_deref_or_ref(expr, found, expected)
         {
@@ -325,9 +329,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 }
             }
             return true;
-        } else if self.suggest_else_fn_with_closure(err, expr, found, expected) {
+        }
+
+        if self.suggest_else_fn_with_closure(err, expr, found, expected) {
             return true;
-        } else if self.suggest_fn_call(err, expr, found, |output| self.can_coerce(output, expected))
+        }
+
+        if self.suggest_fn_call(err, expr, found, |output| self.can_coerce(output, expected))
             && let ty::FnDef(def_id, ..) = *found.kind()
             && let Some(sp) = self.tcx.hir().span_if_local(def_id)
         {
@@ -343,12 +351,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 err.span_label(sp, format!("{descr} `{name}` defined here"));
             }
             return true;
-        } else if self.suggest_cast(err, expr, found, expected, expected_ty_expr) {
+        }
+
+        if self.suggest_cast(err, expr, found, expected, expected_ty_expr) {
             return true;
-        } else {
+        }
-            let methods = self.get_conversion_methods(expr.span, expected, found, expr.hir_id);
+
         if !methods.is_empty() {
-                let mut suggestions = methods.iter()
+            let mut suggestions = methods
+                .iter()
                 .filter_map(|conversion_method| {
                     let receiver_method_ident = expr.method_ident();
                     if let Some(method_ident) = receiver_method_ident
@@ -380,12 +391,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     } else {
                         vec![(expr.span.shrink_to_hi(), format!(".{}()", conversion_method.name))]
                     };
-                        let struct_pat_shorthand_field = self.maybe_get_struct_pattern_shorthand_field(expr);
+                    let struct_pat_shorthand_field =
+                        self.maybe_get_struct_pattern_shorthand_field(expr);
                     if let Some(name) = struct_pat_shorthand_field {
-                            sugg.insert(
+                        sugg.insert(0, (expr.span.shrink_to_lo(), format!("{}: ", name)));
-                                0,
-                                (expr.span.shrink_to_lo(), format!("{}: ", name)),
-                            );
                     }
                     Some(sugg)
                 })
@@ -398,42 +407,100 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 );
                 return true;
             }
-            } else if let ty::Adt(found_adt, found_substs) = found.kind()
-                && self.tcx.is_diagnostic_item(sym::Option, found_adt.did())
-                && let ty::Adt(expected_adt, expected_substs) = expected.kind()
-                && self.tcx.is_diagnostic_item(sym::Option, expected_adt.did())
-                && let ty::Ref(_, inner_ty, _) = expected_substs.type_at(0).kind()
-                && inner_ty.is_str()
-            {
-                let ty = found_substs.type_at(0);
-                let mut peeled = ty;
-                let mut ref_cnt = 0;
-                while let ty::Ref(_, inner, _) = peeled.kind() {
-                    peeled = *inner;
-                    ref_cnt += 1;
         }
-                if let ty::Adt(adt, _) = peeled.kind()
+
-                    && Some(adt.did()) == self.tcx.lang_items().string()
+        if let Some((found_ty_inner, expected_ty_inner, error_tys)) =
+            self.deconstruct_option_or_result(found, expected)
+            && let ty::Ref(_, peeled, hir::Mutability::Not) = *expected_ty_inner.kind()
         {
-                    let sugg = if ref_cnt == 0 {
+            // Suggest removing any stray borrows (unless there's macro shenanigans involved).
-                        ".as_deref()"
+            let inner_expr = expr.peel_borrows();
+            if !inner_expr.span.eq_ctxt(expr.span) {
+                return false;
+            }
+            let borrow_removal_span = if inner_expr.hir_id == expr.hir_id {
+                None
             } else {
-                        ".map(|x| x.as_str())"
+                Some(expr.span.shrink_to_lo().until(inner_expr.span))
             };
+            // Given `Result<_, E>`, check our expected ty is `Result<_, &E>` for
+            // `as_ref` and `as_deref` compatibility.
+            let error_tys_equate_as_ref = error_tys.map_or(true, |(found, expected)| {
+                self.can_eq(self.param_env, self.tcx.mk_imm_ref(self.tcx.lifetimes.re_erased, found), expected)
+            });
+            // FIXME: This could/should be extended to suggest `as_mut` and `as_deref_mut`,
+            // but those checks need to be a bit more delicate and the benefit is diminishing.
+            if self.can_eq(self.param_env, found_ty_inner, peeled) && error_tys_equate_as_ref {
+                err.subdiagnostic(SuggestConvertViaMethod {
+                    span: expr.span.shrink_to_hi(),
+                    sugg: ".as_ref()",
+                    expected,
+                    found,
+                    borrow_removal_span,
+                });
+                return true;
+            } else if let Some((deref_ty, _)) =
+                self.autoderef(expr.span, found_ty_inner).silence_errors().nth(1)
+                && self.can_eq(self.param_env, deref_ty, peeled)
+                && error_tys_equate_as_ref
+            {
+                err.subdiagnostic(SuggestConvertViaMethod {
+                    span: expr.span.shrink_to_hi(),
+                    sugg: ".as_deref()",
+                    expected,
+                    found,
+                    borrow_removal_span,
+                });
+                return true;
+            } else if let ty::Adt(adt, _) = found_ty_inner.peel_refs().kind()
+                && Some(adt.did()) == self.tcx.lang_items().string()
+                && peeled.is_str()
+                // `Result::map`, conversely, does not take ref of the error type.
+                && error_tys.map_or(true, |(found, expected)| {
+                    self.can_eq(self.param_env, found, expected)
+                })
+            {
                 err.span_suggestion_verbose(
                     expr.span.shrink_to_hi(),
                     fluent::hir_typeck_convert_to_str,
-                        sugg,
+                    ".map(|x| x.as_str())",
                     Applicability::MachineApplicable,
                 );
                 return true;
             }
         }
-        }
 
         false
     }
 
+    fn deconstruct_option_or_result(
+        &self,
+        found_ty: Ty<'tcx>,
+        expected_ty: Ty<'tcx>,
+    ) -> Option<(Ty<'tcx>, Ty<'tcx>, Option<(Ty<'tcx>, Ty<'tcx>)>)> {
+        let ty::Adt(found_adt, found_substs) = found_ty.peel_refs().kind() else {
+            return None;
+        };
+        let ty::Adt(expected_adt, expected_substs) = expected_ty.kind() else {
+            return None;
+        };
+        if self.tcx.is_diagnostic_item(sym::Option, found_adt.did())
+            && self.tcx.is_diagnostic_item(sym::Option, expected_adt.did())
+        {
+            Some((found_substs.type_at(0), expected_substs.type_at(0), None))
+        } else if self.tcx.is_diagnostic_item(sym::Result, found_adt.did())
+            && self.tcx.is_diagnostic_item(sym::Result, expected_adt.did())
+        {
+            Some((
+                found_substs.type_at(0),
+                expected_substs.type_at(0),
+                Some((found_substs.type_at(1), expected_substs.type_at(1))),
+            ))
+        } else {
+            None
+        }
+    }
+
     /// When encountering the expected boxed value allocated in the stack, suggest allocating it
     /// in the heap by calling `Box::new()`.
     pub(in super::super) fn suggest_boxing_when_appropriate(

compiler/rustc_infer/messages.ftl

@@ -278,9 +278,6 @@ infer_ril_introduced_by = requirement introduced by this return type
 infer_ril_introduced_here = `'static` requirement introduced here
 infer_ril_static_introduced_by = "`'static` lifetime requirement introduced by the return type
 
-infer_sarwa_option = you can convert from `&Option<T>` to `Option<&T>` using `.as_ref()`
-infer_sarwa_result = you can convert from `&Result<T, E>` to `Result<&T, &E>` using `.as_ref()`
-
 infer_sbfrit_box_return_expr = if you change the return type to expect trait objects, box the returned expressions
 
 infer_sbfrit_change_return_type = you could change the return type to be a boxed trait object

compiler/rustc_infer/src/errors/mod.rs

@@ -1246,30 +1246,6 @@ pub struct FnConsiderCasting {
     pub casting: String,
 }
 
-#[derive(Subdiagnostic)]
-pub enum SuggestAsRefWhereAppropriate<'a> {
-    #[suggestion(
-        infer_sarwa_option,
-        code = "{snippet}.as_ref()",
-        applicability = "machine-applicable"
-    )]
-    Option {
-        #[primary_span]
-        span: Span,
-        snippet: &'a str,
-    },
-    #[suggestion(
-        infer_sarwa_result,
-        code = "{snippet}.as_ref()",
-        applicability = "machine-applicable"
-    )]
-    Result {
-        #[primary_span]
-        span: Span,
-        snippet: &'a str,
-    },
-}
-
 #[derive(Subdiagnostic)]
 pub enum SuggestAccessingField<'a> {
     #[suggestion(

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

@@ -1897,7 +1897,6 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
 
             if should_suggest_fixes {
                 self.suggest_tuple_pattern(cause, &exp_found, diag);
-                self.suggest_as_ref_where_appropriate(span, &exp_found, diag);
                 self.suggest_accessing_field_where_appropriate(cause, &exp_found, diag);
                 self.suggest_await_on_expect_found(cause, span, &exp_found, diag);
                 self.suggest_function_pointers(cause, span, &exp_found, diag);

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

@@ -374,12 +374,18 @@ impl<T> Trait<T> for X {
     ) {
         let tcx = self.tcx;
 
+        // Don't suggest constraining a projection to something containing itself
+        if self.tcx.erase_regions(values.found).contains(self.tcx.erase_regions(values.expected)) {
+            return;
+        }
+
         let msg = || {
             format!(
                 "consider constraining the associated type `{}` to `{}`",
                 values.expected, values.found
             )
         };
+
         let body_owner = tcx.hir().get_if_local(body_owner_def_id);
         let current_method_ident = body_owner.and_then(|n| n.ident()).map(|i| i.name);
 

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

@@ -13,9 +13,9 @@ use rustc_span::{sym, BytePos, Span};
 
 use crate::errors::{
     ConsiderAddingAwait, FnConsiderCasting, FnItemsAreDistinct, FnUniqTypes,
-    FunctionPointerSuggestion, SuggestAccessingField, SuggestAsRefWhereAppropriate,
+    FunctionPointerSuggestion, SuggestAccessingField, SuggestBoxingForReturnImplTrait,
-    SuggestBoxingForReturnImplTrait, SuggestRemoveSemiOrReturnBinding, SuggestTuplePatternMany,
+    SuggestRemoveSemiOrReturnBinding, SuggestTuplePatternMany, SuggestTuplePatternOne,
-    SuggestTuplePatternOne, TypeErrorAdditionalDiags,
+    TypeErrorAdditionalDiags,
 };
 
 use super::TypeErrCtxt;
@@ -289,27 +289,6 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
         }
     }
 
-    /// When encountering a case where `.as_ref()` on a `Result` or `Option` would be appropriate,
-    /// suggests it.
-    pub(super) fn suggest_as_ref_where_appropriate(
-        &self,
-        span: Span,
-        exp_found: &ty::error::ExpectedFound<Ty<'tcx>>,
-        diag: &mut Diagnostic,
-    ) {
-        if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(span)
-            && let Some(msg) = self.should_suggest_as_ref_kind(exp_found.expected, exp_found.found)
-        {
-            // HACK: fix issue# 100605, suggesting convert from &Option<T> to Option<&T>, remove the extra `&`
-            let snippet = snippet.trim_start_matches('&');
-            let subdiag = match msg {
-                SuggestAsRefKind::Option => SuggestAsRefWhereAppropriate::Option { span, snippet },
-                SuggestAsRefKind::Result => SuggestAsRefWhereAppropriate::Result { span, snippet },
-            };
-            diag.subdiagnostic(subdiag);
-        }
-    }
-
     pub(super) fn suggest_function_pointers(
         &self,
         cause: &ObligationCause<'tcx>,

tests/ui/associated-types/dont-suggest-cyclic-constraint.rs

@@ -0,0 +1,11 @@
+use std::fmt::Debug;
+
+fn foo<I: Iterator>(mut iter: I, value: &I::Item)
+where
+    I::Item: Eq + Debug,
+{
+    debug_assert_eq!(iter.next(), Some(value));
+    //~^ ERROR mismatched types
+}
+
+fn main() {}

tests/ui/associated-types/dont-suggest-cyclic-constraint.stderr

@@ -0,0 +1,12 @@
+error[E0308]: mismatched types
+  --> $DIR/dont-suggest-cyclic-constraint.rs:7:35
+   |
+LL |     debug_assert_eq!(iter.next(), Some(value));
+   |                                   ^^^^^^^^^^^ expected `Option<<I as Iterator>::Item>`, found `Option<&<I as Iterator>::Item>`
+   |
+   = note: expected enum `Option<<I as Iterator>::Item>`
+              found enum `Option<&<I as Iterator>::Item>`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.

tests/ui/inference/deref-suggestion.stderr

@@ -84,15 +84,16 @@ LL | fn foo3(_: u32) {}
    |    ^^^^ ------
 
 error[E0308]: mismatched types
-  --> $DIR/deref-suggestion.rs:37:5
+  --> $DIR/deref-suggestion.rs:37:22
    |
 LL |     assert_eq!(3i32, &3i32);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^
+   |                      ^^^^^ expected `i32`, found `&i32`
-   |     |
+   |
-   |     expected `i32`, found `&i32`
+help: consider removing the borrow
-   |     expected because this is `i32`
+   |
+LL -     assert_eq!(3i32, &3i32);
+LL +     assert_eq!(3i32, 3i32);
    |
-   = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0308]: mismatched types
   --> $DIR/deref-suggestion.rs:40:17

tests/ui/issues/issue-100605.stderr

@@ -13,10 +13,6 @@ note: function defined here
    |
 LL | fn takes_option(_arg: Option<&String>) {}
    |    ^^^^^^^^^^^^ ---------------------
-help: you can convert from `&Option<T>` to `Option<&T>` using `.as_ref()`
-   |
-LL |     takes_option(None.as_ref());
-   |                  ~~~~~~~~~~~~~
 help: consider removing the borrow
    |
 LL -     takes_option(&None);
@@ -27,10 +23,8 @@ error[E0308]: mismatched types
   --> $DIR/issue-100605.rs:8:18
    |
 LL |     takes_option(&res);
-   |     ------------ ^^^^
+   |     ------------ ^^^^ expected `Option<&String>`, found `&Option<String>`
-   |     |            |
+   |     |
-   |     |            expected `Option<&String>`, found `&Option<String>`
-   |     |            help: you can convert from `&Option<T>` to `Option<&T>` using `.as_ref()`: `res.as_ref()`
    |     arguments to this function are incorrect
    |
    = note:   expected enum `Option<&String>`
@@ -40,6 +34,11 @@ note: function defined here
    |
 LL | fn takes_option(_arg: Option<&String>) {}
    |    ^^^^^^^^^^^^ ---------------------
+help: try using `.as_ref()` to convert `&Option<String>` to `Option<&String>`
+   |
+LL -     takes_option(&res);
+LL +     takes_option(res.as_ref());
+   |
 
 error: aborting due to 2 previous errors
 

tests/ui/let-else/let-else-ref-bindings.stderr

@@ -6,6 +6,10 @@ LL |     let Some(ref a): Option<&[u8]> = some else { return };
    |
    = note: expected enum `Option<&[u8]>`
               found enum `Option<Vec<u8>>`
+help: try using `.as_deref()` to convert `Option<Vec<u8>>` to `Option<&[u8]>`
+   |
+LL |     let Some(ref a): Option<&[u8]> = some.as_deref() else { return };
+   |                                          +++++++++++
 
 error[E0308]: mismatched types
   --> $DIR/let-else-ref-bindings.rs:20:38
@@ -15,6 +19,11 @@ LL |     let Some(ref a): Option<&[u8]> = &some else { return };
    |
    = note:   expected enum `Option<&[u8]>`
            found reference `&Option<Vec<u8>>`
+help: try using `.as_deref()` to convert `&Option<Vec<u8>>` to `Option<&[u8]>`
+   |
+LL -     let Some(ref a): Option<&[u8]> = &some else { return };
+LL +     let Some(ref a): Option<&[u8]> = some.as_deref() else { return };
+   |
 
 error[E0308]: mismatched types
   --> $DIR/let-else-ref-bindings.rs:24:34
@@ -26,6 +35,10 @@ LL |     let Some(a): Option<&[u8]> = some else { return };
    |
    = note: expected enum `Option<&[u8]>`
               found enum `Option<Vec<u8>>`
+help: try using `.as_deref()` to convert `Option<Vec<u8>>` to `Option<&[u8]>`
+   |
+LL |     let Some(a): Option<&[u8]> = some.as_deref() else { return };
+   |                                      +++++++++++
 
 error[E0308]: mismatched types
   --> $DIR/let-else-ref-bindings.rs:27:34
@@ -37,6 +50,11 @@ LL |     let Some(a): Option<&[u8]> = &some else { return };
    |
    = note:   expected enum `Option<&[u8]>`
            found reference `&Option<Vec<u8>>`
+help: try using `.as_deref()` to convert `&Option<Vec<u8>>` to `Option<&[u8]>`
+   |
+LL -     let Some(a): Option<&[u8]> = &some else { return };
+LL +     let Some(a): Option<&[u8]> = some.as_deref() else { return };
+   |
 
 error[E0308]: mismatched types
   --> $DIR/let-else-ref-bindings.rs:44:46

tests/ui/suggestions/as-ref.rs

@@ -24,4 +24,6 @@ fn main() {
     let multiple_ref_result = &&Ok(Foo);
     multiple_ref_result.map(|arg| takes_ref(arg)); //~ ERROR mismatched types [E0308]
     multiple_ref_result.and_then(|arg| Ok(takes_ref(arg))); //~ ERROR mismatched types [E0308]
+
+    let _: Result<&usize, _> = &Ok(42); //~ ERROR mismatched types [E0308]
 }

tests/ui/suggestions/as-ref.stderr

@@ -74,14 +74,16 @@ error[E0308]: mismatched types
   --> $DIR/as-ref.rs:13:29
    |
 LL |     let y: Option<&usize> = x;
-   |            --------------   ^
+   |            --------------   ^ expected `Option<&usize>`, found `&Option<usize>`
-   |            |                |
+   |            |
-   |            |                expected `Option<&usize>`, found `&Option<usize>`
-   |            |                help: you can convert from `&Option<T>` to `Option<&T>` using `.as_ref()`: `x.as_ref()`
    |            expected due to this
    |
    = note:   expected enum `Option<&usize>`
            found reference `&Option<usize>`
+help: try using `.as_ref()` to convert `&Option<usize>` to `Option<&usize>`
+   |
+LL |     let y: Option<&usize> = x.as_ref();
+   |                              +++++++++
 
 error[E0308]: mismatched types
   --> $DIR/as-ref.rs:15:37
@@ -93,10 +95,10 @@ LL |     let y: Result<&usize, &usize> = x;
    |
    = note:   expected enum `Result<&usize, &usize>`
            found reference `&Result<usize, usize>`
-help: you can convert from `&Result<T, E>` to `Result<&T, &E>` using `.as_ref()`
+help: try using `.as_ref()` to convert `&Result<usize, usize>` to `Result<&usize, &usize>`
    |
 LL |     let y: Result<&usize, &usize> = x.as_ref();
-   |                                     ~~~~~~~~~~
+   |                                      +++++++++
 
 error[E0308]: mismatched types
   --> $DIR/as-ref.rs:19:36
@@ -181,6 +183,22 @@ help: consider using `as_ref` instead
 LL |     multiple_ref_result.as_ref().and_then(|arg| Ok(takes_ref(arg)));
    |                         +++++++++
 
-error: aborting due to 11 previous errors
+error[E0308]: mismatched types
+  --> $DIR/as-ref.rs:28:32
+   |
+LL |     let _: Result<&usize, _> = &Ok(42);
+   |            -----------------   ^^^^^^^ expected `Result<&usize, _>`, found `&Result<{integer}, _>`
+   |            |
+   |            expected due to this
+   |
+   = note:   expected enum `Result<&usize, _>`
+           found reference `&Result<{integer}, _>`
+help: try using `.as_ref()` to convert `&Result<{integer}, _>` to `Result<&usize, _>`
+   |
+LL -     let _: Result<&usize, _> = &Ok(42);
+LL +     let _: Result<&usize, _> = Ok(42).as_ref();
+   |
+
+error: aborting due to 12 previous errors
 
 For more information about this error, try `rustc --explain E0308`.

tests/ui/suggestions/dont-suggest-try_into-in-macros.stderr

@@ -1,13 +1,13 @@
 error[E0308]: mismatched types
-  --> $DIR/dont-suggest-try_into-in-macros.rs:2:5
+  --> $DIR/dont-suggest-try_into-in-macros.rs:2:23
    |
 LL |     assert_eq!(10u64, 10usize);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                       ^^^^^^^ expected `u64`, found `usize`
-   |     |
-   |     expected `u64`, found `usize`
-   |     expected because this is `u64`
    |
-   = note: this error originates in the macro `assert_eq` (in Nightly builds, run with -Z macro-backtrace for more info)
+help: change the type of the numeric literal from `usize` to `u64`
+   |
+LL |     assert_eq!(10u64, 10u64);
+   |                         ~~~
 
 error: aborting due to previous error
 

tests/ui/switched-expectations.stderr

@@ -2,7 +2,9 @@ error[E0308]: mismatched types
   --> $DIR/switched-expectations.rs:3:30
    |
 LL |     let ref string: String = var;
-   |                              ^^^ expected `String`, found `i32`
+   |                              ^^^- help: try using a conversion method: `.to_string()`
+   |                              |
+   |                              expected `String`, found `i32`
 
 error: aborting due to previous error
 

tests/ui/typeck/issue-89856.stderr

@@ -13,7 +13,7 @@ note: function defined here
    |
 LL | fn take_str_maybe(_: Option<&str>) { }
    |    ^^^^^^^^^^^^^^ ---------------
-help: try converting the passed type into a `&str`
+help: try using `.as_deref()` to convert `Option<String>` to `Option<&str>`
    |
 LL |     take_str_maybe(option.as_deref());
    |                          +++++++++++