Rollup merge of #126367 - compiler-errors:point-out-failing-never-obligation, r=WaffleLapkin

Point out failing never obligation for `DEPENDENCY_ON_UNIT_NEVER_TYPE_FALLBACK`

Based on top of #125289, so just need to look at the last commit.

r? `@WaffleLapkin`

compiler/rustc_hir_typeck/messages.ftl

@@ -45,6 +45,7 @@ hir_typeck_convert_using_method = try using `{$sugg}` to convert `{$found}` to `
 hir_typeck_ctor_is_private = tuple struct constructor `{$def}` is private
 
 hir_typeck_dependency_on_unit_never_type_fallback = this function depends on never type fallback being `()`
+    .note = in edition 2024, the requirement `{$obligation}` will fail
     .help = specify the types explicitly
 
 hir_typeck_deref_is_empty = this expression `Deref`s to `{$deref_ty}` which implements `is_empty`

compiler/rustc_hir_typeck/src/errors.rs

@@ -7,7 +7,7 @@ use rustc_errors::{
     SubdiagMessageOp, Subdiagnostic,
 };
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
-use rustc_middle::ty::Ty;
+use rustc_middle::ty::{self, Ty};
 use rustc_span::{
     edition::{Edition, LATEST_STABLE_EDITION},
     symbol::Ident,
@@ -186,7 +186,11 @@ pub enum NeverTypeFallbackFlowingIntoUnsafe {
 #[derive(LintDiagnostic)]
 #[help]
 #[diag(hir_typeck_dependency_on_unit_never_type_fallback)]
-pub struct DependencyOnUnitNeverTypeFallback {}
+pub struct DependencyOnUnitNeverTypeFallback<'tcx> {
+    #[note]
+    pub obligation_span: Span,
+    pub obligation: ty::Predicate<'tcx>,
+}
 
 #[derive(Subdiagnostic)]
 #[multipart_suggestion(

compiler/rustc_hir_typeck/src/fallback.rs

@@ -488,7 +488,7 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         let remaining_errors_if_fallback_to = |fallback| {
             self.probe(|_| {
                 let obligations = self.fulfillment_cx.borrow().pending_obligations();
-                let ocx = ObligationCtxt::new(&self.infcx);
+                let ocx = ObligationCtxt::new_with_diagnostics(&self.infcx);
                 ocx.register_obligations(obligations.iter().cloned());
 
                 for &diverging_vid in diverging_vids {
@@ -506,14 +506,18 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
         // then this code will be broken by the never type fallback change.qba
         let unit_errors = remaining_errors_if_fallback_to(self.tcx.types.unit);
         if unit_errors.is_empty()
-            && let never_errors = remaining_errors_if_fallback_to(self.tcx.types.never)
-            && !never_errors.is_empty()
+            && let mut never_errors = remaining_errors_if_fallback_to(self.tcx.types.never)
+            && let [ref mut never_error, ..] = never_errors.as_mut_slice()
         {
+            self.adjust_fulfillment_error_for_expr_obligation(never_error);
             self.tcx.emit_node_span_lint(
                 lint::builtin::DEPENDENCY_ON_UNIT_NEVER_TYPE_FALLBACK,
                 self.tcx.local_def_id_to_hir_id(self.body_id),
                 self.tcx.def_span(self.body_id),
-                errors::DependencyOnUnitNeverTypeFallback {},
+                errors::DependencyOnUnitNeverTypeFallback {
+                    obligation_span: never_error.obligation.cause.span,
+                    obligation: never_error.obligation.predicate,
+                },
             )
         }
     }