Suggest `;` after bare `match` expression E0308

Fix #72634.

compiler/rustc_hir_typeck/src/_match.rs

@@ -121,6 +121,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         prior_arm_ty,
                         prior_arm_span,
                         scrut_span: scrut.span,
+                        scrut_hir_id: scrut.hir_id,
                         source: match_src,
                         prior_arms: other_arms.clone(),
                         opt_suggest_box_span,

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

@@ -775,6 +775,7 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                 ref prior_arms,
                 opt_suggest_box_span,
                 scrut_span,
+                scrut_hir_id,
                 ..
             }) => match source {
                 hir::MatchSource::TryDesugar(scrut_hir_id) => {
@@ -842,6 +843,18 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                     ) {
                         err.subdiagnostic(subdiag);
                     }
+                    if let Some(hir::Node::Expr(m)) = self.tcx.hir().find_parent(scrut_hir_id)
+                        && let Some(hir::Node::Stmt(stmt)) = self.tcx.hir().find_parent(m.hir_id)
+                        && let hir::StmtKind::Expr(_) = stmt.kind
+                    {
+                        err.span_suggestion_verbose(
+                            stmt.span.shrink_to_hi(),
+                            "consider using a semicolon here, but this will discard any values \
+                             in the match arms",
+                            ";",
+                            Applicability::MaybeIncorrect,
+                        );
+                    }
                     if let Some(ret_sp) = opt_suggest_box_span {
                         // Get return type span and point to it.
                         self.suggest_boxing_for_return_impl_trait(

compiler/rustc_middle/src/traits/mod.rs

@@ -541,6 +541,7 @@ pub struct MatchExpressionArmCause<'tcx> {
     pub prior_arm_ty: Ty<'tcx>,
     pub prior_arm_span: Span,
     pub scrut_span: Span,
+    pub scrut_hir_id: hir::HirId,
     pub source: hir::MatchSource,
     pub prior_arms: Vec<Span>,
     pub opt_suggest_box_span: Option<Span>,

tests/ui/suggestions/issue-81839.stderr

@@ -4,15 +4,22 @@ error[E0308]: `match` arms have incompatible types
 LL | /     match num {
 LL | |         1 => {
 LL | |             cx.answer_str("hi");
-   | |             --------------------
-   | |             |                  |
-   | |             |                  help: consider removing this semicolon
-   | |             this is found to be of type `()`
+   | |             -------------------- this is found to be of type `()`
 LL | |         }
 LL | |         _ => cx.answer_str("hi"),
    | |              ^^^^^^^^^^^^^^^^^^^ expected `()`, found future
 LL | |     }
    | |_____- `match` arms have incompatible types
+   |
+help: consider removing this semicolon
+   |
+LL -             cx.answer_str("hi");
+LL +             cx.answer_str("hi")
+   |
+help: consider using a semicolon here, but this will discard any values in the match arms
+   |
+LL |     };
+   |      +
 
 error: aborting due to previous error
 

tests/ui/wf/wf-unsafe-trait-obj-match.stderr

@@ -11,6 +11,10 @@ LL | |     }
    |
    = note: expected reference `&S`
               found reference `&R`
+help: consider using a semicolon here, but this will discard any values in the match arms
+   |
+LL |     };
+   |      +
 
 error[E0038]: the trait `Trait` cannot be made into an object
   --> $DIR/wf-unsafe-trait-obj-match.rs:26:21