Suggest ; after bare match expression E0308

Fix #72634.

Author: estebank

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

@@ -748,6 +748,18 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         arm_ty,
                         arm_span,
                     );
+                    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(

src/test/ui/suggestions/issue-81839.stderr

@@ -4,10 +4,7 @@ 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 opaque type
@@ -21,6 +18,15 @@ LL |     pub async fn answer_str(&self, _s: &str) -> Test {
    |                                                 ^^^^ checked the `Output` of this `async fn`, found opaque type
    = note: expected unit type `()`
             found opaque type `impl Future<Output = Test>`
+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
 

src/test/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