Provide suggestion when encountering match () { () => 1 } + match () { () => 1 }

```
error[E0308]: mismatched types
  --> $DIR/expr-as-stmt.rs:69:5
   |
LL |     match () { () => 1 } + match () { () => 1 }
   |     ^^^^^^^^^^^^^^^^^^^^ expected `()`, found integer
   |
help: consider using a semicolon here
   |
LL |     match () { () => 1 }; + match () { () => 1 }
   |                         +
help: alternatively, parentheses are required to parse this as an expression
   |
LL |     (match () { () => 1 }) + match () { () => 1 }
   |     +                    +
```

Parentheses are needed for the `match` to be unambiguously parsed as an expression and not a statement when chaining with binops that are also unops.

Author: estebank

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -1914,6 +1914,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 self.check_expr_has_type_or_error(expr, self.tcx.types.unit, |err| {
                     if expr.can_have_side_effects() {
                         self.suggest_semicolon_at_end(expr.span, err);
+                        if let hir::ExprKind::Match(..) = expr.kind {
+                            err.multipart_suggestion(
+                                "alternatively, parentheses are required to parse this as an \
+                                 expression",
+                                vec![
+                                    (expr.span.shrink_to_lo(), "(".to_string()),
+                                    (expr.span.shrink_to_hi(), ")".to_string()),
+                                ],
+                                Applicability::MaybeIncorrect,
+                            );
+                        }
                     }
                 });
             }

tests/ui/parser/expr-as-stmt.stderr

@@ -227,9 +227,16 @@ error[E0308]: mismatched types
   --> $DIR/expr-as-stmt.rs:69:5
    |
 LL |     match () { () => 1 } + match () { () => 1 }
-   |     ^^^^^^^^^^^^^^^^^^^^- help: consider using a semicolon here
-   |     |
-   |     expected `()`, found integer
+   |     ^^^^^^^^^^^^^^^^^^^^ expected `()`, found integer
+   |
+help: consider using a semicolon here
+   |
+LL |     match () { () => 1 }; + match () { () => 1 }
+   |                         +
+help: alternatively, parentheses are required to parse this as an expression
+   |
+LL |     (match () { () => 1 }) + match () { () => 1 }
+   |     +                    +
 
 error[E0308]: mismatched types
   --> $DIR/expr-as-stmt.rs:75:14

tests/ui/suggestions/match-needing-semi.stderr

@@ -28,9 +28,20 @@ LL | |         4 => 1,
 LL | |         3 => 2,
 LL | |         _ => 2
 LL | |     }
-   | |     ^- help: consider using a semicolon here
-   | |_____|
-   |       expected `()`, found integer
+   | |_____^ expected `()`, found integer
+   |
+help: consider using a semicolon here
+   |
+LL |     };
+   |      +
+help: alternatively, parentheses are required to parse this as an expression
+   |
+LL ~     (match 3 {
+LL |         4 => 1,
+LL |         3 => 2,
+LL |         _ => 2
+LL ~     })
+   |
 
 error: aborting due to 2 previous errors