Suggest semicolon removal and boxing when appropriate

Author: estebank

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

@@ -688,12 +688,26 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                     let msg = "`match` arms have incompatible types";
                     err.span_label(outer_error_span, msg);
                     if let Some((sp, boxed)) = semi_span {
-                        if matches!(boxed, StatementAsExpression::NeedsBoxing) {
-                            err.span_suggestion_verbose(
+                        if let (StatementAsExpression::NeedsBoxing, [.., prior_arm]) =
+                            (boxed, &prior_arms[..])
+                        {
+                            err.multipart_suggestion(
+                                "consider removing this semicolon and boxing the expressions",
+                                vec![
+                                    (prior_arm.shrink_to_lo(), "Box::new(".to_string()),
+                                    (prior_arm.shrink_to_hi(), ")".to_string()),
+                                    (arm_span.shrink_to_lo(), "Box::new(".to_string()),
+                                    (arm_span.shrink_to_hi(), ")".to_string()),
+                                    (sp, String::new()),
+                                ],
+                                Applicability::HasPlaceholders,
+                            );
+                        } else if matches!(boxed, StatementAsExpression::NeedsBoxing) {
+                            err.span_suggestion_short(
                                 sp,
-                                "consider removing this semicolon and boxing the expression",
+                                "consider removing this semicolon and boxing the expressions",
                                 String::new(),
-                                Applicability::HasPlaceholders,
+                                Applicability::MachineApplicable,
                             );
                         } else {
                             err.span_suggestion_short(
@@ -727,11 +741,16 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                 }
                 if let Some((sp, boxed)) = semicolon {
                     if matches!(boxed, StatementAsExpression::NeedsBoxing) {
-                        err.span_suggestion_verbose(
-                            sp,
+                        err.multipart_suggestion(
                             "consider removing this semicolon and boxing the expression",
-                            String::new(),
-                            Applicability::HasPlaceholders,
+                            vec![
+                                (then.shrink_to_lo(), "Box::new(".to_string()),
+                                (then.shrink_to_hi(), ")".to_string()),
+                                (else_sp.shrink_to_lo(), "Box::new(".to_string()),
+                                (else_sp.shrink_to_hi(), ")".to_string()),
+                                (sp, String::new()),
+                            ],
+                            Applicability::MachineApplicable,
                         );
                     } else {
                         err.span_suggestion_short(

src/test/ui/suggestions/match-prev-arm-needing-semi.stderr

@@ -24,10 +24,13 @@ help: consider `await`ing on the `Future`
    |
 LL |         false => async_dummy().await,
    |                               ^^^^^^
-help: consider removing this semicolon and boxing the expression
+help: consider removing this semicolon and boxing the expressions
+   |
+LL |             Box::new(async_dummy())
+LL |
+LL |         }
+LL |         false => Box::new(async_dummy()),
    |
-LL |             async_dummy()
-   |                         --
 
 error[E0308]: `match` arms have incompatible types
   --> $DIR/match-prev-arm-needing-semi.rs:39:18
@@ -55,10 +58,13 @@ help: consider `await`ing on the `Future`
    |
 LL |         false => async_dummy2().await,
    |                                ^^^^^^
-help: consider removing this semicolon and boxing the expression
+help: consider removing this semicolon and boxing the expressions
+   |
+LL |             Box::new(async_dummy())
+LL |
+LL |         }
+LL |         false => Box::new(async_dummy2()),
    |
-LL |             async_dummy()
-   |                         --
 
 error[E0308]: `match` arms have incompatible types
   --> $DIR/match-prev-arm-needing-semi.rs:50:18
@@ -70,10 +76,10 @@ LL |       let _ = match true {
    |  _____________-
 LL | |         true => async_dummy(),
    | |                 ------------- this is found to be of type `impl Future`
+LL | |
 LL | |         false => async_dummy2(),
    | |                  ^^^^^^^^^^^^^^ expected opaque type, found a different opaque type
-LL | |
-LL | |
+...  |
 LL | |
 LL | |     };
    | |_____- `match` arms have incompatible types
@@ -84,6 +90,7 @@ LL | |     };
 help: consider `await`ing on both `Future`s
    |
 LL |         true => async_dummy().await,
+LL |
 LL |         false => async_dummy2().await,
    |