Provide boxing suggestion for type Alias = dyn Trait return type

Author: estebank

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -1892,6 +1892,17 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             // type T = dyn Trait;
             // fn foo() -> impl T { .. }
             err.span_note(self.tcx.def_span(def_id), "this type alias is unsized");
+            err.multipart_suggestion(
+                format!(
+                    "consider boxing the return type, and wrapping all of the returned values in \
+                    `Box::new`",
+                ),
+                vec![
+                    (ty.span.shrink_to_lo(), "Box<".to_string()),
+                    (ty.span.shrink_to_hi(), ">".to_string()),
+                ],
+                Applicability::MaybeIncorrect,
+            );
             return false;
         }
 

tests/ui/type-alias/dyn-trait-type-alias-return-type.fixed

@@ -0,0 +1,18 @@
+//@ run-rustfix
+type T = dyn core::fmt::Debug;
+//~^ NOTE this type alias is unsized
+
+fn f() -> Box<T> { loop {} }
+//~^ ERROR the size for values of type `(dyn Debug + 'static)` cannot be known at compilation time
+//~| HELP the trait `Sized` is not implemented for `(dyn Debug + 'static)`
+//~| NOTE doesn't have a size known at compile-time
+//~| NOTE the return type of a function must have a statically known size
+//~| HELP consider boxing the return type
+
+fn main() {
+    f();
+    //~^ ERROR the size for values of type `(dyn Debug + 'static)` cannot be known at compilation time
+    //~| HELP the trait `Sized` is not implemented for `(dyn Debug + 'static)`
+    //~| NOTE doesn't have a size known at compile-time
+    //~| NOTE the return type of a function must have a statically known size
+}

tests/ui/type-alias/dyn-trait-type-alias-return-type.rs

@@ -1,3 +1,4 @@
+//@ run-rustfix
 type T = dyn core::fmt::Debug;
 //~^ NOTE this type alias is unsized
 
@@ -6,5 +7,12 @@ fn f() -> T { loop {} }
 //~| HELP the trait `Sized` is not implemented for `(dyn Debug + 'static)`
 //~| NOTE doesn't have a size known at compile-time
 //~| NOTE the return type of a function must have a statically known size
+//~| HELP consider boxing the return type
 
-fn main() {}
+fn main() {
+    f();
+    //~^ ERROR the size for values of type `(dyn Debug + 'static)` cannot be known at compilation time
+    //~| HELP the trait `Sized` is not implemented for `(dyn Debug + 'static)`
+    //~| NOTE doesn't have a size known at compile-time
+    //~| NOTE the return type of a function must have a statically known size
+}

tests/ui/type-alias/dyn-trait-type-alias-return-type.stderr

@@ -1,17 +1,30 @@
 error[E0277]: the size for values of type `(dyn Debug + 'static)` cannot be known at compilation time
-  --> $DIR/dyn-trait-type-alias-return-type.rs:4:11
+  --> $DIR/dyn-trait-type-alias-return-type.rs:5:11
    |
 LL | fn f() -> T { loop {} }
    |           ^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `(dyn Debug + 'static)`
 note: this type alias is unsized
-  --> $DIR/dyn-trait-type-alias-return-type.rs:1:1
+  --> $DIR/dyn-trait-type-alias-return-type.rs:2:1
    |
 LL | type T = dyn core::fmt::Debug;
    | ^^^^^^
    = note: the return type of a function must have a statically known size
+help: consider boxing the return type, and wrapping all of the returned values in `Box::new`
+   |
+LL | fn f() -> Box<T> { loop {} }
+   |           ++++ +
+
+error[E0277]: the size for values of type `(dyn Debug + 'static)` cannot be known at compilation time
+  --> $DIR/dyn-trait-type-alias-return-type.rs:13:5
+   |
+LL |     f();
+   |     ^^^ doesn't have a size known at compile-time
+   |
+   = help: the trait `Sized` is not implemented for `(dyn Debug + 'static)`
+   = note: the return type of a function must have a statically known size
 
-error: aborting due to 1 previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0277`.