Make impl Trait suggestion in E0746 MachineApplicable

Author: estebank

src/librustc/traits/error_reporting.rs

@@ -1840,7 +1840,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                                 last_ty, trait_obj,
                             ),
                             format!("impl {}", trait_obj),
-                            Applicability::MaybeIncorrect,
+                            Applicability::MachineApplicable,
                         );
                         err.note(impl_trait_msg);
                     } else {

src/test/ui/error-codes/E0746.fixed

@@ -0,0 +1,18 @@
+// run-rustfix
+#![allow(dead_code)]
+struct Struct;
+trait Trait {}
+impl Trait for Struct {}
+impl Trait for u32 {}
+
+fn foo() -> impl Trait { Struct }
+//~^ ERROR E0746
+
+fn bar() -> impl Trait { //~ ERROR E0746
+    if true {
+        return 0;
+    }
+    42
+}
+
+fn main() {}

src/test/ui/error-codes/E0746.rs

@@ -1,3 +1,5 @@
+// run-rustfix
+#![allow(dead_code)]
 struct Struct;
 trait Trait {}
 impl Trait for Struct {}

src/test/ui/error-codes/E0746.stderr

@@ -1,5 +1,5 @@
 error[E0746]: return type cannot have a bare trait because it must be `Sized`
-  --> $DIR/E0746.rs:6:13
+  --> $DIR/E0746.rs:8:13
    |
 LL | fn foo() -> dyn Trait { Struct }
    |             ^^^^^^^^^ doesn't have a size known at compile-time
@@ -11,7 +11,7 @@ LL | fn foo() -> impl Trait { Struct }
    |             ^^^^^^^^^^
 
 error[E0746]: return type cannot have a bare trait because it must be `Sized`
-  --> $DIR/E0746.rs:9:13
+  --> $DIR/E0746.rs:11:13
    |
 LL | fn bar() -> dyn Trait {
    |             ^^^^^^^^^ doesn't have a size known at compile-time