Add test for type Alias = dyn Trait in return type

Author: estebank

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

@@ -0,0 +1,8 @@
+type T = dyn core::fmt::Debug;
+ 
+fn f() -> T { loop {} }
+//~^ ERROR return type cannot be a trait object without pointer indirection
+//~| HELP 
+//~| HELP 
+
+fn main() {}

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

@@ -0,0 +1,18 @@
+error[E0746]: return type cannot be a trait object without pointer indirection
+  --> $DIR/dyn-trait-type-alias-return-type.rs:3:11
+   |
+LL | fn f() -> T { loop {} }
+   |           ^ doesn't have a size known at compile-time
+   |
+help: consider returning an `impl Trait` instead of a `dyn Trait`
+   |
+LL | fn f() -> impl T { loop {} }
+   |           ++++
+help: alternatively, box the return type, and wrap all of the returned values in `Box::new`
+   |
+LL | fn f() -> Box<dyn T> { Box::new(loop {}) }
+   |           +++++++  +   +++++++++       +
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0746`.