Wishlist: compiler should give hint when trying to use Box<Trait> as Trait

If I have a function:

fn foo<F: Foo>(f: F) { ... }

and I pass it Box<Foo>, this fails because Box<Foo> doesn't actually implement the Foo trait. Ideally it should, but as a stop-gap the compiler should hint at that problem, because it wasn't obvious to me that a boxed trait object doesn't implement that trait.

1. Perhaps this would be better as a clippy lint.

   Clar Fon at 2017-06-01 18:36:33

2. Well, its going to be reporting an error anyway; I'm just suggesting that it explicitly call out that that Box<Trait> doesn't implement Trait.

   I haven't used clippy much, because it's still unstable only (right?) - but I assume it points out issues on code which is correct enough to compile, but are not necessarily ideal stylistically.

   Jeremy Fitzhardinge at 2017-06-01 18:39:42

3. Triage: Current output:

   error[E0277]: the trait bound `std::boxed::Box<dyn Foo>: Foo` is not satisfied
     --> src/main.rs:11:5
      |
   11 |     foo(x);
      |     ^^^ the trait `Foo` is not implemented for `std::boxed::Box<dyn Foo>`
      |
   note: required by `foo`
     --> src/main.rs:1:1
      |
   1  | fn foo<F: Foo>(f: F) { panic!() }
      | ^^^^^^^^^^^^^^^^^^^^
   

   ---

   Update:

   error[E0277]: the trait bound `std::boxed::Box<dyn Foo>: Foo` is not satisfied
     --> src/main.rs:11:9
      |
   1  | fn foo<F: Foo>(f: F) { panic!() }
      |    ---    --- required by this bound in `foo`
   ...
   11 |     foo(x);
      |         ^ the trait `Foo` is not implemented for `std::boxed::Box<dyn Foo>`
   

   ---

   Update:

   error[E0277]: the trait bound `Box<dyn Foo>: Foo` is not satisfied
     --> src/main.rs:11:9
      |
   11 |     foo(x);
      |     --- ^ the trait `Foo` is not implemented for `Box<dyn Foo>`
      |     |
      |     required by a bound introduced by this call
      |
      = help: the trait `Foo` is implemented for `Bar`
   note: required by a bound in `foo`
     --> src/main.rs:1:11
      |
   1  | fn foo<F: Foo>(f: F) { panic!() }
      |           ^^^ required by this bound in `foo`
   

   Esteban Kuber at 2019-09-03 00:56:12