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

[jsgf]

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.

[clarfonthey]

Perhaps this would be better as a clippy lint.

[jsgf]

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.

[estebank]

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`

[estebank]

Current output:

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`
  --> src/main.rs:7:1
   |
 7 | impl Foo 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`