Don't require associated types with Self: Sized bounds in `dyn Trait` objects
Trait objects require *all* associated types to be specified, even if the associated type has an explicit `where Self: Sized` bound. The following snippet does not compile on master, but does with this PR.
```rust
fn _assert_is_object_safe(_: &dyn Foo) {}
pub trait Foo {
type Bar where Self: Sized;
}
```
In contrast, if a `Self: Sized` bound is added to a method, the methodjust isn't callable on trait objects, but the trait can be made object safe just fine.
```rust
fn _assert_is_object_safe(_: &dyn Foo) {}
pub trait Foo {
fn foo() where Self: Sized;
}
```
This PR closes this inconsistency (though it still exists for associated constants).
Additionally this PR adds a new lint that informs users they can remove associated type bounds from their trait objects if those associated type bounds have a `where Self: Sized` bound, and are thus useless.
r? `@compiler-errors`
99f7d368c0