E0277: suggest `?Sized` to relax implicit `Sized` bound on associated types

[tlyu]

Given the following code: (playground)

trait TraitWithAType {
    type Item;
}
trait Trait {}
struct A {}
impl TraitWithAType for A {
    type Item = dyn Trait;
}

The current output is:

   Compiling playground v0.0.1 (/playground)
error[E0277]: the size for values of type `(dyn Trait + 'static)` cannot be known at compilation time
 --> src/lib.rs:7:5
  |
2 |     type Item;
  |     ---------- required by this bound in `TraitWithAType::Item`
...
7 |     type Item = dyn Trait;
  |     ^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `(dyn Trait + 'static)`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground`

To learn more, run the command again with --verbose.

Ideally the output should look like:

  |
2 |     type Item;
  |     ---------- required by this bound in `TraitWithAType::Item`
...
7 |     type Item = dyn Trait;
  |     ^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `(dyn Trait + 'static)`
help: consider relaxing the implicit `Sized` restriction
  |
2 |     type Item: ?Sized
  |              ^^^^^^^^

A similar error message with an unsized generic type argument does mention the implicit Sized bound. (playground) Unfortunately, it also doesn't explain the implicit bound up front in the "required by this bound" text. (It mentions it when suggesting to relax the implicit bound, but I think it would be better to mention it up front as well.)

Both error messages could benefit from changing the "required by this bound" text to "required by an implicit Sized bound".

[tlyu]

@rustbot label +A-associated-items +A-traits +A-typesystem +D-papercut +D-terse

[tlyu]

I'm working on a pull request for this issue.

@rustbot claim

[estebank]

@tlyu Oh! Very sorry, if you spent too much time on this already, but I have done some work around this already in #85947. If you want, you can take over it and bring it to completion, there are some cases, like #85799 (comment) that still need to be addressed and I haven't yet figured out. Alternatively, if you can find a way to accomplish the same result without touching the TraitPredicate and instead deal only with a new ObligationCauseCode, that would be preferable to my approach.

[tlyu]

I've pushed what I have so far to https://github.com/tlyu/rust/tree/suggest-unsized , sorry if it conflicts with stuff you've already done. If you'd like to take a look at it, that would be great. I'll take a look at what you have as well.

There are some issues to be worked out with conflicting suggestions for associated types for ?Sized vs the existing suggestion for adding an explicit Sized to a trait's bounds, in test cases such as type Foo = Self; within a trait declaration.