Coherence hint wrong for a blanked implementation bounded by a private trait

[nagisa]

Given code like this:

trait Private {
}

impl<T: Private> Send for T {
}

the diagnostic will read as such:

error[E0119]: conflicting implementations of trait `std::marker::Send` for type `&_`:
 --> src/lib.rs:4:1
  |
4 | impl<T: Private> Send for T {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: conflicting implementation in crate `core`:
          - impl<T> std::marker::Send for &T
            where T: std::marker::Sync, T: ?Sized;
  = note: downstream crates may implement trait `Private` for type `&_`

the last note here is incorrect because the only thing that can implement this trait is the crate which declared the private Private trait.

[fmckeogh]

I'd like to work on this if that's ok? :)

@rustbot claim

[arielb1]

reopening because of #63145 - we need to come up with a fix to this issue that actually makes things clearer.

[donchoi25]

Hi! I'd like to work on this.
@rustbot claim

[nico-abram]

I don't know when this was fixed, but I see the following output on the playground (1.58) https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=f0b6bc2bf8ccf0f3eb3f563f8b3287a1 :

trait Private {
}

unsafe impl<T: Private> Send for T {
}

error[E0210]: type parameter `T` must be used as the type parameter for some local type (e.g., `MyStruct<T>`)
 --> src/lib.rs:4:13
  |
4 | unsafe impl<T: Private> Send for T {
  |             ^ type parameter `T` must be used as the type parameter for some local type
  |
  = note: implementing a foreign trait is only possible if at least one of the types for which it is implemented is local
  = note: only traits defined in the current crate can be implemented for a type parameter

I think this can be closed