query cycle: specialisation cycle detected when building specialization graph of trait

[matthiaskrgr]

Code

#![feature(specialization)]

trait Trait<T> {
    type Assoc;
}

impl<T> Trait<T> for Vec<T> {}

impl Trait<u8> for Vec<u32> {
    type Assoc = u8;
}

impl<T> Trait<T> for String {
    type Assoc = ();
}

impl Trait<<Vec<u8> as Trait<u8>>::Assoc> for String {}

fn main() {}

Current output

warning: the feature `specialization` is incomplete and may not be safe to use and/or cause compiler crashes
 --> 43273EB829A15349E16F5931020795BF70C210EEDA25D591DFCCF621A1677EA0.rs:1:12
  |
1 | #![feature(specialization)]
  |            ^^^^^^^^^^^^^^
  |
  = note: see issue #31844 <https://github.com/rust-lang/rust/issues/31844> for more information
  = help: consider using `min_specialization` instead, which is more stable and complete
  = note: `#[warn(incomplete_features)]` on by default

error[E0391]: cycle detected when building specialization graph of trait `Trait`
 --> 43273EB829A15349E16F5931020795BF70C210EEDA25D591DFCCF621A1677EA0.rs:3:1
  |
3 | trait Trait<T> {
  | ^^^^^^^^^^^^^^
  |
  = note: ...which immediately requires building specialization graph of trait `Trait` again
note: cycle used when coherence checking all impls of trait `Trait`
 --> 43273EB829A15349E16F5931020795BF70C210EEDA25D591DFCCF621A1677EA0.rs:3:1
  |
3 | trait Trait<T> {
  | ^^^^^^^^^^^^^^
  = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

error: aborting due to 1 previous error; 1 warning emitted

For more information about this error, try `rustc --explain E0391`.

Desired output

We should have a proper diagnostic here pointing out where the cyclic problem is

Rationale and extra context

cc #119321

Other cases

No response

Anything else?

No response

[compiler-errors]

specialization is not expected to ever be stabilized, for the record

[matthiaskrgr]

Should we mark it as internal then?