"Cycle computing supertraits" error could be more helpful

[joshlf]

Consider the following code:

trait Foo<F> {}

trait Bar: Foo<Self::F> {
    type F;
}

Trying to compile it, we get the following error:

error[E0391]: cycle detected when computing the supertraits of `Bar`
 --> src/lib.rs:3:1
  |
3 | trait Bar: Foo<Self::F> {
  | ^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: ...which again requires computing the supertraits of `Bar`, completing the cycle
note: cycle used when collecting item types in top-level module
 --> src/lib.rs:3:1
  |
3 | trait Bar: Foo<Self::F> {
  | ^^^^^^^^^^^^^^^^^^^^^^^

To most programmers, this just looks like Rust won't let you have a trait bound which depends on an associated type in the trait being defined. That's what I thought for a long time. But that's not actually true - what's actually going on is that in order to figure out which F is being referred to in Self::F, the supertraits need to be computed. If we help the compiler out by disambiguating which F we mean, the error goes away:

trait Foo<F> {}

trait Bar: Foo<<Self as Bar>::F> {
    type F;
}

I suggest that we improve the error message in this case to suggest disambiguating any associated types in trait bounds in order to steer users towards this solution.

[samsartor]

@rustbot claim

[joshlf]

I would just like to say that I just got bitten by this again and, even though I ran into this just last week, I initially forgot and thought that I was screwed until I remembered this. We definitely need a better error message :)

[samsartor]

I'm on it!

[JohnTitor]

Triage: I'm going to release assignment due to inactivity.
@samsartor If you're still interested in this, feel free to re-claim.
@rustbot release-assignment

[samsartor]

Sorry about that. I worked on it a bit but then totally forgot I even had it assigned! Someone else should give it a shot.

[naiveai]

I'm new to the compiler, and I don't fully know whether this is a good issue to jump in on, but I find it interesting so I'll give it a shot. I'll set a reminder to unassign myself if I can't figure it out so I don't forget about it.

@rustbot claim