Unclear TAIT error on auto trait solving

[oli-obk]

I am trying to use TAIT to poll a future constructed by an async fn, but running into this error, I am unsure where the issue exactly is, would appreciate any guidance
Example Code: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=b2a9d6f2493a79743f66e7852aeca81a

Error

error[[E0391]](https://doc.rust-lang.org/nightly/error-index.html#E0391): cycle detected when computing type of `BarFuture::{opaque#0}`
  --> src/lib.rs:7:18
   |
7  | type BarFuture = impl Future<Output = usize> + Send + Sync + 'static;
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: ...which requires type-checking `<impl at src/lib.rs:17:1: 17:11>::bar_project`...
  --> src/lib.rs:23:40
   |
23 |         let bar: &mut BarFuture = &mut self.bar;
   |                                        ^^^^
   = note: ...which requires evaluating trait selection obligation `core::pin::Pin<&'a mut State>: core::ops::deref::DerefMut`...
   = note: ...which again requires computing type of `BarFuture::{opaque#0}`, completing the cycle
note: cycle used when checking item types in top-level module
  --> src/lib.rs:1:1
   |
1  | / #![feature(type_alias_impl_trait)]
2  | |
3  | | [use std::future::Future;](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=b2a9d6f2493a79743f66e7852aeca81a#)
4  | | [use std::pin::Pin;](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=b2a9d6f2493a79743f66e7852aeca81a#)
...  |
33 | |     }
34 | | }
   | |_^

Originally posted by @dignifiedquire in #86733 (comment)

We should make cycle errors due to opaque types tell the user about the fact that they can control what's in the defining scope and what isn't

[oli-obk]

Unfortunately cycle errors are some of the hardest to debug. Usually cycle errors with TAITs come from auto traits like Send and Sync. In this case, due to the involvement of Pin I am guessing it's about the Unpin auto trait. Basically in order to prove that Pin<&'a mut State>: DerefMut we need to prove that State: Unpin. You can prove this by adding Unpin to the list of traits on your TAIT, but then you'll just run into new errors. These may be clearer to you.

Either way, I'll look into improving these cycle error.

[dignifiedquire]

You can prove this by adding Unpin to the list of traits on your TAIT, but then you'll just run into new errors.

I believe that will not work due to the fact that the futures from async fns are !Unpin.

[oli-obk]

Ah right, you want !Pin. Either way, the way to break the cycle is to change what is in the defining scope: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=d6eab067c3324f59b718b8a29892f02b