Mixing async functions and &mut leads to overflow evaluating requirement

[agrif]

Code:

#![feature(async_await, await_macro, futures_api)]

// Consider this a stand-in for tokio::io::AsyncRead
trait Foo { }
impl Foo for () { }
impl<'a, T> Foo for &'a mut T where T: Foo { }

// Consider this a stand-in for tokio::io::read
async fn foo_async<T>(_v: T) -> u8 where T: Foo {
    0
}

// this should just act exactly like foo_async...
async fn bad<T>(v: T) -> u8 where T: Foo {
    await!(foo_async(v))
}

// let's see if it is!
async fn async_main() {
    let mut v = ();
    
    // compiler error:
    let _ = await!(bad(&mut v));
    // totally fine:
    let _ = await!(foo_async(&mut v));
    // also totally fine:
    let _ = await!(bad(v));
}

fn main() {
    let _ = async_main();
}

Playground.

This fails to compile:

Compiling playground v0.0.1 (/playground)
error[E0275]: overflow evaluating the requirement `[static generator@src/main.rs:9:49: 11:2 {}]: std::marker::Freeze`
  |
  = help: consider adding a `#![recursion_limit="128"]` attribute to your crate
  = note: required because of the requirements on the impl of `std::marker::Freeze` for `[static generator@src/main.rs:9:49: 11:2 {}]`
[ ... line repeated many, many times ... ]
  = note: required because it appears within the type `impl std::future::Future`
  = note: required because it appears within the type `impl std::future::Future`
  = note: required because it appears within the type `{impl std::future::Future, ()}`
  = note: required because it appears within the type `[static generator@src/main.rs:14:42: 16:2 v:&mut () {impl std::future::Future, ()}]`
  = note: required because it appears within the type `std::future::GenFuture<[static generator@src/main.rs:14:42: 16:2 v:&mut () {impl std::future::Future, ()}]>`
  = note: required because it appears within the type `impl std::future::Future`
  = note: required because it appears within the type `impl std::future::Future`
  = note: required because it appears within the type `for<'r, 's> {(), impl std::future::Future, impl std::future::Future, impl std::future::Future}`
  = note: required because it appears within the type `[static generator@src/main.rs:19:23: 28:2 for<'r, 's> {(), impl std::future::Future, impl std::future::Future, impl std::future::Future}]`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0275`.
error: Could not compile `playground`.

This might be related to #50674, except that the code that triggers it seems more likely to be valid. I ran into this bug when trying to use tokio to read data and pass it off to serde. At the very least, the error message is extremely confusing.

Slightly modifying this code will occasionally trigger another compiler error (something about DeBruijn indices) but I have lost the exact code that triggers it.

[paracat]

i have produced the same error in a project:

https://github.com/paracat/reflow/commit/d25aa1f4364c70f512d58cb6946d56faece5b5d8

though i haven't extracted a minimal snippet, the cause of the problem is in the latest commit that only changes several lines

i have also found the code that produces the DeBruijn error. It's the commit before this one. It seems whether ? is used for error handling matters

apart from the two last commits that produce these two different errors, previous commits compile fine

i believe this is related to #53989

[nikomatsakis]

Maybe related to #55809 ?

[eminence]

@nikomatsakis that link is just back to this same issue. What issue did you actually mean to link?

[nikomatsakis]

@eminence good point, I don't remember anymore =)

[nikomatsakis]

Marking as blocking -- we should be trying to figure out what is happening here, at least, and deciding whether to block on it.

[nikomatsakis]

Assigning to myself to try and do some investigation.

[davidtwco]

This code compiles successfully when I check locally against nightly and a build of master, I've submitted #59156 with a regression test.