Implementation of trait "not general enough" in nightly, works fine on stable

[ejmount]

edit/summary by @lcnr

This is a regression caused by the unsoundness fix in #124336 which causes uses of impls to now have to prove the outlives bounds of the trait. In this case, this requires proving the 'static bound of Role whenever we're using an impl.

We expected that this only causes unintended breakage due to higher ranked goals. However, this can also trigger #110338. This is a general issue with the way generator interiors are handled. We deeply care about fixing this issue and doing so should cause this pattern to compile again.

Until then, we believe the fallout to be minor enough for this to be acceptable. For this to trigger we need to

• prove an auto trait bound for a generator
• the generator contains a field for which proving the auto trait bound requires using an impl of a trait with a region bound
• this either requires an impl like impl<T: Role> Send for MyType<T> or a field whose normalization requires proving the trait

---

Code

I tried this (reduced) code:

use tokio::sync::mpsc::{unbounded_channel,UnboundedSender};

pub trait Role: 'static + Sync + Send {
    type Return : Send;
}

pub trait BasicRole: 'static + Send + Sync {}
impl Role for dyn BasicRole + '_ {
    type Return = ();
}

pub struct ReturnEnvelope<R: Role + ?Sized> {
	pub return_path: Option<R::Return>,
}

pub fn start() {
    let (_basic_role_input, mut basic_role_output): (UnboundedSender<ReturnEnvelope<dyn BasicRole>>, _) = unbounded_channel();

    let event_loop = async move {
        let _ = basic_role_output.recv().await;
    };
    let _ = ::tokio::task::spawn(event_loop);
}

(playground)

I expected to see this happen: successful compile.

Instead, this happened: I received this error:

error: implementation of `Role` is not general enough
  --> src/lib.rs:22:13
   |
22 |     let _ = ::tokio::task::spawn(event_loop);
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Role` is not general enough
   |
   = note: `Role` would have to be implemented for the type `(dyn BasicRole + '0)`, for any lifetime `'0`...
   = note: ...but `Role` is actually implemented for the type `(dyn BasicRole + '1)`, for some specific lifetime `'1`

This can also be seen on the playground configured to use nightly.

I am aware that handling this construct correctly seems to be an outstanding issue, I'm specifically reporting the inconsistency that it does work on stable and not nightly.

Version it worked on

It most recently worked on: 1.81.0 stable (most recent)

Version with regression

rustc --version --verbose:

rustc 1.83.0-nightly (eb4e23467 2024-10-09)
binary: rustc
commit-hash: eb4e2346748e1760f74fcaa27b42431e0b95f8f3
commit-date: 2024-10-09
host: x86_64-pc-windows-msvc
release: 1.83.0-nightly
LLVM version: 19.1.1

However, this has been an issue since at least the 2024-08-17 nightly (feeba19)

@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged

[lqd]

cc @compiler-errors (if bisection is accurate)

[compiler-errors]

That bisection was wrong. It's likely due to #124336, and the correct fix would be to change:

impl Role for dyn BasicRole + '_ {
    type Return = ();
}

to:

impl Role for dyn BasicRole + 'static {
    type Return = ();
}

Because BasicRole requires that the self type outlives 'static.

[compiler-errors]

Actually, wait, this is not fixed by that change. This may be a knock-on issue related to #110338 :(

[ejmount]

Just to clarify, the older nightly version wasn't from a bisect, it was just what I was using when I first encountered this problem

[compiler-errors]

The only fix I expect to maybe work is if you remove the 'static supertrait bound from Role and BasicRole. I don't know if that's possible given your unminimized code.

[ejmount]

The only fix I expect to maybe work is if you remove the 'static supertrait bound from Role and BasicRole. I don't know if that's possible given your unminimized code.

Unfortunately that doesn't seem to be an option because I want to have Arc<dyn BasicRole> objects, which seems to imply that BasicRole must implement Any and therefore must be 'static. (Testing this immediately told me it was required, but the compiler never articulated where the 'static bound was coming from)

[bjorn3]

If you want to avoid the 'static bound, use Arc<dyn BasicRole + '_>. If a trait object is used without explicit lifetime bound, it is inferred to be 'static.

[ejmount]

I checked more carefully just now, and removing the 'static constraint from Role actually broke a number of functions taking a generic Arc<R: Role> and wanting to spawn tasks with it. Fixing this by putting the constraint into the signature so it took a Arc<R: Role + 'static> meant the code now built, including some unit tests that created Arc<dyn BasicRole> values (without having to specify Arc<dyn + '_>), so it seems that my project works for now.

[lqd]

This actually bisects to #124336. It fixed soundness issues, so some breakage was expected, but also some unintended ones that we didn't encounter on crater. I'll let Michael chime in on whether this is the latter.