Regression in Beta/Nightly: implementation of Trait is not general enough

[msrd0]

Code

I tried this code:

https://github.com/gotham-rs/gotham/tree/3da82b63e36db81f73f0d88b7610a3979c62289a/examples/handlers/simple_async_handlers_await

I expected to see this happen: Code compiles (as it does on stable: https://github.com/gotham-rs/gotham/actions/runs/3873554934/jobs/6603712754)

Instead, this happened: Code fails to compile (on beta: https://github.com/gotham-rs/gotham/actions/runs/3873554934/jobs/6603712995 and nightly: https://github.com/gotham-rs/gotham/actions/runs/3873554934/jobs/6603713263)

Version it worked on

It most recently worked on: 1.66.0 (69f9c33 2022-12-12)

Version with regression

1.67.0-beta.6 (51b0345 2022-12-31) and rustc 1.68.0-nightly (cc47b06 2023-01-08)

@rustbot modify labels: +regression-from-stable-to-beta

[albertlarsan68]

searched nightlies: from nightly-2022-10-29 to nightly-2022-12-10
regressed nightly: nightly-2022-12-10
searched commit range: 7632db0...dfe3fe7
regressed commit: 7701a7e (#105456)

bisected with cargo-bisect-rustc v0.6.5

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --access=github --start=1.66.0 --end=2022-12-10 -- build -p=gotham_examples_handlers_simple_async_handlers_await

@rustbot label +regression-from-stable-to-beta -regression-untriaged

[lqd]

@albertlarsan68 The rollup can itself be checked with these artifacts. I'm pretty sure it's going to be #105255 again.

[albertlarsan68]

• Remove UnsafetyState #105441
• Fix warning when libcore is compiled with no_fp_fmt_parse #105434
• CI: add missing line continuation marker #105433
• Use Symbol for the crate name instead of String/str #105423
• Add help for #![feature(impl_trait_in_fn_trait_return)] #105408
• Stop passing -export-dynamic to wasm-ld. #105405
• make retagging work even with 'unstable' places #105317
• Make nested RPIT inherit the parent opaque's generics. #105255
• kmc-solid: std::sys code maintenance #105120
• Detect long types in E0308 and write them to disk #104922

[albertlarsan68]

It is in fact #105255 that makes this error, cc @cjgillot @compiler-errors

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-high T-compiler

[apiraino]

Can this be closed since #106759 or it needs more work?

(beta backport in #107071)

[wesleywiser]

I think we still would like an mcve and regression test to be added. Lowering priority since this was reverted.

[msrd0]

Here's a smaller version that fails to compile on nightly-2023-01-08 but compiles on rustc 1.67.1 (d5a82bbd2 2023-02-07) (Arch Linux rust 1:1.67.1-1):

mod gotham {
	use std::future::Future;

	pub struct State;

	pub trait IntoResponse {}

	pub struct Response;

	impl IntoResponse for Response {}

	mod private {
		use super::*;

		pub trait HandlerMarker {}

		pub trait AsyncHandlerFn<'a> {
			type Res;
		}

		impl<'a, Fut, R, F> AsyncHandlerFn<'a> for F
		where
			F: FnOnce(&'a mut State) -> Fut,
			Fut: Future<Output = R> + Send + 'a
		{
			type Res = R;
		}

		impl<F, R> HandlerMarker for F
		where
			R: IntoResponse + 'static,
			for<'a> F: AsyncHandlerFn<'a, Res = R> + Send
		{
		}
	}

	pub fn to_async_borrowing<F>(handler: F)
	where
		F: private::HandlerMarker
	{
		_ = handler;
	}
}

use gotham::*;

async fn sleep_handler(_: &mut State) -> impl IntoResponse {
	Response
}

fn main() {
	to_async_borrowing(sleep_handler);
}

[Mark-Simulacrum]

Untagging as a regression since I believe we expect this to already be fixed on beta + nightly as of #106759 (and its backport).

[msrd0]

I managed to get the code that triggers the regression even shorter:

use std::future::Future;

trait AsyncCallback<'a> {
    type Out;
}

impl<'a, Fut, T, F> AsyncCallback<'a> for F
where
    F: FnOnce(&'a mut ()) -> Fut,
    Fut: Future<Output = T> + Send + 'a,
{
    type Out = T;
}

trait CallbackMarker {}
impl<F, T> CallbackMarker for F
where
    T: 'static,
    for<'a> F: AsyncCallback<'a, Out = T> + Send,
{
}

fn do_sth<F: CallbackMarker>(_: F) {}

async fn callback(_: &mut ()) -> impl Send {}

fn main() {
    do_sth(callback);
}

Do you want me to create a PR that adds a test for this code? If so, is there some guide on how to add regression tests?

[kpreid]

Triage: this has got a MCVE.
@rustbot label -E-needs-mcve

Do you want me to create a PR that adds a test for this code? If so, is there some guide on how to add regression tests?

@msrd0 I'm not an experienced compiler dev but I believe you simply add a test case file to tests/ui/ like in this commit for example.