Nightly compiler bug: "lifetime mismatch"

[annmarie-switzer]

Nightly 2022-02-13 seems to have introduced a bug. Code that compiles properly on stable and nightly-2022-02-09 does not compile with nightly-2022-02-13:

$ cargo +nightly run run
   Compiling detect_core v0.0.0 (C:\git\datadetect\core)
error[E0308]: mismatched types
   --> core\src\analysis.rs:217:17
    |
217 |                   tokio::spawn(async move {
    |                   ^^^^^^^^^^^^ lifetime mismatch       
...
395 |       reqs.then(|req| async move {
    |  ________________________________-
396 | |         let data = serde_json::to_vec(&req).unwrap();
397 | |         let subj = format!("HASH_REQ.{}", &req.connector);
398 | |
...   |
447 | |         vec![header, doc]
448 | |     })
    | |     -
    | |     |
    | |_____the expected `async` block
    |       the found `async` block
    |
   ::: core\src\reporting\scan.rs:18:59
    |
18  |   pub async fn send_report(task_type: &str, stat: ScanStat) {
    |                                                             -
    |                                                             |
    |                                                             one of the expected opaque types
    |                                                             one of the found opaque types
    |
   ::: C:\Users\aswitzer\.cargo\registry\src\github.com-1ecc6299db9ec823\nats-0.16.0\src\asynk.rs:158:74
    |
158 |       pub async fn request(&self, subject: &str, msg: impl AsRef<[u8]>) -> io::Result<Message> {
    |                                                                            -------------------
    |                                                                            |
    |                                                                            one of the expected opaque types
    |                                                                            one of the found opaque types
    |
   ::: C:\Users\aswitzer\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library\core\src\future\mod.rs:65:43
    |
65  |   pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
    |                                             -------------------------------
    |                                             |
    |                                             one of the expected opaque types
    |                                             one of the found opaque types
    |
note: while checking the return type of the `async fn`
   --> core\src\lib.rs:101:24
    |
101 | pub async fn nats() -> &'static nats::asynk::Connection {
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ checked the `Output` of this `async fn`, one of the expected opaque types
note: while checking the return type of the `async fn`
    |
101 | pub async fn nats() -> &'static nats::asynk::Connection {
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ checked the `Output` of this `async fn`, one of the found opaque types
    = note: expected opaque type `impl futures::Future<Output = [async output]>`
               found opaque type `impl futures::Future<Output = [async output]>`
note: the lifetime requirement is introduced here
   --> C:\Users\aswitzer\.cargo\registry\src\github.com-1ecc6299db9ec823\tokio-1.15.0\src\task\spawn.rs:127:21
    |
127 |         T: Future + Send + 'static,
    |                     ^^^^

For more information about this error, try `rustc --explain E0308`.
error: could not compile `detect_core` due to previous error

[ehuss]

Thanks for the report! Is this a public project you can share? Or perhaps you can put together a minimum reproduction? And perhaps you can narrow down to one specific nightly where it starts happening?

[andrewbanchich]

Seems like this may be related to this PR which was part of Tokio 1.16, although I'm not sure why it the same code would compile on stable and not nightly.

[annmarie-switzer]

@ehuss It is broken on nightly-2022-02-13 and nightly-2022-02-15, but works on nightly-2022-02-09. I can not share the repo but I will try to put together reproduction soon.

[annmarie-switzer]

Sorry I misunderstood. We tested on nightly 2022-02-12 and it compiles. So something broke on the release on the 13th.

[annmarie-switzer]

Sorry again - I thought I was on nightly-2022-02-13 when in reality I was not. It actually appears to be the release on the 14th that broke things.

9th - works
10th - works
11th - works
12th - works
13th - works
14th - BROKEN
15th - BROKEN
16th - BROKEN

[ehuss]

I think it is likely caused by #91403, cc @cjgillot

[oli-obk]

Considering this regression and the perf regression, we should probably revert and try again in a new PR

[apiraino]

Assigning priority as discussed in the Zulip thread of the Prioritization Working Group.

@rustbot label -I-prioritize +P-high

[pnkfelix]

@rustbot assign @cjgillot

to own reverting PR #91403 and ensuring that the revert is part of the soon-to-be-cut beta.

[Dirbaio]

I have a standalone repro of something that builds on nightly-2022-02-13, fails on nightly-2022-02-14 as well. I'm not sure if it's the same bug. playground

use core::future::Future;

trait Callback<'a> {}

impl<'a, T, Fut> Callback<'a> for T
where
    T: FnOnce(&'a mut ()) -> Fut,
    Fut: Future<Output = ()> + 'a,
{
}

async fn transaction<Cb>(f: Cb)
where
    Cb: for<'a> Callback<'a>,
{
}

async fn my_callback(bus: &mut ()) {}

async fn boom() {
    transaction(my_callback).await;
}

error: implementation of `Callback` is not general enough
  [--> src/lib.rs:21:5
](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#)   |
21 |     transaction(my_callback).await;
   |     ^^^^^^^^^^^ implementation of `Callback` is not general enough
   |
   = note: `fn(&mut ()) -> impl Future<Output = ()> {my_callback::<'_>}` must implement `Callback<'0>`, for any lifetime `'0`...
   = note: ...but it actually implements `Callback<'1>`, for some specific lifetime `'1`