warn(must_not_suspend) started being raised incorrectly when moving from stable to nightly

[finnbear]

Code

I tried this code:

use std::cell::RefCell;

fn main() {
    
}

async fn bug(ref_cell: &RefCell<Option<bool>>) {
    let borrow = ref_cell.borrow();
    if let Some(_) = borrow.as_ref() {
        drop(borrow);
        
        async {
            
        }.await;
    }
}

Note: The if let Some(_) = borrow.as_ref() { } block is not necessary to reproduce (only its contents are needed), but this is how my actual code is structured, so I want to ensure it is considered

Playground link: here

I expected to see this compile without errors.

Instead, this happened:

warning: `Ref` held across a suspend point, but should not be
  --> src/main.rs:9:9
   |
9  |       let borrow = ref_cell.borrow();
   |           ^^^^^^
...
13 | /         async {
14 | |             
15 | |         }.await;
   | |_______________- the value is held across this suspend point
   |
   = note: `#[warn(must_not_suspend)]` on by default
note: holding a Ref across suspend points can cause BorrowErrors
  --> src/main.rs:9:9
   |
9  |     let borrow = ref_cell.borrow();
   |         ^^^^^^
help: consider using a block (`{ ... }`) to shrink the value's scope, ending before the suspend point
  --> src/main.rs:9:9
   |
9  |     let borrow = ref_cell.borrow();
   |         ^^^^^^

Version it worked on

It most recently worked on: 1.56.0 stable

Version with regression

rustc --version --verbose:

1.58.0-nightly 2021-10-29 e99963c554e4d12010c2

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

[apiraino]

Bisection seems to point to commit 598d89b

searched nightlies: from nightly-2021-01-01 to nightly-2021-10-29
regressed nightly: nightly-2021-10-02
searched commits: from aa7aca3 to c02371c
regressed commit: 598d89b

bisected with cargo-bisect-rustc v0.6.0

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

cargo bisect-rustc --start=2021-01-01 --end=2021-10-29 --script=./script.sh 

[Mark-Simulacrum]

Regression is actually in 1.57 based on that bisection, but we're planning to switch the lint to allow by default for now regardless.

[camelid]

This issue is marked with the 1.58.0 milestone, but beta is currently 1.57.0, so should the milestone be updated?

[Mark-Simulacrum]

Yeah, I think I misclicked.

[camelid]

#89826 has landed on master now (not beta yet).

[pnkfelix]

PR #89826 is in beta now. Closing as fixed.