significant_drop_in_scrutinee fires when matching on async function result

[jakubdabek]

Summary

When calling an async function that returns a MutexGuard the lint fires.

Lint Name

significant_drop_in_scrutinee

Reproducer

I tried this code:

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=9193048ab64be887f2aa4cb7af02e490

use std::sync::Mutex;
use std::sync::MutexGuard;

async fn foo_async(mutex: &Mutex<i32>) -> Option<MutexGuard<'_, i32>> {
    Some(mutex.lock().unwrap())
}

#[warn(clippy::significant_drop_in_scrutinee)]
async fn bar_async() {
    let mutex = Mutex::new(1);
    let locked = match foo_async(&mutex).await {
        Some(locked) => locked,
        None => panic!("unavailable"),
    };
    
    println!("{}", *locked);
}

I saw this happen:

warning: temporary with significant `Drop` in `match` scrutinee will live until the end of the `match` expression
  --> src/main.rs:12:42
   |
12 |     let locked = match foo_async(&mutex).await {
   |                                          ^^^^^
...
15 |     };
   |      - temporary lives until here
   |
   = note: this might lead to deadlocks or other unexpected behavior
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#significant_drop_in_scrutinee
note: the lint level is defined here
  --> src/main.rs:9:8
   |
9  | #[warn(clippy::significant_drop_in_scrutinee)]
   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: try moving the temporary above the match
   |
12 ~     let locked = await;
13 ~     match foo_async(&mutex).() {
   |

I expected to see this happen:
No lint emitted, we're destructuring a value with the MutexGuard itself and assigning it.
At first I thought this was a problem with the code that has only that, i.e. matching on some fn foo(mutex: &Mutex<T>) -> Option<MutexGuard<'_, T>>, but that's fine.

Secondly, I think this has something to do with the await desugaring, it also breaks the suggestion:

help: try moving the temporary above the match
   |
12 ~     let locked = await;
13 ~     match foo_async(&mutex).() {
   |

It even fires without the Option, but that's kinda degenerate code:

    let locked = match foo(&mutex).await {
        locked => locked,
    };

Version

Clippy 0.1.85 (2024-12-19 9e136a30a9) via The Playground

Additional Labels

@rustbot label +I-suggestion-causes-error