while_let_loop fails to see that temporary value needs to be dropped

[tijsvd]

Summary

There may be important temporary values in the if/while expression, like a mutex lock guard. In case of an if-statement-with-break, these are dropped after the statement. In case of a while-let loop, these are retained for the whole loop body.

I would expect that the lint might recognise this kind of thing and not warn. Seems hard though. But blindly "fixing" can lead to bad errors.

Lint Name

while_let_loop

Reproducer

I tried this code:

use std::sync::{Arc, Mutex};

fn handle_items(q: Arc<Mutex<Vec<i32>>>) {
    loop {
        let item = match q.lock().unwrap().pop() {
            Some(item) => item,
            None => break,
        };
        // lock guard dropped here
        println!("process: {}", item);
        if item > 10 {
            // deadlocks here if while-let
            q.lock().unwrap().insert(0, item - 1);
        }
    }
}

fn main() {
    let q = Arc::new(Mutex::new(vec![1, 2, 3, 12, 7]));
    handle_items(q);
}

I saw this happen:

warning: this loop could be written as a `while let` loop
  --> src/main.rs:4:2
   |
4  | /     loop {
5  | |         let item = match q.lock().unwrap().pop() {
6  | |             Some(item) => item,
7  | |             None => break,
...  |
14 | |         }
15 | |     }
   | |_____^ help: try: `while let Some(item) = q.lock().unwrap().pop() { .. }`
   |
   = note: `#[warn(clippy::while_let_loop)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#while_let_loop

I expected to see this happen:

Nothing. The suggested code will deadlock.

Version

rustc 1.62.0 (a8314ef7d 2022-06-27)
binary: rustc
commit-hash: a8314ef7d0ec7b75c336af2c9857bfaf43002bfc
commit-date: 2022-06-27
host: x86_64-unknown-linux-gnu
release: 1.62.0
LLVM version: 14.0.5

Additional Labels

No response

[Jarcho]

This doesn't lint on the latest nightly.

[tijsvd]

This doesn't lint on the latest nightly.

Right. If that's deliberate and not accidental, then I guess this can be closed.

[Jarcho]

Was changed in #8666.

[tijsvd]

Thank you, closing.