False positive `significant_drop_in_scrutinee` if matching on a `MutexGuard` even if it is still used afterwards

[sdroege]

Summary

See reproducer below

Lint Name

significant_drop_in_scrutinee

Reproducer

I tried this code:

fn main() {
    let x = std::sync::Mutex::new(vec![1, 2, 3]);
    let x_guard = x.lock().unwrap();
    match x_guard[0] {
        1 => println!("1!"),
        x => println!("{x}"),
    }
    drop(x_guard); // Some "usage"
}

I saw this happen:

warning: temporary with significant drop in match scrutinee
 --> src/main.rs:4:11
  |
4 |     match x_guard[0] {
  |           ^^^^^^^^^^
  |
  = note: `#[warn(clippy::significant_drop_in_scrutinee)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#significant_drop_in_scrutinee
help: try moving the temporary above the match
  |
4 ~     let value = x_guard[0];
5 ~     match value {
  |

I expected to see this happen: No warning

Version

rustc 1.63.0-beta.2 (6c1f14289 2022-06-28)
binary: rustc
commit-hash: 6c1f1428998a08864495bb599840a5997ec175f4
commit-date: 2022-06-28
host: x86_64-unknown-linux-gnu
release: 1.63.0-beta.2
LLVM version: 14.0.5

Additional Labels

@rustbot label +I-false-positive

[sdroege]

Arguably even without the later usage this shouldn't produce a warning because the MutexGuard is not only temporary during the match but outlives it.

[fxdave]

Here it is another example:

let mut db = db.lock().unwrap();
match db.remove(&path.0) {
    Ok(removed) => HttpResponse::Ok().json(DeleteSuccessResponse { removed }),
    Err(error) => HttpResponse::InternalServerError().json(GenericError {
        error: error.to_string(),
    }),
}

However, the fix was easy in this case:

let mut db = db.lock().unwrap();
+ let result = db.remove(&path.0);
+ match result {
- match db.remove(&path.0) {
    Ok(removed) => HttpResponse::Ok().json(DeleteSuccessResponse { removed }),
    Err(error) => HttpResponse::InternalServerError().json(GenericError {
        error: error.to_string(),
    }),
}