question_mark suggesting incorrect code when type is behind Deref (missing parentheses)

[SirMangler]

Summary

The question_mark lint suggests incorrect code when the return type is behind Deref. I first noticed this when using parking_lot::Mutex<Option<_>>::lock() which returns a MutexGuard<'_, RawMutex, Option<_>>. Specifically, it forgets to include the parentheses to dereference the type before applying the question mark operator.

Here is a minimal reproduction in rust playground:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=1d714214fc7983d0785b99ffd7d3eab4

Thanks!

Reproducer

I tried this code:

use parking_lot::Mutex;

fn main() {
    let a = Mutex::new(Some(0));
    println!("{:?}", format_a(a));
}

fn format_a(a: Mutex<Option<u32>>) -> Option<String> {
    let Some(a) = *a.lock() else {
        return None;
    };
    
    Some(format!("{}", a))
}

I expected to see this happen:

warning: this `let...else` may be rewritten with the `?` operator
  --> src/main.rs:9:5
   |
9  | /     let Some(a) = *a.lock() else {
10 | |         return None;
11 | |     };
   | |______^ help: replace it with: `let a = (*a.lock())?;`

Instead, this happened:

warning: this `let...else` may be rewritten with the `?` operator
  --> src/main.rs:9:5
   |
9  | /     let Some(a) = *a.lock() else {
10 | |         return None;
11 | |     };
   | |______^ help: replace it with: `let a = *a.lock()?;`

error[E0277]: the `?` operator can only be applied to values that implement `Try`
 --> src/main.rs:9:14
  |
9 |     let a = *a.lock()?;
  |              ^^^^^^^^^ the `?` operator cannot be applied to type `parking_lot::lock_api::MutexGuard<'_, parking_lot::RawMutex, Option<u32>>`
  |
  = help: the trait `Try` is not implemented for `parking_lot::lock_api::MutexGuard<'_, parking_lot::RawMutex, Option<u32>>`

Version

rustc 1.87.0-nightly (ce36a966c 2025-02-17)
binary: rustc
commit-hash: ce36a966c79e109dabeef7a47fe68e5294c6d71e
commit-date: 2025-02-17
host: x86_64-unknown-linux-gnu
release: 1.87.0-nightly
LLVM version: 20.1.0

Additional Labels

No response

[alex-semenyuk]

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