Closed
Description
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