Incorrect suggestion for has_significant_drop #10413
Description
Summary
The has_significant_drop lint only (seems to) considers the first usage of a locked value and suggests an incorrect correction of the code.
Lint Name
has_significant_drop
Reproducer
The code comes from https://github.com/Thomasdezeeuw/heph/blob/8be6cfa97d6870cc7e086e63e32608976c6d76f1/inbox/src/lib.rs#L519-L529. The relevant part:
// self.registered_waker: Option<task::Waker>
// self.channel.sender_wakers: Mutex<Vec<task::Waker>>
if let Some(waker) = self.registered_waker.take() {
let mut sender_wakers = self.channel.sender_wakers.lock().unwrap();
let idx = sender_wakers.iter().position(|w| w.will_wake(&waker));
if let Some(idx) = idx {
let waker = sender_wakers.swap_remove(idx);
unlock(sender_wakers);
drop(waker);
}
}The suggestion Clippy makes:
error: temporary with significant `Drop` can be early dropped
--> inbox/src/lib.rs:523:21
|
522 | if let Some(waker) = self.registered_waker.take() {
| ___________________________________________________________-
523 | | let mut sender_wakers = self.channel.sender_wakers.lock().unwrap();
| | ^^^^^^^^^^^^^
524 | | let idx = sender_wakers.iter().position(|w| w.will_wake(&waker));
525 | | if let Some(idx) = idx {
... |
529 | | }
530 | | }
| |_________- temporary `sender_wakers` is currently being dropped at the end of its contained scope
|
= note: this might lead to unnecessary resource contention
= help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#significant_drop_tightening
= note: `-D clippy::significant-drop-tightening` implied by `-D clippy::nursery`
help: merge the temporary construction with its single usage
|
523 ~
524 + let idx = self.channel.sender_wakers.lock().unwrap().position(|w| w.will_wake(&waker));
|
help: remove separated single usage
|
524 - let idx = sender_wakers.iter().position(|w| w.will_wake(&waker));
524 +
|
We can't move the lock into the line with idx as sender_wakers (the locked value) is used later on. Clippy however doesn't seem to see/understand this.
I guess the basic problem is that Clippy is only considering a single use of the locked value, while in the code above their are two uses.
Version
rustc 1.69.0-nightly (d962ea578 2023-02-26)
binary: rustc
commit-hash: d962ea57899d64dc8a57040142c6b498a57c8064
commit-date: 2023-02-26
host: x86_64-apple-darwin
release: 1.69.0-nightly
LLVM version: 15.0.7
Additional Labels
@rustbot label+I-suggestion-causes-error