FP: `clippy::significant_drop_tightening` shows incorrect drop point with `RwLockWriteGuard::downgrade`

[edwloef]

Summary

When consuming the RwLockWriteGuard via downgrade, the lint still suggests consuming the lock earlier, and then points at the end of the scope as the drop point and emits a broken suggestion.

Lint Name

significant_drop_tightening

Reproducer

I tried this code:

use std::sync::{RwLock, RwLockWriteGuard};

fn main() {
    let lock = RwLock::new(42);
    let guard = lock.write().unwrap();
    let guard = RwLockWriteGuard::downgrade(guard);
    drop(guard);
}

I saw this happen:

warning: temporary with significant `Drop` can be early dropped
 --> src/main.rs:5:9
  |
3 |   fn main() {
  |  ___________-
4 | |     let lock = RwLock::new(42);
5 | |     let guard = lock.write().unwrap();
  | |         ^^^^^
6 | |     let guard = RwLockWriteGuard::downgrade(guard);
7 | |     drop(guard);
8 | | }
  | |_- temporary `guard` 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/rust-1.92.0/index.html#significant_drop_tightening
  = note: `-W clippy::significant-drop-tightening` implied by `-W clippy::nursery`
  = help: to override `-W clippy::nursery` add `#[allow(clippy::significant_drop_tightening)]`
help: merge the temporary construction with its single usage
  |
5 ~     
6 +     let guard = lock.write().unwrap().;
7 ~     
  |

I expected to see this happen:
No lint suggesting to drop the write guard earlier.
No broken suggestion.

Version

rustc 1.92.0 (ded5c06cf 2025-12-08)
binary: rustc
commit-hash: ded5c06cf21d2b93bffd5d884aa6e96934ee4234
commit-date: 2025-12-08
host: x86_64-unknown-linux-gnu
release: 1.92.0
LLVM version: 21.1.3

Additional Labels

@rustbot label +I-suggestion-causes-error