rust-lang / rust-lang/rust-clippy
`double_ended_iterator_last` should not emit on move FnMut closure
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Description
Summary
This lint is not emitted on a non-move closure, but it fails to see the order dependence when the closure is marked as move.
Lint Name
double_ended_iterator_last
Reproducer
I tried this code: (playground)
(this is a little contrived, but it's a simplified version of the code I found it on)
#[deny(clippy::double_ended_iterator_last)]
fn find_best<'a>(a: &'a [usize], b: &[i32]) -> Option<(usize, &'a usize)> {
let mut best_val = 0;
a
.iter()
.enumerate()
.rev()
.filter(move |&(i, &val)| {
match b[val].cmp(&best_val) {
Ordering::Less => false,
Ordering::Equal if i < val => false,
Ordering::Equal | Ordering::Greater => {
best_val = b[val];
true
}
}
})
.last()
}
I saw this happen:
error: called `Iterator::last` on a `DoubleEndedIterator`; this will needlessly iterate the entire iterator
--> src/main.rs:6:5
|
6 | / a
7 | | .iter()
8 | | .enumerate()
9 | | .rev()
... |
19 | | })
20 | | .last()
| |_______________^
|
I expected to see this happen:
No lint to be emitted, as what happens if the closure is made not move
Version
rustc 1.98.0 (88d9e12ae 2026-08-18)
binary: rustc
commit-hash: 88d9e12ae178fab0fb5cc050a94da85685d449ea
commit-date: 2026-08-18
host: x86_64-unknown-linux-gnu
release: 1.98.0
LLVM version: 22.1.8
Additional Labels
@rustbot label +I-suggestion-causes-bug
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by running the reproducer in src/main.rs and inspect the double_ended_iterator_last lint's handling of the move FnMut closure around filter and last. Compare it with the non-move closure behavior. Done means the lint is not emitted for this order-dependent case while existing applicable cases remain covered.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- rust
- Domain
- tooling
- Issue type
- Bug
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Activity status
- Active
- Clarity
- Mostly clear
- Newbie friendliness
- 58/100