`double_ended_iterator_last` triggered when using FnMut closure in iterator method

[saecki]

Summary

The double_ended_iterator_last lint is triggered but using next_back() instead of last() would change the behavior of the code because the inspect() closure mutates a captured variable.

Lint Name

double_ended_iterator_last

Reproducer

I tried this code:

let mut line = 0;
let line_start = memchr::memchr_iter(b'\n', bytes)
    .inspect(|_| line += 1)
    .last();

I saw this happen:

   --> crates/typst-library/src/diag.rs:787:26
    |
787 |           let line_start = memchr::memchr_iter(b'\n', bytes)
    |  __________________________^
788 | |             .inspect(|_| line += 1)
789 | |             .last()
    | |______________-----^
    |                |
    |                help: try: `next_back()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#double_ended_iterator_last
    = note: `-D clippy::double-ended-iterator-last` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::double_ended_iterator_last)]`

I expected to see this happen:

No warning because Iterator::inspect takes an FnMut closure.

Version

rustc 1.87.0 (17067e9ac 2025-05-09)
binary: rustc
commit-hash: 17067e9ac6d7ecb70e50f92c1944e545188d2359
commit-date: 2025-05-09
host: x86_64-unknown-linux-gnu
release: 1.87.0
LLVM version: 20.1.1

Additional Labels

No response

[saecki]

Already closed in #14490