needless_pass_by_ref_mut false positive in closure

[cuviper]

Summary

I have a closure that captures &mut impl FnMut and calls it, and this lint thinks it doesn't need to be mutable.

I can reproduce this on 1.73.0-beta.7 and 1.74.0-nightly (13e6f24b9 2023-09-23), as well as my own compilation of clippy master (aa137a7e5705 2023-09-24).

Lint Name

needless_pass_by_ref_mut

Reproducer

I tried this code:

pub fn filter_copy<T: Copy>(predicate: &mut impl FnMut(T) -> bool) -> impl FnMut(&T) -> bool + '_ {
    move |&item| predicate(item)
}

I saw this happen:

warning: this argument is a mutable reference, but not used mutably
 --> src/lib.rs:1:40
  |
1 | pub fn filter_copy<T: Copy>(predicate: &mut impl FnMut(T) -> bool) -> impl FnMut(&T) -> bool + '_ {
  |                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&impl FnMut(T) -> bool`
  |
  = warning: changing this function will impact semver compatibility
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut
  = note: `#[warn(clippy::needless_pass_by_ref_mut)]` on by default

I expected to see this happen:
No lint, as it must be &mut to be callable in the closure.

The suggested change to &impl FnMut is an error:

error[E0596]: cannot borrow `*predicate` as mutable, as it is behind a `&` reference
 --> src/lib.rs:2:18
  |
2 |     move |&item| predicate(item)
  |                  ^^^^^^^^^ cannot borrow as mutable

Version

rustc 1.74.0-nightly (13e6f24b9 2023-09-23)
binary: rustc
commit-hash: 13e6f24b9adda67852fb86538541adaa68aff6e8
commit-date: 2023-09-23
host: x86_64-unknown-linux-gnu
release: 1.74.0-nightly
LLVM version: 17.0.0

Additional Labels

No response

[cuviper]

@rustbot label +I-suggestion-causes-error

[GuillaumeGomez]

Fixed in #11621. I'm adding a regression test in it just in case.