`needless_pass_by_ref_mut` false positive because of type change

[smoelius]

Summary

A function may require an &mut argument so that it has a particular type.

Lint Name

needless_pass_by_ref_mut

Reproducer

I tried this code (note that foo needs to have the &mut argument to be passed to bar):

fn foo(y: &mut i32) -> i32 {
    12 + *y
}

fn bar(_: impl Fn(&mut i32) -> i32) {}

fn main() {
    bar(foo);
}

I saw this happen:

warning: this argument is a mutable reference, but not used mutably
 --> src/main.rs:1:11
  |
1 | fn foo(y: &mut i32) -> i32 {
  |           ^^^^^^^^ help: consider changing to: `&i32`
  |
  = 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 warning.

Version

rustc 1.73.0-nightly (da6b55cc5 2023-07-17)
binary: rustc
commit-hash: da6b55cc5eaf76ed6acb7dc2f7d611e32af7c9a7
commit-date: 2023-07-17
host: x86_64-unknown-linux-gnu
release: 1.73.0-nightly
LLVM version: 16.0.5

Additional Labels

No response

[Centri3]

cc @GuillaumeGomez

This would require some work, but should be possible to fix. My lint single_call_fn does something similar, use a visitor to see where the function's accessed and suppress the warning if it's passed to a function that expects a closure taking &mut.

Note that this would not cover cases where it's first assigned to a local however; you'd need to visit local usage too then, but that's likely rare enough that it's not really too big of a deal (perhaps it could just be noted in "Known issues")

You'd likely want to store the function LocalDefIds in the lint's struct then have a check_crate_post to get both the usage and to do the linting

(PS: I think this will need span_lint_hir_and_then! I need to fix that on my lint...)

[taiki-e]

@rustbot label +I-suggestion-causes-error