ptr_arg should not warn about &mut Vec<T> for underscore prefixed parameters

[cmrschwarz]

Summary

Clippy only warns about parameters of type &mut Vec<T> being replaceable by &mut [T] if none of the Vec APIs are used.
For example, you can't use &mut [T] instead if you want to push into the Vec.

A leading _ indicates intentionally unused parameters. The compiler should essentially assume (for the purpose of warnings) that the parameter is being used, but in a way that it cannot see. This is usually done for todo!s, #[cfg] stuff or api requirements.

If there's a usage that we cannot see, than we should not assume anything about that usage. So we should not assume that it isn't e.g. a push into the Vec.

Therefore, the warning about &mut Vec<T> should not be emitted for such parameters.

Lint Name

clippy::ptr_arg

Reproducer

I tried this code:

fn foo(_x: &mut Vec<i32>) {
    todo!();
}

I saw this happen:

warning: writing `&mut Vec` instead of `&mut [_]` involves a new object where a slice will do
 --> src/main.rs:1:12
  |
1 | fn foo(_x: &mut Vec<i32>) {
  |            ^^^^^^^^^^^^^ help: change this to: `&mut [i32]`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg
  = note: `#[warn(clippy::ptr_arg)]` on by default

I expected to see this happen:
No diagnostic.

Version

rustc 1.81.0 (eeb90cda1 2024-09-04)
binary: rustc
commit-hash: eeb90cda1969383f56a2637cbd3037bdf598841c
commit-date: 2024-09-04
host: x86_64-unknown-linux-gnu
release: 1.81.0
LLVM version: 18.1.7

Additional Labels

No response