needless_pass_by_ref_mut false positive in closure in async

[mkroening]

Summary

Another variant of #11179 and #11299.

Found here: hermit-os/kernel/src/fd/socket/tcp.rs#L69-L94

Lint Name

needless_pass_by_ref_mut

Reproducer

I tried this code:

async fn foo(x: &mut i32) {
    (|| *x += 1)();
}

I saw this happen:

warning: this argument is a mutable reference, but not used mutably
 --> foo.rs:1:17
  |
1 | async fn foo(x: &mut 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 needless_pass_by_ref_mut.

Version

rustc 1.74.0-nightly (ef85656a1 2023-08-21)
binary: rustc
commit-hash: ef85656a10657ba5e4f7fe2931a4ca6293138d51
commit-date: 2023-08-21
host: aarch64-apple-darwin
release: 1.74.0-nightly
LLVM version: 17.0.0

Additional Labels

No response

[mkroening]

@GuillaumeGomez, would this be important to fix before the lint lands in stable? I am not sure about the release process, but it sounded to me like it would be relevant for beta: #11314 (comment)

[GuillaumeGomez]

It'd be better indeed. I started looking at it but I'm a bit under the water currently, too many "more pressing" things to do. Hopefully I should be able to get back to it this week-end or next week.

[fasterthanlime]

Caught this in real-world code yesterday:

https://github.com/hapsoc/hring/blob/a2ffb643447b1e4bcdb673f3837b67cf3e02a19c/crates/hring/src/h1/client.rs#L34

(Not a showstopper, just thought I'd add a data point!)

[GuillaumeGomez]

I really need to find time to fix this...

[fasterthanlime]

I really need to find time to fix this...

@GuillaumeGomez is this a good first issue? Is there a place one could get guidance to fix this?

[GuillaumeGomez]

Honestly I don't think it's a good first issue at all. You can see the code here. If you want to start contributing on clippy, I'll be more than happy to help getting started but in this case, the problem is that the code generated by rustc for async is quite tricky to handle, especially variables being passed through which change IDs etc. Lots of fun. :3

[GuillaumeGomez]

I opened #11492 which fixes this issue.