needless_pass_by_ref_mut false positive

[iamsauravsharma]

Summary

I got

error: this argument is a mutable reference, but not used mutably
  --> src/migrator/any.rs:22:17
   |
22 |     connection: &mut <Any as sqlx::Database>::Connection,
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&<Any as sqlx::Database>::Connection`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut
note: the lint level is defined here
  --> src/lib.rs:3:9
   |
3  | #![deny(clippy::all)]
   |         ^^^^^^^^^^^
   = note: `#[deny(clippy::needless_pass_by_ref_mut)]` implied by `#[deny(clippy::all)]`

even when connection is used as mutable https://github.com/iamsauravsharma/sqlx_migrator/blob/master/src/migrator/any.rs#L36
GitHub actions error https://github.com/iamsauravsharma/sqlx_migrator/actions/runs/5583309753/jobs/10203493083

Lint Name

needless_pass_by_ref_mut

Reproducer

1. clone repo at https://github.com/iamsauravsharma/sqlx_migrator
2. run cargo clippy --features any,postgres you will see clippy error

error: this argument is a mutable reference, but not used mutably
  --> src/migrator/any.rs:22:17
   |
22 |     connection: &mut <Any as sqlx::Database>::Connection,
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider changing to: `&<Any as sqlx::Database>::Connection`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_ref_mut

Expects no error since connection is used mutably and using as suggestion doesn't compile code

Version

rustc 1.73.0-nightly (da6b55cc5 2023-07-17)
binary: rustc
commit-hash: da6b55cc5eaf76ed6acb7dc2f7d611e32af7c9a7
commit-date: 2023-07-17
host: aarch64-apple-darwin
release: 1.73.0-nightly
LLVM version: 16.0.5

Additional Labels

No response

[Centri3]

cc @GuillaumeGomez

[y21]

Minimal reproducible example:

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

Looks like it doesn't work correctly in async fns.

[iamsauravsharma]

Also, It works properly if you add extra parameter which are not stored in heap? (not sure)

// no issue
async fn _f(x: &mut i32, _p: i32) {
    *x += 1;
}

and doesn't work with heap based parameter? (not sure)

async fn _g(x: &mut i32, _p: String) {
    *x += 1;
}

async fn _h(x: &mut i32, _p: Vec<i32>, _q: Box<u64>) {
    *x += 1;
}

[GuillaumeGomez]

Thanks! Taking a look.

[GuillaumeGomez]

I opened #11184 to fix this issue.

[taiki-e]

@rustbot label +I-suggestion-causes-error