needless_pass_by_ref_mut false positive for required mut reference in async function #11216
Closed as not planned
Description
Summary
When passing a mut reference into an async function, the needless_pass_by_ref_mut warning is always triggered, even if the reference is used to call a function that requires it to be mutable. If the suggestion is taken, the resulting code does not compile.
Removing the async modifier from the requires_mut function in the reproducer (and replacing block_on with a direct function call) will produce correct output. In practice I discovered this bug with a trait parameter for an async_trait, but it presents on struct methods as well.
Lint Name
needless_pass_by_ref_mut
Reproducer
I tried this code:
struct S;
impl S {
fn mut_fn(&mut self) {
todo!();
}
}
async fn uses_mut(s: &mut S) {
s.mut_fn();
}
fn main() {
let mut s = S;
futures::executor::block_on(uses_mut(&mut s));
}I saw this happen:
warning: this argument is a mutable reference, but not used mutably
--> src/main.rs:9:22
|
9 | async fn uses_mut(s: &mut S) {
| ^^^^^^ help: consider changing to: `&S`
|
= 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 generated, the mut reference is required.
Version
rustc 1.73.0-nightly (1d56e3a6d 2023-07-22)
binary: rustc
commit-hash: 1d56e3a6d943062e41165bf07fea5ea8324ae011
commit-date: 2023-07-22
host: x86_64-unknown-linux-gnu
release: 1.73.0-nightly
LLVM version: 16.0.5
Additional Labels
@rustbot label +I-suggestion-causes-error