`needless_pass_by_value` lint does not trigger on `async fn`

[skr2005]

Summary

The needless_pass_by_value lint does not fire for async fn, which makes it ineffective in projects that use async functions extensively. I am not sure whether this behavior is intended though.

Lint Name

needless_pass_by_value

Reproducer

I tried this code:

#![forbid(clippy::needless_pass_by_value)]

use std::hint::black_box;

fn main() {
    black_box(take_value_1);
    black_box(take_value_2);
}

struct Foo;

async fn take_value_1(x: Foo) {
    black_box(&x);
}

fn take_value_2(x: Foo) {
    black_box(&x);
}

I expected clippy to emit an error for both take_value_1 and take_value_2, since neither function consumes its Foo argument.

However, only take_value_2 triggers the lint. take_value_1 does not:

error: this argument is passed by value, but not consumed in the function body
  --> src\main.rs:16:20
   |
16 | fn take_value_2(x: Foo) {
   |                    ^^^
   |
help: or consider marking this type as `Copy`
  --> src\main.rs:10:1
   |
10 | struct Foo;
   | ^^^^^^^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_pass_by_value
note: the lint level is defined here
  --> src\main.rs:1:11
   |
1  | #![forbid(clippy::needless_pass_by_value)]
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: consider taking a reference instead
   |
16 | fn take_value_2(x: &Foo) {
   |                    +

error: could not compile `tt` (bin "tt") due to 1 previous error

Version

rustc 1.87.0 (17067e9ac 2025-05-09)
binary: rustc
commit-hash: 17067e9ac6d7ecb70e50f92c1944e545188d2359
commit-date: 2025-05-09
host: x86_64-pc-windows-gnu
release: 1.87.0
LLVM version: 20.1.1

[samueltardieu]

Using a reference in async fn take_value_1(x: &Foo) would restrict the lifetime of the returned anonymous future to the one of x, as its value would not be moved into the future. For example the following code would no longer compile:

use std::hint::black_box;

fn run_future(f: impl std::future::Future<Output = ()> + 'static) {
    black_box(f);
}

fn main() {
    let x = Foo;
    run_future(take_value_1(x));
}

struct Foo;

async fn take_value_1(x: Foo) {
    black_box(x);
}

So unless we know for sure that the future will never be passed to something that require a lifetime greater than the one of its argument, we cannot suggest using a reference there.

[skr2005]

Yes. Now I understand. It seems non-trivial for clippy to check lifetimes in which the futures are used, so feel free to close this issue as not planned.