async closure does not implement FnMut because it captures state from its environment for async move closures capturing copyable values, while closures returning async move blocks work fine

[ProbstDJakob]

async move closures do not implement FnMut (and therefore also not Fn) when capturing a copyable value, but closures returning a async move block do.

I tried this code:

fn main() {}

fn needs_fn_mut<T, F: FnMut() -> T>(mut f: F) -> T {
    (f)()
}

fn hello() -> impl Future<Output = u32> {
    let copyable = 0;

    needs_fn_mut(async move || { copyable })
}

Same with Fn

fn main() {}

fn needs_fn<T, F: Fn() -> T>(f: F) -> T {
    (f)()
}

fn hello() -> impl Future<Output = u32> {
    let copyable = 0;

    needs_fn(async move || { copyable })
}

Working code with async move blocks:

fn main() {}

fn needs_fn_mut<T, F: FnMut() -> T>(mut f: F) -> T {
    (f)()
}

fn hello() -> impl Future<Output = u32> {
    let copyable = 0;

    needs_fn_mut(|| async move { copyable })
}

Same with Fn

fn main() {}

fn needs_fn<T, F: Fn() -> T>(f: F) -> T {
    (f)()
}

fn hello() -> impl Future<Output = u32> {
    let copyable = 0;

    needs_fn(|| async move { copyable })
}

Meta

rustc --version --verbose:

rustc 1.86.0 (05f9846f8 2025-03-31)
binary: rustc
commit-hash: 05f9846f893b09a1be1fc8560e33fc3c815cfecb
commit-date: 2025-03-31
host: x86_64-unknown-linux-gnu
release: 1.86.0
LLVM version: 19.1.7

Backtrace

error: async closure does not implement `FnMut` because it captures state from its environment
  --> <anon>:10:18
   |
10 |     needs_fn_mut(async move || { copyable })
   |     ------------ ^^^^^^^^^^^^^
   |     |
   |     required by a bound introduced by this call
   |
note: required by a bound in `needs_fn_mut`
  --> <anon>:3:23
   |
3  | fn needs_fn_mut<T, F: FnMut() -> T>(mut f: F) -> T {
   |                       ^^^^^^^^^^^^ required by this bound in `needs_fn_mut`

error: aborting due to 1 previous error

This issue may be linked to #125247.