Maybe-sized RPIT type is unexpectedly allowed in `async fn`

[obi1kenobi]

I tried this code: playground

use std::fmt::Debug;

fn breaks() -> impl Debug + ?Sized {
    123
}
//
// error[E0277]: the size for values of type `impl Debug + ?Sized` cannot be known at compilation time
//  --> src/lib.rs:3:16
//   |
// 3 | fn breaks() -> impl Debug + ?Sized {
//   |                ^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
//   |
//   = help: the trait `Sized` is not implemented for `impl Debug + ?Sized`
//   = note: the return type of a function must have a statically known size

async fn works() -> impl Debug + ?Sized {
    123
}
// no error!

I expected to see this happen: return types must always be sized, so -> impl Debug + ?Sized is not valid for any function regardless if it's async or not.

Instead, this happened: the compiler unexpectedly accepts it for async functions.

Meta

rustc --version --verbose:

rustc 1.91.1 (ed61e7d7e 2025-11-07)
binary: rustc
commit-hash: ed61e7d7e242494fb7057f2657300d9e77bb4fcb
commit-date: 2025-11-07
host: x86_64-unknown-linux-gnu
release: 1.91.1
LLVM version: 21.1.2

Also happens on the 2025-11-27 nightly on the playground.

[theemathas]

I even found a way to call and use the output of this function. (More straightforward ways of doing this makes the compiler complain that the return value isn't sized.)

use std::fmt::Debug;

#[tokio::main]
async fn main() {
    wut(works()).await;
}

async fn works() -> impl Debug + ?Sized {
    123
}

async fn wut<T: Future<Output: Debug>>(fut: T) {
    let q = fut.await;
    println!("{q:?}");
}

This compiles and prints 123.

[jhpratt]

For context:

impl Future<Output = impl Debug + ?Sized> is also accepted, though manually implementing Future requires the Sized bound is upheld (as it doesn't opt out of the bound). Similarly, Poll<T> does not opt out, so it requires the type (returned by fn poll) to be sized.

While this probably should be rejected, simple attempts to use this method still errors. (playground)

async fn foo() {
    works().await;
}

   Compiling playground v0.0.1 (/playground)
error[E0277]: the size for values of type `impl Debug + ?Sized` cannot be known at compilation time
  --> src/lib.rs:21:5
   |
21 |     works().await;
   |     ^^^^^^^^^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `impl Debug + ?Sized`
   = note: all local variables must have a statically known size

error[E0277]: the size for values of type `impl Debug + ?Sized` cannot be known at compilation time
  --> src/lib.rs:21:13
   |
21 |     works().await;
   |             ^^^^^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `impl Debug + ?Sized`
note: required by an implicit `Sized` bound in `Poll`
  --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/task/poll.rs:14:15
   |
14 | pub enum Poll<T> {
   |               ^ required by the implicit `Sized` requirement on this type parameter in `Poll`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (lib) due to 2 previous errors

Though as I'm about to post this, I'm sniped and proven that this can in fact be called. It takes some effort, at least.