[BUG] Stable (warning): Async traits Self return requires type specification

[Arthurdw]

When using the #![feature(async_fn_in_trait)] flag, I stumbled upon a warning that I don't understand and assume to be a bug/missed implementation.

If this is the intended behavior, could anyone please explain why?

This is simplified code that generates the warning:

#![feature(async_fn_in_trait)]

trait Repository {
    async fn new() -> Self;
}

struct MyRepository {}

impl Repository for MyRepository {
    async fn new() -> Self {
        todo!()
    }
}

fn main() {
    todo!()
}

warning: opaque type `impl Future<Output = Self>` does not satisfy its associated type bounds
  --> src\main.rs:4:23
   |
4  |     async fn new() -> Self;
   |                       ^^^^
   |
  ::: C:\Users\arthu\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib/rustlib/src/rust\library\core\src\future\future.rs:41:5
   |
41 |     type Output;
   |     ------------ this associated type bound is unsatisfied for `Self`
   |
   = note: `#[warn(opaque_hidden_inferred_bound)]` on by default

But when adding a type restriction on self (in this case Sized) the warning does not appear anymore.

#![feature(async_fn_in_trait)]

trait Repository
where
    Self: Sized,
{
    async fn new() -> Self;
}

struct MyRepository {}

impl Repository for MyRepository {
    async fn new() -> Self {
        todo!()
    }
}

fn main() {
    todo!()
}

Meta

rustc --version --verbose:

rustc 1.72.0-nightly (cb80ff132 2023-07-07)
binary: rustc
commit-hash: cb80ff132a0e9aa71529b701427e4e6c243b58df
commit-date: 2023-07-07
host: x86_64-pc-windows-msvc
release: 1.72.0-nightly
LLVM version: 16.0.5

[compiler-errors]

This is in fact a bug.

[Arthurdw]

FYI: still present on rustc 1.75.0-nightly

[Arthurdw]

Update: This issue is still present, and has now sneaked into stable with the release of 1.75; as such I'm updating the title to reflect that.

active toolchain
----------------

stable-x86_64-unknown-linux-gnu (default)
rustc 1.75.0 (82e1608df 2023-12-21)

$ cargo check
warning: opaque type `impl Future<Output = Self>` does not satisfy its associated type bounds
  --> src/main.rs:2:5
   |
2  |     async fn new() -> Self;
   |     ^^^^^^^^^^^^^^^^^^^^^^^
   |
  ::: /var/home/arthur/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/future/future.rs:41:5
   |
41 |     type Output;
   |     ------------ this associated type bound is unsatisfied for `Self`
   |
   = note: `#[warn(opaque_hidden_inferred_bound)]` on by default

warning: `testing-async` (bin "testing-async") generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.00s

[BaxHugh]

This is similar to the bug report issue-119893: calling async trait fn gives compiler error 'type annotations needed' when type annotation is already provided

[compiler-errors]

@BaxHugh: This isn't the same bug. This just has to do with a false warning of the opaque_hidden_inferred_bound lint.

[buildwithzephyr]

👍 on this, also encountering this bug

[xiyu1984]

Also encountering with rustc 1.74.0-nightly