Implementation is not general enough error with no context

[Noratrieb]

Code

use std::{any::Any, marker::PhantomData};

pub struct HasDropImpl;
impl core::ops::Drop for HasDropImpl {
    fn drop(&mut self) {}
}

pub struct Wrapper<Context = dyn Any>
where
    Context: ObjectMarker + ?Sized,
{
    raw: HasDropImpl,
    _param: PhantomData<Context>,
}

unsafe impl<C> Send for Wrapper<C> where C: ObjectMarker + ?Sized {}

pub(crate) trait ObjectMarker {}
impl ObjectMarker for dyn Any {}

fn fails() -> Result<(), Wrapper> {
    Ok(())
}

async fn fut1() {}

fn main() {
    let fut = async {
        let _across_await = fails();
        fut1().await;
    };

    require_send(fut);
}

fn require_send<F>(_: F)
where
    F: Future + Send,
{
}

Current output

error: implementation of `ObjectMarker` is not general enough
  --> src/main.rs:33:5
   |
33 |     require_send(fut);
   |     ^^^^^^^^^^^^^^^^^ implementation of `ObjectMarker` is not general enough
   |
   = note: `(dyn Any + '0)` must implement `ObjectMarker`, for any lifetime `'0`...
   = note: ...but `ObjectMarker` is actually implemented for the type `(dyn Any + 'static)`

Desired output

Rationale and extra context

This is a minimization from the rootcause library. The original error was exactly the same, with no more information than this.

Other cases

Rust Version

rustc 1.93.0-nightly (27b076af7 2025-11-21)
binary: rustc
commit-hash: 27b076af7e3e7a363975443d81dfa9ecee5a74ec
commit-date: 2025-11-21
host: x86_64-unknown-linux-gnu
release: 1.93.0-nightly
LLVM version: 21.1.5

Anything else?

No response

[Noratrieb]

I think what is happening here is that rustc tries to prove

• F: Send
  • which requires proving _across_await Result<(), Wrapper<dyn Any>: Send
    • which is satisfied by impl<C> Send for Wrapper<C> where C: ObjectMarker + ?Sized
      • which requires dyn Any: ObjectMarker
        • ... which should be satisfied by impl ObjectMarker for dyn Any 🤔

[Noratrieb]

I suspect this is related to object lifetime defaults... https://doc.rust-lang.org/reference/lifetime-elision.html#default-trait-object-lifetimes
the impl is, as the error points out, for dyn Any + 'static. But I think Wrapper<dyn Any> should also be Wrapper<dyn Any + 'static>? With Any it's even more funny since dyn Any: 'static should always hold, and indeed

    fn stat<V>(_: V) where V: 'static {}
    stat(fails());

holds.

And so does

stat(fut);

So something is going wrong with the coroutine witness type

[Noratrieb]

This is another issue cause by #110338. The lifetime comes from the coroutine interior and gets erased, losing its 'staticness.

[Noratrieb]

This issue would be fixed by -Zhigher-ranked-assumptions from #143545.