"Impl not general enough" error for using an associated type with drop glue over an await point

I tried this code:

trait Trait<'cx>: Sized {
    type State: Default;
}

struct Problem<'cx, T: Trait<'cx>> {
    state: T::State,
    cx: &'cx (),
}

impl<'cx, T: Trait<'cx>> Problem<'cx, T> {
    fn new() -> Self {
        Self {
            state: Default::default(),
            cx: &(),
        }
    }
}

struct Impler<'cx>(&'cx ());

impl<'cx> Trait<'cx> for Impler<'cx> {
    type State = String;
}

fn problem<'cx>() -> impl Future<Output = ()> + Send {
    async move {
        let _problem: Problem<'cx, Impler<'cx>> = Problem::new();
        async {}.await;
    }
}

I expected to see this happen: successful compilation

Instead, this happened: compilation error

error: implementation of `Trait` is not general enough
  --> src/lib.rs:26:5
   |
26 | /     async move {
27 | |         let _problem: Problem<'cx, Impler<'cx>> = Problem::new();
28 | |         async {}.await;
29 | |     }
   | |_____^ implementation of `Trait` is not general enough
   |
   = note: `Trait<'1>` would have to be implemented for the type `Impler<'0>`, for any two lifetimes `'0` and `'1`...
   = note: ...but `Trait<'2>` is actually implemented for the type `Impler<'2>`, for some specific lifetime `'2`

error: could not compile `tmp` (lib) due to 1 previous error

With -Znext-solver the error is:

error[E0277]: `{coroutine witness@src/lib.rs:26:5: 26:15}` cannot be sent between threads safely
  --> src/lib.rs:25:1
   |
25 | fn problem<'cx>() -> impl Future<Output = ()> + Send {
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `{coroutine witness@src/lib.rs:26:5: 26:15}` cannot be sent between threads safely
26 |     async move {
   |     ---------- within this `{async block@src/lib.rs:26:5: 26:15}`
   |
   = help: within `{async block@src/lib.rs:26:5: 26:15}`, the trait `Send` is not implemented for `{coroutine witness@src/lib.rs:26:5: 26:15}`
note: required because it's used within this `async` block
  --> src/lib.rs:26:5
   |
26 |     async move {
   |     ^^^^^^^^^^

error[E0277]: `{coroutine witness@src/lib.rs:26:5: 26:15}` cannot be sent between threads safely
  --> src/lib.rs:26:5
   |
26 |       async move {
   |       ^---------
   |       |
   |  _____within this `{async block@src/lib.rs:26:5: 26:15}`
   | |
27 | |         let _problem: Problem<'cx, Impler<'cx>> = Problem::new();
28 | |         async {}.await;
29 | |     }
   | |_____^ `{coroutine witness@src/lib.rs:26:5: 26:15}` cannot be sent between threads safely
   |
   = help: within `{async block@src/lib.rs:26:5: 26:15}`, the trait `Send` is not implemented for `{coroutine witness@src/lib.rs:26:5: 26:15}`
note: required because it's used within this `async` block
  --> src/lib.rs:26:5
   |
26 |     async move {
   |     ^^^^^^^^^^

error[E0277]: `{coroutine witness@src/lib.rs:26:5: 26:15}` cannot be sent between threads safely
  --> src/lib.rs:25:54
   |
25 |   fn problem<'cx>() -> impl Future<Output = ()> + Send {
   |  ______________________________________________________^
26 | |     async move {
   | |     ---------- within this `{async block@src/lib.rs:26:5: 26:15}`
27 | |         let _problem: Problem<'cx, Impler<'cx>> = Problem::new();
28 | |         async {}.await;
29 | |     }
30 | | }
   | |_^ `{coroutine witness@src/lib.rs:26:5: 26:15}` cannot be sent between threads safely
   |
   = help: within `{async block@src/lib.rs:26:5: 26:15}`, the trait `Send` is not implemented for `{coroutine witness@src/lib.rs:26:5: 26:15}`
note: required because it's used within this `async` block
  --> src/lib.rs:26:5
   |
26 |     async move {
   |     ^^^^^^^^^^

For more information about this error, try `rustc --explain E0277`.
error: could not compile `tmp` (lib) due to 3 previous errors

There are five simple modifications to the code that make it compile (also on `-Znext-solver):

• remove the Send bound on the output future of fn problem()
• replace the State of Impler with a type that doesn't need drop glue
• move the await point above the creation of Problem
• remove the lifetime of Trait
• remove the lifetime of Impler

Another modification that makes it compile is to split Trait into a supertrait without a lifetime:

trait Super {
    type State: Default;
}

trait Trait<'cx>: Super + Sized {}

This fix is what I ended up using in my codebase. It also made me think that this might be a compiler bug.

Meta

rustc --version --verbose:

rustc 1.95.0-nightly (842bd5be2 2026-01-29)
binary: rustc
commit-hash: 842bd5be253e17831e318fdbd9d01d716557cc75
commit-date: 2026-01-29
host: x86_64-unknown-linux-gnu
release: 1.95.0-nightly
LLVM version: 22.1.0

(feel free to improve the title)

Start with the supplied src/lib.rs reproducer and run it on the reported nightly, both with and without -Znext-solver. Compare the diagnostics across the listed code modifications, then trace the compiler behavior responsible for the await-point, associated-type, and lifetime interaction. Done means the original example compiles as expected without requiring one of the workarounds.