impl Trait in return position implicitly captures non-'static type parameter despite + 'static

[progval]

Code

trait MyTrait {}
struct MyStruct;
impl MyTrait for MyStruct {}

trait DeserTrait<'de> {}
struct DeserStruct;
impl<'de> DeserTrait<'de> for &'de DeserStruct {}

pub fn testfn<'de, D: DeserTrait<'de>>(_deserializer: D) -> impl MyTrait + 'static {
    MyStruct
}

fn test() -> impl Send {
    let deserializer = DeserStruct;
    let graph = testfn(&deserializer);
    graph
}

(playground)

Current output

error[E0597]: `deserializer` does not live long enough
  --> src/lib.rs:15:24
   |
14 |     let deserializer = DeserStruct;
   |         ------------ binding `deserializer` declared here
15 |     let graph = testfn(&deserializer);
   |                 -------^^^^^^^^^^^^^-
   |                 |      |
   |                 |      borrowed value does not live long enough
   |                 argument requires that `deserializer` is borrowed for `'static`
16 |     graph
17 | }
   | - `deserializer` dropped here while still borrowed

Desired output

I'm not sure, but adding + use<> to the function's return type explains what the issue is actually that impl Trait implicitly captured the trait bound:

error: `impl Trait` must mention all type parameters in scope in `use<...>`
 --> src/lib.rs:9:61
  |
9 | pub fn testfn<'de, D: DeserTrait<'de>>(_deserializer: D) -> impl MyTrait + use<> + 'static {
  |                    -                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |                    |
  |                    type parameter is implicitly captured by this `impl Trait`
  |
  = note: currently, all type parameters are required to be mentioned in the precise captures list

Rationale and extra context

It's impossible to debug (or google) otherwise, and I only figured it out because I remembered + use<> from the stabilization blog post

Other cases

No response

Rust Version

rustc 1.82.0

Anything else?

No response

[compiler-errors]

This is a known limitation, see #116733 Limitation: Opaque hidden types.

[compiler-errors]

I'd recommend using + use<> syntax, which doesn't have this problem. edit: Actually, not totally sure if that will work for your usecase.

[progval]

I understand, but I wrote issue about the diagnostic that is unclear, not about limitations of the type system