i didn't "find" it. As i said, it's a well known one : https://github.com/rust-lang/rust/issues/110338

Simply using axum with code using multiple layers of async was enough.

But then again, it looked like this bug (the error message is the same), however at this point i'm really unsure if it's exactly the same. The error message and the method signature was so atrocious that i just gave up and found a simpler design using macros that dodged the bullet.