Error using async trait helper methods with async trait bound on associated type

[inanna-malick]

I'm not experienced enough with async/await to be sure that this is a library issue and not a user error, but I didn't expect to see this type of type error in this use case. (minimized example)

#[async_trait]
pub trait TestCap {
    async fn foo(&self) -> String;
}

#[async_trait]
pub trait HasTestCap {
    type Output: TestCap;

    fn test_caps(&self) -> &Self::Output;

    async fn test_foo(&self) -> String {
        self.test_caps().foo()
    }
}

results in:

error[E0271]: type mismatch resolving `<impl core::future::future::Future as core::future::future::Future>::Output == std::string::String`
  --> src/capabilities/mod.rs:26:33
   |
26 |     async fn test_foo(&self) -> String {
   |                                 ^^^^^^ expected struct `std::pin::Pin`, found struct `std::string::String`
   |
   = note: expected type `std::pin::Pin<std::boxed::Box<dyn core::future::future::Future<Output = std::string::String> + std::marker::Send>>`
              found type `std::string::String`
   = note: the return type of a function must have a statically known size

[dtolnay]

• foo() is async so it evaluates to a future, not a String. You need to await it.

  self.test_caps().foo().await

• Also the value of type &Self::Output is live across the await point so you must require that it is threadsafe.

  type Output: TestCap + Sync;

[inanna-malick]

awesome, thanks - I had assumed you didn't need to .await unless you wanted to use the result of an async function, such that (eg) having an async function as the last expression of an async function wouldn't require await