Rust trait alias

Question

I am working on a actor project which need alias for the trait because it is so long, but even with the nightly feature #![feature(trait_alias)] seems can not achieve.

In short I write a playground: I want alias A<T> to be shorter causing I have many generic type on A in real case; and the same time I want to access the type Output = Self; from its implementations B. Appreciate for any help.

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=5a9bb8d3f76112c0b73ea1da8af34959

#![feature(trait_alias)]

trait A<T> {
    type Output;
    fn test(a: T) -> Self::Output;
}

//To alias the trait, real situation longer than this.
//attempt 1:
trait B: A<String>{}

//attempt 2:
//trait B : A<String, Output=Self> where Self: std::marker::Sized {}
//impl<T> B for T where T: A<String, Output=T> {}

//attempt 3 with trait_alias:
//trait B = A<String>;

struct SA;

impl B for SA {
    type Output = Self;
}

Answer 1

trait alias is only mean to be:

used wherever traits would normally be used as either bounds or trait objects. Source

So, your use case doesn't match, you can't do that.