Async trait methods for no_std

Features like async-trait, avoid using Box and dyn.

This crate is ready for #![no_std] when PR69033 merged.

Thanks to crate async-trait, some code from these.

WARNING: This crate use some unstable even incomplete feature.

Features

• define async fn in trait without default implementation.
• support default implementations in trait.
• test more feature support.

Works

• Solve lifetime for self.
• Find a way to process default implementations in trait.
  • Don't define type alias as associated type.
  • Error: type parameter Self is part of concrete type but not used in parameter list for the impl Trait type alias

Usage

Please enable feature type_alias_impl_trait and generic_associated_types;

#![feature(type_alias_impl_trait)]
#![feature(generic_associated_types)]

use async_trait_static::async_trait;

async fn hello() -> u8 {
    1
}

#[async_trait]
trait AsyncFnTrait {
    async fn run(&self);
}

struct AsyncStruct;

#[async_trait]
impl AsyncFnTrait for AsyncStruct {
    async fn run(&self) {
        hello().await;
    }
}