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. You will get some warning from compiler.

Features

• Self
  • Self by reference.
  • Self by value.
  • Self by mut reference.
  • no Self.
  • any type of Self.
• Any number of arguments, any return value.
  • Arguments.
    • As value.
    • As reference without lifetime.
  • Return value expect reference (return reference at Lifetime return).
• Lifetime parameters.
  • Lifetime arguments.
  • Lifetime return.
• Associated types support.
• Having async and non-async functions in the same trait.

Incomplete Feature

These feature all require generic_associated_types support type arguments.

Following issue: #44265

• support default async fn implementations in trait (Wait feature generic_associated_types support type arguments).
• Generic type parameters.
  • Generic arguments (Wait for feature generic_associated_types support type arguments).
  • Generic return (Wait for feature generic_associated_types support type arguments).
  • impl trait in arguments (Wait for feature generic_associated_types support type arguments).

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;
    }
}