---
tags: async, rustc
---

# Static async fns in traits tests

Example #1 (trivial example implementing for primitive type)

// check-pass

trait MyTrait {
    async fn foo(&self) -> i32;
}

impl MyTrait for i32 {
    async fn foo(&self) -> i32 {
        *self
    }
}

Example #2 (needs async fn)

trait MyTrait {
    async fn foo(&self) -> i32;
}

impl MyTrait for i32 {
    fn foo(&self) -> i32 {
        // ~^ ERROR: async fn
        *self
    }
}

Example #3 (needs async fn, can't fake it like this)

trait MyTrait {
    async fn foo(&self) -> i32;
}
    
impl MyTrait for i32 {
    fn foo(&self) -> impl Future<Output = i32> { {
    // ~^ ERROR: async fn
        async {
            *self
        }
    }
}

Example #4 (generics):

// check-pass

trait MyTrait<T, U> {
    async fn foo(&self) -> &(T, U);
}

impl<T, U> MyTrait<T, U> for (T, U) {
    async fn foo(&self) -> &(T, U) {
        self
    }
}

Example #5 (generics + bounds):

// check-pass
use std::fmt::Debug;
use std::hash::Hash;

trait MyTrait<T, U> {
    async fn foo(&self) -> &(T, U) where T: Debug + Sized, U: Hash;
}

impl<T, U> MyTrait<T, U> for (T, U) {
    async fn foo(&self) -> &(T, U) {
        self
    }
}

Example #6 (lifetimes):

// check-pass

trait MyTrait<'a, 'b, T> {
    async fn foo(&'a self, key: &'b T) -> (&'a Self, &'b T);
}

impl<'a, 'b, T, U> MyTrait<'a, 'b, T> for U {
    async fn foo(&'a self, key: &'b T) -> (&'a U, &'b T) {
        (self, key)
    }
}

Example #7 (lifetimes + bounds):

// check-pass
use std::fmt::Debug;

trait MyTrait<'a, 'b, T> {
    async fn foo(&'a self, key: &'b T) -> (&'a Self, &'b T) where T: Debug + Sized;
}

impl<'a, 'b, T, U> MyTrait<'a, 'b, T> for U {
    async fn foo(&'a self, key: &'b T) -> (&'a U, &'b T) {
        (self, key)
    }
}

Example #8 (associated types)

use std::fmt::Debug;

trait MyTrait<'a, 'b, T> where Self: 'a, T: Debug + Sized + 'b {
    type MyAssoc;// = (&'a T, &'b U);

    async fn foo(&'a self, key: &'b T) -> Self::MyAssoc;
}

impl<'a, 'b, T: Debug + Sized + 'b, U: 'a> MyTrait<'a, 'b, T> for U {
    type MyAssoc = (&'a U, &'b T);

    async fn foo(&'a self, key: &'b T) -> (&'a U, &'b T) {
        (self, key)
    }
}

Example #9 (you can fake it with an assoc type)

// check-pass

trait MyTrait {
    type Fut<'a>: Future<Output = i32>
    where
        Self: 'a;

    fn foo(&self) -> Fut;
}

impl MyTrait for i32 {
    type Fut<'a> = impl Future + 'a
    where
        Self: 'a;

    fn foo<'a>(&'a self) -> Fut<'a> {
        async {
            self
        }
    }
}

Example #10 (Use desugared version in both places)

trait MyTrait {
    type Fut<'a>: Future<Output = i32>
    where
        Self: 'a;

    async fn foo(&self) -> Fut;
}

impl MyTrait for i32 {
    type Fut<'a> = impl Future + 'a
    where
        Self: 'a;

    fn foo<'a>(&'a self) -> Fut<'a> {
    // ~^ ERROR async fn
        async {
            self
        }
    }
}