async-trait playground

Context

Rust 1.75 stabilized impl trait in return positions of trait functions. That means it's possible to use specific async trait implementations depending on static feature flags. Before that the async-trait macro had to be used for async trait fn's, which used dynamic dispatch in the background.

A trait like this can be defined and implemented now:

trait Frobnicate {
    async fn frobnicate(&self) -> &str;
}

If we require further trait bounds (e.g we have to add Send b/c we want to call the fn in a multithreaded async runtime) we can desuger the async expression and add the bound:

trait Frobnicate {
    fn frobnicate(&self) -> impl Future<Output = &str> + Send;
}

Sample

cargo r --features=bar -- ohai &
curl http://localhost:3000
ohai

cargo r -- notfoo &
curl http://localhost:3000
foo