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;
}cargo r --features=bar -- ohai &
curl http://localhost:3000
ohaicargo r -- notfoo &
curl http://localhost:3000
foo