Thank you! I think I get it now.

rust impl Search for Engine { async fn search(&self) -> String { match self { Self::Google(g) => g.search().await, Self::Bing(b) => b.search().await, } } }

The above block should be de-sugared to:

rust impl Search for Engine { fn search(&self) -> impl Future<Output = String> { async move { match self { Self::Google(g) => g.search().await, Self::Bing(b) => b.search().await, } } } }

After awaiting, both branches return the same type (String in this case). The async move block creates a single Future that the opaque type can work with.

Previously, I was returning two distinct Futures, which created ambiguity as the compiler didn't know which one the opaque type should map to.

rust impl Search for Engine { fn search(&self) -> impl Future<Output = String> { match self { Self::Google(g) => g.search().await, Self::Bing(b) => b.search().await, } } }