Designing a public async API in Rust forces you to make a difficult trade-off immediately: how do you return a Future? For years, the ecosystem relied on the async-trait macro (dynamic dispatch). With the stabilization of async fn in traits (Rust 1.75+), library authors now have native options. However, this introduces a new complexity: choosing between the flexibility of dynamic dispatch ( Box<dyn Future> ) and the performance of static dispatch ( impl Future ). This choice dictates your library's performance profile, thread-safety guarantees, and how painful it is for downstream users to integrate your code. The Root Cause: Sizedness and State Machines To make the right architectural decision, you must understand what happens inside the compiler when you write async fn . Rust implements async/await using compiler-generated state machines . When you compile an async function, Rust creates a unique, anonymous enum ...
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