Async functions in traits

Level: 301 · deep dive

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The bullets below are the questions the finished page has to answer.

One line: A trait may declare async fn since Rust 1.75 ↗, but the future each method returns carries no Send promise — so a spawned actor that is generic over the trait is refused, until the trait states the bound itself with -> impl Future<Output = …> + Send.

What it has to cover

• A Durability trait with an async fn append, and three implementations: the file log from chapter 9, an in-memory log for tests, and a no-op
• The error. Making the store actor generic over D: Durability and spawning it — the full compiler message, and why it points at tokio::spawn rather than at the trait
• Why the trait cannot promise Send by itself: each implementation's future is a different type, and the bound has to be written where callers can rely on it
• The fix: impl Future<Output = …> + Send in return position in the trait, and what it costs implementations that would not have been Send
• The other options, and when each is worth it: the trait-variant crate, a boxed future (async-trait), and naming the bound at the call site
• What dyn does and does not allow. A trait with async fn is not dyn compatible; Box<dyn Durability> needs boxed futures — or an enum of the three backends instead

The trap it exists for

Reading the error as a problem with the spawn. The spawn is correct; the promise is missing from a trait in another file, and no amount of editing near tokio::spawn adds it.

What minidb gains

Durability chosen at startup — file, memory or none — behind one trait, which chapter 15 also relies on.

See also

• Static vs dynamic dispatch — dyn compatibility, and the enum as a third strategy
• Returning a trait — impl Trait in return position outside a trait
• Send and Sync — the bound that is missing
• Tasks — where Send + 'static first appeared

Po polsku

Od Rusta 1.75 trait może deklarować async fn, ale future zwracany przez taką metodę nie obiecuje, że jest Send. Aktor uogólniony po takim traicie i uruchomiony przez tokio::spawn zostaje więc odrzucony, a komunikat błędu wskazuje na spawn, nie na trait, w którym brakuje obietnicy. Rozwiązaniem jest zapisanie ograniczenia w samym traicie: -> impl Future<Output = …> + Send. Trait z async fn nie nadaje się też do dyn bez opakowywania future’ów w Box.

Szukaj po polsku: async fn w traitach · rust async fn in trait send bound · trait_variant make send · async trait dyn compatible