Do I have to use Pin when returning a boxed trait object from an async method?

Question

async fn execute(&self, partition: usize) -> Result<Pin<Box<dyn BatchStream + Send + Sync>>>;

Answer 1

In general: No. If you have a function in the form async fn f() -> Box<dyn Trait>, there is no need for Pin.

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However, returning a Future or Stream is a special case. So if BatchStream is an alias, or supertrait, or just mimics the Stream trait, then the Pin is likely necessary for the return value to be useful.

See other Q&As that explain where Pin is needed with Streams:

• How to use map for Box<dyn Stream<Item=()>>?
• Traits returning a trait: In some cases works, in others it doesn't
• `FuturesUnordered` does not satisfy `Stream`?