With respect to traits, there’s a somewhat “clever” thing that I can imagine “doing the right thing” in 95% of scenarios: a future returned from an async method (or gen fn method in the future) is Send IFF all the arguments to the method are Send and Sync (Sync because you want to also be able to hold references to the argument types within the Future), unless someone has manually annotated the async fn in the trait definition as Send or !Send (I don’t care particularly what the syntax for this is).
This will “do the right thing” in nearly all scenarios, excepting where a !Send type is acquired from “somewhere” outside of the arguments to the function. it’s never the case that an async fn with a !Send argument is Send (since at the very least the argument is held onto and dropped at the start of the Future). !Sync arguments are a bit trickier, but I’d imagine these would be extraordinarily rare in scenarios where the Future itself was expected to be Send.
WDYT?