How often do you want non-send futures?

[cramertj]

@newpavlov

I also find it really strange that this code:

fn foo() -> impl Foo { .. }

fn bar(f: impl Foo + Send) { .. } 

bar(foo())

Compiles if foo returns Send type and fails with compilation error otherwise. I believe such leakage of an inner implementation detail goes against Rust principles and we should use explicit impl Foo + Send in situations like this. Was this an intentional design? And if so was it discussed and what is rationale behind it?

Yes, this was always the design of impl Trait. Auto trait leakage was an intentional part of the design, and is necessary for many usecases where today it is impossible to represent a bound like impl SendIfAndOnlyIf<T: Send> explicitly.