lint idea: public Futures should be Send

[rkuhn]

One possible result of this discussion on Rust internals could be a lint that helps library maintainers in the large and the small (i.e. also between modules of the same crate) produce useful Future implementations when multithreaded executors are a possible runtime target. The lint would be disabled when targeting embedded systems where Futures are multiplexed onto the current thread only.

Informally specified, the rule would be that for any public function or method that returns an implementation of Future the return type must also be Send.

Is this feasible? Is this desirable?