spawn_unsafe should require that F: Send about crossbeam HOT 4 CLOSED

The bounds were deliberately left off to provide a maximally flexible, unsafe spawning function -- i.e., to cover even the case where you wanted to work with a non-Send closure unsafely.

That said, the flexibility isn't needed for scoped threads, and of course does add to the overall danger of this function. Arguably, this treatment of Send might be better accomplished through a newtype wrapper anyway. So I'm open to adding the bound.

from crossbeam.

An alternative might be to provide two unsafe functions instead of one, where one requires Send, and its use is recommended where possible.

from crossbeam.

Given that there's not a pressing need for a version without Send, I'd accept a PR that just added Send to the current method.

from crossbeam.

PR submitted: #55.

from crossbeam.