Expose a non-allocating way to access the current task name.

[reem]

Currently, std::task::name allocates a new String for the current task name, but this is unnecessary and slow.

Note: this probably requires a 'thread lifetime or a reference type specific to thread-local values, perhaps struct ThreadRef<T> { nosend: NoSend, ref: &'static T }, but I haven't really thought through all the use cases.

[thestinger]

It needs more than that because changing the task name will invalidate it.

[reem]

Maybe this is an appropriate use-case for dynamic borrow checking. We could return a RefCell<ThreadRef<str>>, though.... ugh.

[reem]

Actually that would require full shared ownership, with Rc too, which would be sad for such a simple thing.

[steveklabnik]

And even then, Rc boxes its contents, so it's going to end up being heap allocated anyway. Given the constaints, I'm not sure this is possible.

[reem]

I think I have a non-allocating solution using &'static RefCell<&'static str> and some !Send impls, but it's much too complicated to be ergonomically used here. String is fine.