Make it possible to compare threads

[tomaka]

Objects of type Thread can't be compared. There's no way to know if you are still in the same thread as you were before.

I don't know if this should just be an implementation of Eq on Thread, or if a new ThreadId type would need to be created.

[sfackler]

Should be pretty simple - pthread_equals on not-Windows and GetThreadId on Windows.

[nagisa]

There has been a bunch of attempts to implement this already:
rust-lang/rust#29457
rust-lang/rust#29448
rust-lang/rust#29447

Also see:
rust-lang/rust#21507

[ranma42]

@alexcrichton your comment in one of the implementation attempts suggests that [Partial]Eq should be implemented on a ThreadId type and not on Thread itself.
What are the advantages of that approach? What other operations (besides Eq) would you expect to be implemented for ThreadId?

[alexcrichton]

Threads themselves are somewhat more opaque and it's in general just a little more flexible to deal with ids instead of threads themselves. We could also lower the ThreadId to the underlying primitive if that's the implementation detail.

[tomaka]

ThreadId now exists.