make rustdoc show whether types are affine, Send and/or Freeze

[thestinger]

It's not possible to tell whether a type is going to have move semantics or Send/Freeze without looking into each layer of the potentially private implementation details.

[thestinger]

cc @alexcrichton

[alexcrichton]

I don't think that this would be too hard to do. There's a few tough parts about this though:

• What information is shown for generic types? (how do display all of it)
• This would involve building a cache of information that's sendable because the tcx is destroyed after compilation (it's not sendable across tasks).

I would imagine this would just query all types ahead of time and then place little badges on the relevant pages about what qualities the type has (although again, with generics I'm not sure how this would work).

[alexcrichton]

Closing in favor of RFC #3 (opt-in builtin traits). When opting in, you'll have to have impl Send for ... somewhere, and the Send trait will show up in the "Implemented Traits" section of the documentation.