RFC: `'task` lifetime

[huonw]

Add a 'task lifetime that represents things that live as long as the task, e.g. borrowing a @T would give a &'task T.

I haven't met any instance where I really needed this; but @bblum said he'd be interested, and I have met a few lifetime issues with borrowing things out of @ in a chain (but this is probably just #3511).

[bblum]

My use case would be in reading a task's name. It would be fn() -> &'task str.

[bblum]

I don't think it would be safe for borrowing a @T into a &'task T but you could definitely put the latter inside one of the former.

The main extra capability would be to have a lifetime that fulfills 'static but does not fulfill Send.

[graydon]

Yeah, certain tls interfaces (that internally promise not to delete their referent) would also qualify.

[emberian]

cc @nikomatsakis @pnkfelix

[nikomatsakis]

seems like a post 1.0 thing to me, but I'm not particularly opposed to the idea

[eddyb]

Would be nice to be able to do type IdentInterner = &'task StrInterner; and remove that @.
The TLD only really needs 'static without Send, maybe there's a simple way of achieving that?

[rust-highfive]

This issue has been moved to the RFCs repo: rust-lang/rfcs#295