Make the 'a lifetime on TyCtxt useless

[Zoxc]

cc @rust-lang/compiler

r? @nikomatsakis

[nikomatsakis]

I love the idea of simplifying the TyCtxt setup!

[michaelwoerister]

Would we be able to get rid of 'a entirely in a subsequent change? That would be awesome!

[Zoxc]

@michaelwoerister Yes.

I also propose a rename to this:

type QueryCx<'qx> = TyCtxt<'qx, 'qx, 'qx>;
type LocalCx<'qx, 'lx> = TyCtxt<'lx, 'qx, 'lx>;

[eddyb]

@Zoxc Note that pretty much all APIs must take the more general version ("LocalCx"), as to avoid callers having to call (the equivalent of) .global_tcx().

Because of that, I don't consider LocalCx "local" but rather "potentially-local", so it's more that the other one is the "global" context.

[nikomatsakis]

I like GlobalCx and QueryCx -- I personally would prefer the lifetimes to be ordered "small to large", so QueryCx<'local, 'global> and GlobalCx<'global>. But anyway that's for after this PR, I suppose.

[nikomatsakis]

Maybe we should assert that this is None and set it back to None before we return, then?

[nikomatsakis]

I guess it hardly matters, it's just the interning maps.

[nikomatsakis]

Can we add a comment here:

---

The interners parameter is a reference to a storage slot with 'tcx lifetime that we can use. It is expected to be None on entry and will be reset to None on return. This allows us to create a &'tcx reference to the interners while the tcx is active.

---

that presumes, of course, that you accept my proposal above =)

[nikomatsakis]

In any case, I'm 👍 on this change, I'd like some kind of comment to make the role of the Option a bit clearer, primarily. Not sure where is the best place to put it though.

[nikomatsakis]

@bors r+

Good enough

[bors]

📌 Commit d0190d3 has been approved by nikomatsakis

[bors]

⌛ Testing commit d0190d3 with merge 0a4a4ff...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing 0a4a4ff to master...

[Zoxc]

I have found two drawbacks to doing this:

• GlobalCtxt's destructor may not access references with the 'tcx lifetime. So we need to use #[may_dangle] on Drop impls.
• After creating the GlobalCtxt we can no longer gain ownership or even mutable access of it.