🔬 refactor structure of `ty::Predicate`

[nikomatsakis]

The current Predicate conservatively assumes that every variant may bind higher-ranked lifetimes:

rust/src/librustc/ty/mod.rs

Lines 905 to 941 in 322d7f7

	#[derive(Clone, Copy, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable)]
	pub enum Predicate<'tcx> {
	/// Corresponds to `where Foo : Bar<A,B,C>`. `Foo` here would be
	/// the `Self` type of the trait reference and `A`, `B`, and `C`
	/// would be the type parameters.
	Trait(PolyTraitPredicate<'tcx>),
	/// where `T1 == T2`.
	Equate(PolyEquatePredicate<'tcx>),
	/// where 'a : 'b
	RegionOutlives(PolyRegionOutlivesPredicate<'tcx>),
	/// where T : 'a
	TypeOutlives(PolyTypeOutlivesPredicate<'tcx>),
	/// where <T as TraitRef>::Name == X, approximately.
	/// See `ProjectionPredicate` struct for details.
	Projection(PolyProjectionPredicate<'tcx>),
	/// no syntax: T WF
	WellFormed(Ty<'tcx>),
	/// trait must be object-safe
	ObjectSafe(DefId),
	/// No direct syntax. May be thought of as `where T : FnFoo<...>`
	/// for some substitutions `...` and T being a closure type.
	/// Satisfied (or refuted) once we know the closure's kind.
	ClosureKind(DefId, ClosureSubsts<'tcx>, ClosureKind),
	/// `T1 <: T2`
	Subtype(PolySubtypePredicate<'tcx>),
	/// Constant initializer must evaluate successfully.
	ConstEvaluatable(DefId, &'tcx Substs<'tcx>),
	}

This forces us to account for higher-ranked binders in a lot more places in the code than we really should have to. It would be nice to restructure Predicate to avoid that.

In the past, I tried two approaches. In one, I separated out Predicate into Predicate and PredicateAtom. In the other, I introduced a Poly variant into Predicate. In the latter case, this made the Predicate type recursive, so as an initial refactoring, I converted Predicate to be a &'tcx PredicateKind, introducing indirection.

It's been long enough that my memory is a bit dim, but I think that the second refactoring was done precisely because the first one turned out be rather intrusive. I am not sure just now which one I prefer, but I think we will want to do one of them sooner or later!

(NB. This will conflict with the work that @sgrif is doing in #48536, though the conflicts ought to be possible to resolve.)

[nikomatsakis]

I think that the way to start here would be first to convert Predicate to be interned, so that it can have a recursive structure. The starting point would be to cherry-pick these commits from my repo:

• nikomatsakis@8cea848
• nikomatsakis@99bbc5a

We would probably want to measure the performance of this, though. =)

[sgrif]

Given that this conflicts with what I'm already working on, I'd like to take this one.

[ishitatsuyuki]

@sgrif Do you have any updates on this?

[leodasvacas]

This forces us to account for higher-ranked binders in a lot more places in the code than we really should have to.

Do you remember examples of places where this is a problem, to help guide the refactoring?

[leodasvacas]

This was discussed in a WG-traits meeting. @nikomatsakis didn't remember why he originally wanted this and it seems less relevant with chalk, making it a low priority.

[nikomatsakis]

I'm going to close this issue altogether, since it seems like we are going more the route of building up a "parallel structure" for Chalk purposes.