From 3a5bbf89b2229c629c6f01bdd87354cba136d133 Mon Sep 17 00:00:00 2001 From: Niko Matsakis Date: Tue, 11 Apr 2017 17:16:57 -0400 Subject: [PATCH] avoid unneeded subtype obligations in lub/glb In some specific cases, the new scheme was failing to learn as much from a LUB/GLB operaiton as the old code, which caused coercion to go awry. A slight ordering hack fixes this. --- src/librustc/infer/lattice.rs | 28 +++++++++++++++++++++++++++- 1 file changed, 27 insertions(+), 1 deletion(-) diff --git a/src/librustc/infer/lattice.rs b/src/librustc/infer/lattice.rs index f7b26a918b3..d4d090f0153 100644 --- a/src/librustc/infer/lattice.rs +++ b/src/librustc/infer/lattice.rs @@ -44,6 +44,10 @@ pub trait LatticeDir<'f, 'gcx: 'f+'tcx, 'tcx: 'f> : TypeRelation<'f, 'gcx, 'tcx> // Relates the type `v` to `a` and `b` such that `v` represents // the LUB/GLB of `a` and `b` as appropriate. + // + // Subtle hack: ordering *may* be significant here. This method + // relates `v` to `a` first, which may help us to avoid unecessary + // type variable obligations. See caller for details. fn relate_bound(&mut self, v: Ty<'tcx>, a: Ty<'tcx>, b: Ty<'tcx>) -> RelateResult<'tcx, ()>; } @@ -74,7 +78,29 @@ pub fn super_lattice_tys<'a, 'gcx, 'tcx, L>(this: &mut L, Ok(v) } - (&ty::TyInfer(TyVar(..)), _) | + // If one side is known to be a variable and one is not, + // create a variable (`v`) to represent the LUB. Make sure to + // relate `v` to the non-type-variable first (by passing it + // first to `relate_bound`). Otherwise, we would produce a + // subtype obligation that must then be processed. + // + // Example: if the LHS is a type variable, and RHS is + // `Box`, then we current compare `v` to the RHS first, + // which will instantiate `v` with `Box`. Then when `v` + // is compared to the LHS, we instantiate LHS with `Box`. + // But if we did in reverse order, we would create a `v <: + // LHS` (or vice versa) constraint and then instantiate + // `v`. This would require further processing to achieve same + // end-result; in partiular, this screws up some of the logic + // in coercion, which expects LUB to figure out that the LHS + // is (e.g.) `Box`. A more obvious solution might be to + // iterate on the subtype obligations that are returned, but I + // think this suffices. -nmatsakis + (&ty::TyInfer(TyVar(..)), _) => { + let v = infcx.next_ty_var(TypeVariableOrigin::LatticeVariable(this.cause().span)); + this.relate_bound(v, b, a)?; + Ok(v) + } (_, &ty::TyInfer(TyVar(..))) => { let v = infcx.next_ty_var(TypeVariableOrigin::LatticeVariable(this.cause().span)); this.relate_bound(v, a, b)?;