Merge rustc_infer::infer::relate::{glb,lub}.

Most of the code in these two modules is duplicated in the other module.
This commit eliminates the duplication by replacing them with a new
module `lattice_op`. The new `LatticeOpKind` enum is used to distinguish
between glb and lub in the few places where the behaviour differs.

Author: nnethercote

compiler/rustc_infer/src/infer/relate/combine.rs

@@ -1,4 +1,4 @@
-//! There are four type combiners: [TypeRelating], [Lub], and [Glb],
+//! There are four type combiners: [TypeRelating], `Lub`, and `Glb`,
 //! and `NllTypeRelating` in rustc_borrowck, which is only used for NLL.
 //!
 //! Each implements the trait [TypeRelation] and contains methods for
@@ -26,8 +26,7 @@ use rustc_middle::ty::{self, InferConst, IntType, Ty, TyCtxt, TypeVisitableExt,
 pubuse rustc_next_trait_solver::relate::combine::*;
 use tracing::debug;
 
-usesuper::glb::Glb;
-usesuper::lub::Lub;
+usesuper::lattice::{LatticeOp,LatticeOpKind};
 usesuper::type_relating::TypeRelating;
 usesuper::{RelateResult,StructurallyRelateAliases};
 usecrate::infer::{DefineOpaqueTypes,InferCtxt,TypeTrace, relate};
@@ -298,12 +297,12 @@ impl<'infcx, 'tcx> CombineFields<'infcx, 'tcx> {
 TypeRelating::new(self,StructurallyRelateAliases::No, ty::Contravariant)
 }
 
-pubfnlub<'a>(&'amutself) -> Lub<'a,'infcx,'tcx>{
-Lub::new(self)
+pub(crate)fnlub<'a>(&'amutself) -> LatticeOp<'a,'infcx,'tcx>{
+LatticeOp::new(self,LatticeOpKind::Lub)
 }
 
-pubfnglb<'a>(&'amutself) -> Glb<'a,'infcx,'tcx>{
-Glb::new(self)
+pub(crate)fnglb<'a>(&'amutself) -> LatticeOp<'a,'infcx,'tcx>{
+LatticeOp::new(self,LatticeOpKind::Glb)
 }
 
 pubfnregister_obligations(