Remove xform submodule.

It used to be bigger, with an `Xform` trait, but now it has just a
single function.

compiler/rustc_hir_analysis/src/variance/mod.rs

@@ -27,9 +27,6 @@ mod solve;
 
 pub(crate) mod dump;
 
-/// Code for transforming variances.
-mod xform;
-
 pub(crate) fn provide(providers: &mut Providers) {
     *providers = Providers { variances_of, crate_variances, ..*providers };
 }

compiler/rustc_hir_analysis/src/variance/solve.rs

@@ -12,8 +12,26 @@ use tracing::debug;
 use super::constraints::*;
 use super::terms::VarianceTerm::*;
 use super::terms::*;
-use super::xform::*;
 
+fn glb(v1: ty::Variance, v2: ty::Variance) -> ty::Variance {
+    // Greatest lower bound of the variance lattice as defined in The Paper:
+    //
+    //       *
+    //    -     +
+    //       o
+    match (v1, v2) {
+        (ty::Invariant, _) | (_, ty::Invariant) => ty::Invariant,
+
+        (ty::Covariant, ty::Contravariant) => ty::Invariant,
+        (ty::Contravariant, ty::Covariant) => ty::Invariant,
+
+        (ty::Covariant, ty::Covariant) => ty::Covariant,
+
+        (ty::Contravariant, ty::Contravariant) => ty::Contravariant,
+
+        (x, ty::Bivariant) | (ty::Bivariant, x) => x,
+    }
+}
 struct SolveContext<'a, 'tcx> {
     terms_cx: TermsContext<'a, 'tcx>,
     constraints: Vec<Constraint<'a>>,