Auto merge of #142090 - compiler-errors:perf-stable-root-var, r=lcnr

Make root vars more stable

Never resolve a ty/ct vid to a higher vid as its root. This should make the optimization in #141500 more "stable" when there are a lot of vars flying around.

r? `@ghost`

Author: bors

compiler/rustc_infer/src/infer/type_variable.rs

@@ -238,6 +238,9 @@ impl<'tcx> ut::UnifyKey for TyVidEqKey<'tcx> {
     fn tag() -> &'static str {
         "TyVidEqKey"
     }
+    fn order_roots(a: Self, _: &Self::Value, b: Self, _: &Self::Value) -> Option<(Self, Self)> {
+        if a.vid.as_u32() < b.vid.as_u32() { Some((a, b)) } else { Some((b, a)) }
+    }
 }
 
 impl<'tcx> ut::UnifyValue for TypeVariableValue<'tcx> {

compiler/rustc_infer/src/infer/unify_key.rs

@@ -137,6 +137,9 @@ impl<'tcx> UnifyKey for ConstVidKey<'tcx> {
     fn tag() -> &'static str {
         "ConstVidKey"
     }
+    fn order_roots(a: Self, _: &Self::Value, b: Self, _: &Self::Value) -> Option<(Self, Self)> {
+        if a.vid.as_u32() < b.vid.as_u32() { Some((a, b)) } else { Some((b, a)) }
+    }
 }
 
 impl<'tcx> UnifyValue for ConstVariableValue<'tcx> {

compiler/rustc_type_ir/src/ty_kind.rs

@@ -771,23 +771,6 @@ pub enum InferTy {
     FreshFloatTy(u32),
 }
 
-/// Raw `TyVid` are used as the unification key for `sub_relations`;
-/// they carry no values.
-impl UnifyKey for TyVid {
-    type Value = ();
-    #[inline]
-    fn index(&self) -> u32 {
-        self.as_u32()
-    }
-    #[inline]
-    fn from_index(i: u32) -> TyVid {
-        TyVid::from_u32(i)
-    }
-    fn tag() -> &'static str {
-        "TyVid"
-    }
-}
-
 impl UnifyValue for IntVarValue {
     type Error = NoError;