Cache TermRef#isStable

Author: mbovel

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -161,7 +161,7 @@ object Types {
       * Rationale: If an expression has a stable type, the expression must be idempotent, so stable types
       * must be singleton types of stable expressions. */
     final def isStable(using Context): Boolean = stripTypeVar match {
-      case tp: TermRef => tp.symbol.isStableMember && tp.prefix.isStable || tp.info.isStable
+      case tp: TermRef => tp.cachedIsStable
       case _: SingletonType | NoPrefix => true
       case tp: RefinedOrRecType => tp.parent.isStable
       case tp: ExprType => tp.resultType.isStable
@@ -2665,6 +2665,19 @@ object Types {
     override def designator: Designator = myDesignator
     override protected def designator_=(d: Designator): Unit = myDesignator = d
 
+    var myIsStablePeriod: Period = Nowhere
+    var myIsStable: Boolean = false
+
+    private[Types] def cachedIsStable(using Context): Boolean =
+      if myIsStablePeriod != ctx.period then
+        val isStable: Boolean = symbol.isStableMember && prefix.isStable || info.isStable
+        if !isProvisional then
+          myIsStablePeriod = ctx.period
+          myIsStable = isStable
+        isStable
+      else
+        myIsStable
+
     //assert(name.toString != "<local Coder>")
     override def underlying(using Context): Type = {
       val d = denot