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
@@ -175,7 +175,7 @@ object Types {
         // see: tests/explicit-nulls/pos/flow-stable.scala.disabled
         tp.tp1.isStable && (realizability(tp.tp2) eq Realizable) ||
         tp.tp2.isStable && (realizability(tp.tp1) eq Realizable)
-      case tp: AppliedType => tp.appliedTypeIsStable
+      case tp: AppliedType => tp.cachedIsStable
       case _ => false
     }
 
@@ -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
+
+    inline 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
@@ -4163,12 +4176,11 @@ object Types {
     private var myGround: Byte = 0
     private var myIsStable: Byte = 0
 
-
     def isGround(acc: TypeAccumulator[Boolean])(using Context): Boolean =
       if myGround == 0 then myGround = if acc.foldOver(true, this) then 1 else -1
       myGround > 0
 
-    inline def appliedTypeIsStable(using Context): Boolean =
+    inline def cachedIsStable(using Context): Boolean =
       if myIsStable == 0 then
         val isStable: Byte = tycon match
           case tycon: TypeRef if defn.isCompiletimeAppliedType(tycon.symbol) && args.forall(_.isStable) => 1