Avoid forcing infos of some symbols in InfoTransforms

Provides the "mayChange" hook to exclude symbols from being completed
prior to a transformInfo.

4 files changed, 21 insertions, 5 deletions

diff --git a/src/dotty/tools/dotc/core/DenotTransformers.scala b/src/dotty/tools/dotc/core/DenotTransformers.scala
index 0e31d87e6..02d27ea33 100644
--- a/src/dotty/tools/dotc/core/DenotTransformers.scala
+++ b/src/dotty/tools/dotc/core/DenotTransformers.scala
@@ -38,13 +38,23 @@ object DenotTransformers {
     def transformInfo(tp: Type, sym: Symbol)(implicit ctx: Context): Type
 
     def transform(ref: SingleDenotation)(implicit ctx: Context): SingleDenotation = {
-      val info1 = transformInfo(ref.info, ref.symbol)
-      if (info1 eq ref.info) ref
-      else ref match {
-        case ref: SymDenotation => ref.copySymDenotation(info = info1)
-        case _ => ref.derivedSingleDenotation(ref.symbol, info1)
+      val sym = ref.symbol
+      if (sym.exists && !mayChange(sym)) ref
+      else {
+        val info1 = transformInfo(ref.info, ref.symbol)
+        if (info1 eq ref.info) ref
+        else ref match {
+          case ref: SymDenotation => ref.copySymDenotation(info = info1)
+          case _ => ref.derivedSingleDenotation(ref.symbol, info1)
+        }
       }
     }
+
+    /** Denotations with a symbol where `mayChange` is false are guaranteed to be
+     *  unaffected by this transform, so `transformInfo` need not be run. This
+     *  can save time, and more importantly, can help avoid forcing symbol completers.
+     */
+    protected def mayChange(sym: Symbol)(implicit ctx: Context): Boolean = true
   }
 
   /** A transformer that only transforms SymDenotations */
diff --git a/src/dotty/tools/dotc/transform/ElimByName.scala b/src/dotty/tools/dotc/transform/ElimByName.scala
index 40fd48327..7adee3a8a 100644
--- a/src/dotty/tools/dotc/transform/ElimByName.scala
+++ b/src/dotty/tools/dotc/transform/ElimByName.scala
@@ -116,4 +116,6 @@ class ElimByName extends MiniPhaseTransform with InfoTransformer { thisTransform
     case ExprType(rt) if exprBecomesFunction(sym) => defn.FunctionType(Nil, rt)
     case _ => tp
   }
+
+  override def mayChange(sym: Symbol)(implicit ctx: Context): Boolean = sym.isTerm
 }
diff --git a/src/dotty/tools/dotc/transform/ElimRepeated.scala b/src/dotty/tools/dotc/transform/ElimRepeated.scala
index eb44c3e2c..03fd28a26 100644
--- a/src/dotty/tools/dotc/transform/ElimRepeated.scala
+++ b/src/dotty/tools/dotc/transform/ElimRepeated.scala
@@ -28,6 +28,8 @@ class ElimRepeated extends MiniPhaseTransform with InfoTransformer { thisTransfo
   def transformInfo(tp: Type, sym: Symbol)(implicit ctx: Context): Type =
     elimRepeated(tp)
 
+  override def mayChange(sym: Symbol)(implicit ctx: Context): Boolean = sym is Method
+
   private def elimRepeated(tp: Type)(implicit ctx: Context): Type = tp.stripTypeVar match {
     case tp @ MethodType(paramNames, paramTypes) =>
       val resultType1 = elimRepeated(tp.resultType)
diff --git a/src/dotty/tools/dotc/transform/ExplicitOuter.scala b/src/dotty/tools/dotc/transform/ExplicitOuter.scala
index 436d9bcf7..0810f7d9c 100644
--- a/src/dotty/tools/dotc/transform/ExplicitOuter.scala
+++ b/src/dotty/tools/dotc/transform/ExplicitOuter.scala
@@ -55,6 +55,8 @@ class ExplicitOuter extends MiniPhaseTransform with InfoTransformer { thisTransf
       tp
   }
 
+  override def mayChange(sym: Symbol)(implicit ctx: Context): Boolean = sym.isClass
+
   /** First, add outer accessors if a class does not have them yet and it references an outer this.
    *  If the class has outer accessors, implement them.
    *  Furthermore, if a parent trait might have an outer accessor,