Use foreachSubTree instead of new TreeTraverser

compiler/src/dotty/tools/dotc/ast/TreeMapWithImplicits.scala

@@ -30,16 +30,10 @@ class TreeMapWithImplicits extends tpd.TreeMapWithPreciseStatContexts {
 
   private def patternScopeCtx(pattern: Tree)(using Context): Context = {
     val nestedCtx = ctx.fresh.setNewScope
-    new TreeTraverser {
-      def traverse(tree: Tree)(using Context): Unit = {
-        tree match {
-          case d: DefTree if d.symbol.isOneOf(GivenOrImplicitVal) =>
-            nestedCtx.enter(d.symbol)
-          case _ =>
-        }
-        traverseChildren(tree)
-      }
-    }.traverse(pattern)
+    pattern.foreachSubTree {
+      case d: DefTree if d.symbol.isOneOf(GivenOrImplicitVal) => nestedCtx.enter(d.symbol)
+      case _ =>
+    }
     nestedCtx
   }
 

compiler/src/dotty/tools/dotc/inlines/Inliner.scala

@@ -963,29 +963,24 @@ class Inliner(val call: tpd.Tree)(using Context):
         bindingOfSym(binding.symbol) = binding
       }
 
-      val countRefs = new TreeTraverser {
-        override def traverse(t: Tree)(using Context) = {
-          def updateRefCount(sym: Symbol, inc: Int) =
-            for (x <- refCount.get(sym)) refCount(sym) = x + inc
-          def updateTermRefCounts(t: Tree) =
-            t.typeOpt.foreachPart {
-              case ref: TermRef => updateRefCount(ref.symbol, 2) // can't be inlined, so make sure refCount is at least 2
-              case _ =>
-            }
-
-          t match {
-            case t: RefTree =>
-              updateRefCount(t.symbol, 1)
-              updateTermRefCounts(t)
-            case _: New | _: TypeTree =>
-              updateTermRefCounts(t)
-            case _ =>
-          }
-          traverseChildren(t)
+      def updateRefCount(sym: Symbol, inc: Int) =
+        for (x <- refCount.get(sym)) refCount(sym) = x + inc
+      def updateTermRefCounts(tree: Tree) =
+        tree.typeOpt.foreachPart {
+          case ref: TermRef => updateRefCount(ref.symbol, 2) // can't be inlined, so make sure refCount is at least 2
+          case _ =>
         }
-      }
-      countRefs.traverse(tree)
-      for (binding <- bindings) countRefs.traverse(binding)
+      def countRefs(tree: Tree) =
+        tree.foreachSubTree {
+          case t: RefTree =>
+            updateRefCount(t.symbol, 1)
+            updateTermRefCounts(t)
+          case t @ (_: New | _: TypeTree) =>
+            updateTermRefCounts(t)
+          case _ =>
+        }
+      countRefs(tree)
+      for (binding <- bindings) countRefs(binding)
 
       def retain(boundSym: Symbol) = {
         refCount.get(boundSym) match {

compiler/src/dotty/tools/dotc/quoted/PickledQuotes.scala

@@ -275,9 +275,7 @@ object PickledQuotes {
           QuotesCache(pickled) = tree
 
           // Make sure trees and positions are fully loaded
-          new TreeTraverser {
-            def traverse(tree: Tree)(using Context): Unit = traverseChildren(tree)
-          }.traverse(tree)
+          tree.foreachSubTree(identity)
 
           quotePickling.println(i"**** unpickled quote\n$tree")
 

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -1193,15 +1193,11 @@ trait Checking {
    */
   def checkNoForwardDependencies(vparams: List[ValDef])(using Context): Unit = vparams match {
     case vparam :: vparams1 =>
-      val check = new TreeTraverser {
-        def traverse(tree: Tree)(using Context) = tree match {
-          case id: Ident if vparams.exists(_.symbol == id.symbol) =>
-            report.error(em"illegal forward reference to method parameter", id.srcPos)
-          case _ =>
-            traverseChildren(tree)
-        }
+      vparam.tpt.foreachSubTree {
+        case id: Ident if vparams.exists(_.symbol == id.symbol) =>
+          report.error(em"illegal forward reference to method parameter", id.srcPos)
+        case _ =>
       }
-      check.traverse(vparam.tpt)
       checkNoForwardDependencies(vparams1)
     case Nil =>
   }