diff --git a/compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala b/compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala
index 8e054c9c2dd5..2040b39c3071 100644
--- a/compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala
+++ b/compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala
@@ -30,7 +30,7 @@ import Decorators._
 import tpd._
 
 import scala.tools.asm
-import StdNames.nme
+import StdNames.{nme, str}
 import NameOps._
 import NameKinds.DefaultGetterName
 import dotty.tools.dotc.core
@@ -161,7 +161,8 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
   def boxMethods: Map[Symbol, Symbol] = defn.ScalaValueClasses().map{x => // @darkdimius Are you sure this should be a def?
     (x, Erasure.Boxing.boxMethod(x.asClass))
   }.toMap
-  def unboxMethods: Map[Symbol, Symbol] = defn.ScalaValueClasses().map(x => (x, Erasure.Boxing.unboxMethod(x.asClass))).toMap
+  def unboxMethods: Map[Symbol, Symbol] =
+    defn.ScalaValueClasses().map(x => (x, Erasure.Boxing.unboxMethod(x.asClass))).toMap
 
   override def isSyntheticArrayConstructor(s: Symbol) = {
     s eq defn.newArrayMethod
@@ -244,7 +245,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
           case ClazzTag => av.visit(name, const.typeValue.toTypeKind(bcodeStore)(innerClasesStore).toASMType)
           case EnumTag =>
             val edesc = innerClasesStore.typeDescriptor(const.tpe.asInstanceOf[bcodeStore.int.Type]) // the class descriptor of the enumeration class.
-            val evalue = const.symbolValue.name.toString // value the actual enumeration value.
+            val evalue = const.symbolValue.name.mangledString // value the actual enumeration value.
             av.visitEnum(name, edesc, evalue)
         }
       case t: TypeApply if (t.fun.symbol == Predef_classOf) =>
@@ -252,7 +253,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
       case t: tpd.Select =>
         if (t.symbol.denot.is(Flags.Enum)) {
           val edesc = innerClasesStore.typeDescriptor(t.tpe.asInstanceOf[bcodeStore.int.Type]) // the class descriptor of the enumeration class.
-          val evalue = t.symbol.name.toString // value the actual enumeration value.
+          val evalue = t.symbol.name.mangledString // value the actual enumeration value.
           av.visitEnum(name, edesc, evalue)
         } else {
           assert(toDenot(t.symbol).name.is(DefaultGetterName)) // this should be default getter. do not emmit.
@@ -262,8 +263,8 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
         for(arg <- t.elems) { emitArgument(arrAnnotV, null, arg, bcodeStore)(innerClasesStore) }
         arrAnnotV.visitEnd()
 
-      case Apply(fun, args) if (fun.symbol == defn.ArrayClass.primaryConstructor ||
-        (toDenot(fun.symbol).owner == defn.ArrayClass.linkedClass && fun.symbol.name == nme_apply)) =>
+      case Apply(fun, args) if fun.symbol == defn.ArrayClass.primaryConstructor ||
+        toDenot(fun.symbol).owner == defn.ArrayClass.linkedClass && fun.symbol.name == nme_apply =>
         val arrAnnotV: AnnotationVisitor = av.visitArray(name)
 
         var actualArgs = if (fun.tpe.isInstanceOf[ImplicitMethodType]) {
@@ -311,7 +312,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
   private def emitAssocs(av: asm.AnnotationVisitor, assocs: List[(Name, Object)], bcodeStore: BCodeHelpers)
                         (innerClasesStore: bcodeStore.BCInnerClassGen) = {
     for ((name, value) <- assocs)
-      emitArgument(av, name.toString, value.asInstanceOf[Tree], bcodeStore)(innerClasesStore)
+      emitArgument(av, name.mangledString, value.asInstanceOf[Tree], bcodeStore)(innerClasesStore)
     av.visitEnd()
   }
 
@@ -360,9 +361,8 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
     else clazz.getName
   }
 
-  def requiredClass[T](implicit evidence: ClassTag[T]): Symbol = {
+  def requiredClass[T](implicit evidence: ClassTag[T]): Symbol =
     ctx.requiredClass(erasureString(evidence.runtimeClass).toTermName)
-  }
 
   def requiredModule[T](implicit evidence: ClassTag[T]): Symbol = {
     val moduleName = erasureString(evidence.runtimeClass)
@@ -404,16 +404,15 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
     def newAnyRefMap[K <: AnyRef, V](): mutable.AnyRefMap[K, V] = new mutable.AnyRefMap[K, V]()
     def newWeakMap[K, V](): mutable.WeakHashMap[K, V] = new mutable.WeakHashMap[K, V]()
     def recordCache[T <: Clearable](cache: T): T = cache
-    def newWeakSet[K <: AnyRef](): WeakHashSet[K] = new WeakHashSet[K]()
+    def newWeakSet[K >: Null <: AnyRef](): WeakHashSet[K] = new WeakHashSet[K]()
     def newMap[K, V](): mutable.HashMap[K, V] = new mutable.HashMap[K, V]()
     def newSet[K](): mutable.Set[K] = new mutable.HashSet[K]
   }
 
+  val MODULE_INSTANCE_FIELD: String = str.MODULE_INSTANCE_FIELD
 
-
-  val MODULE_INSTANCE_FIELD: String = nme.MODULE_INSTANCE_FIELD.toString
-
-  def internalNameString(offset: Int, length: Int): String = new String(Names.chrs, offset, length)
+  def dropModule(str: String) =
+    if (!str.isEmpty && str.last == '$') str.take(str.length - 1) else str
 
   def newTermName(prefix: String): Name = prefix.toTermName
 
@@ -424,7 +423,6 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
       Flags.Bridge | Flags.VBridge | Flags.Private | Flags.Macro
   }.bits
 
-
   def isQualifierSafeToElide(qual: Tree): Boolean = tpd.isIdempotentExpr(qual)
   def desugarIdent(i: Ident): Option[tpd.Select] = {
     i.tpe match {
@@ -537,32 +535,25 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
     }
   }
 
-
   implicit def nameHelper(n: Name): NameHelper = new NameHelper {
-    def toTypeName: Name = n.toTypeName
     def isTypeName: Boolean = n.isTypeName
-    def toTermName: Name = n.toTermName
-    def dropModule: Name = n.stripModuleClassSuffix
-
-    def len: Int = n.toSimpleName.length
-    def offset: Int = n.toSimpleName.start
     def isTermName: Boolean = n.isTermName
     def startsWith(s: String): Boolean = n.startsWith(s)
+    def mangledString: String = n.mangledString
   }
 
-
   implicit def symHelper(sym: Symbol): SymbolHelper = new SymbolHelper {
     // names
     def fullName(sep: Char): String = sym.showFullName
     def fullName: String = sym.showFullName
     def simpleName: Name = sym.name
-    def javaSimpleName: Name = toDenot(sym).name // addModuleSuffix(simpleName.dropLocal)
-    def javaBinaryName: Name = javaClassName.replace('.', '/').toTypeName // TODO: can we make this a string? addModuleSuffix(fullNameInternal('/'))
-    def javaClassName: String = toDenot(sym).fullName.toString// addModuleSuffix(fullNameInternal('.')).toString
+    def javaSimpleName: String = toDenot(sym).name.mangledString // addModuleSuffix(simpleName.dropLocal)
+    def javaBinaryName: String = javaClassName.replace('.', '/') // TODO: can we make this a string? addModuleSuffix(fullNameInternal('/'))
+    def javaClassName: String = toDenot(sym).fullName.mangledString // addModuleSuffix(fullNameInternal('.')).toString
     def name: Name = sym.name
-    def rawname: Name = {
+    def rawname: String = {
       val original = toDenot(sym).initial
-      sym.name(ctx.withPhase(original.validFor.phaseId))
+      sym.name(ctx.withPhase(original.validFor.phaseId)).mangledString
     }
 
     // types
@@ -675,9 +666,7 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
     }
     def enclClass: Symbol = toDenot(sym).enclosingClass
     def linkedClassOfClass: Symbol = linkedClass
-    def linkedClass: Symbol = {
-      toDenot(sym)(ctx).linkedClass(ctx)
-    } //exitingPickler(sym.linkedClassOfClass)
+    def linkedClass: Symbol = toDenot(sym)(ctx).linkedClass(ctx) //exitingPickler(sym.linkedClassOfClass)
     def companionClass: Symbol = toDenot(sym).companionClass
     def companionModule: Symbol = toDenot(sym).companionModule
     def companionSymbol: Symbol = if (sym is Flags.Module) companionClass else companionModule
diff --git a/compiler/src/dotty/tools/backend/jvm/GenBCode.scala b/compiler/src/dotty/tools/backend/jvm/GenBCode.scala
index 25a8061ac05f..90f13f83a791 100644
--- a/compiler/src/dotty/tools/backend/jvm/GenBCode.scala
+++ b/compiler/src/dotty/tools/backend/jvm/GenBCode.scala
@@ -202,7 +202,7 @@ class GenBCodePipeline(val entryPoints: List[Symbol], val int: DottyBackendInter
 
         if (claszSymbol.isClass) // @DarkDimius is this test needed here?
           for (binary <- ctx.compilationUnit.pickled.get(claszSymbol.asClass)) {
-            val dataAttr = new CustomAttr(nme.TASTYATTR.toString, binary)
+            val dataAttr = new CustomAttr(nme.TASTYATTR.mangledString, binary)
             val store = if (mirrorC ne null) mirrorC else plainC
             store.visitAttribute(dataAttr)
             if (ctx.settings.emitTasty.value) {
diff --git a/compiler/src/dotty/tools/dotc/config/Config.scala b/compiler/src/dotty/tools/dotc/config/Config.scala
index 46b1896f1add..709a5233b89e 100644
--- a/compiler/src/dotty/tools/dotc/config/Config.scala
+++ b/compiler/src/dotty/tools/dotc/config/Config.scala
@@ -49,6 +49,13 @@ object Config {
    */
   final val checkNoSkolemsInInfo = false
 
+  /** Check that Name#toString is not called directly from backend by analyzing
+   *  the stack trace of each toString call on names. This is very expensive,
+   *  so not suitable for continuous testing. But it can be used to find a problem
+   *  when running a specific test.
+   */
+  final val checkBackendNames = false
+
   /** Type comparer will fail with an assert if the upper bound
    *  of a constrained parameter becomes Nothing. This should be turned
    *  on only for specific debugging as normally instantiation to Nothing
diff --git a/compiler/src/dotty/tools/dotc/core/Denotations.scala b/compiler/src/dotty/tools/dotc/core/Denotations.scala
index fd42bde3660f..ffc58f2e8d2f 100644
--- a/compiler/src/dotty/tools/dotc/core/Denotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/Denotations.scala
@@ -582,7 +582,7 @@ object Denotations {
     def hasUniqueSym: Boolean
     protected def newLikeThis(symbol: Symbol, info: Type): SingleDenotation
 
-    final def signature(implicit ctx: Context): Signature = {
+    final def signature(implicit ctx: Context): Signature =
       if (isType) Signature.NotAMethod // don't force info if this is a type SymDenotation
       else info match {
         case info: MethodicType =>
@@ -594,7 +594,6 @@ object Denotations {
           }
         case _ => Signature.NotAMethod
       }
-    }
 
     def derivedSingleDenotation(symbol: Symbol, info: Type)(implicit ctx: Context): SingleDenotation =
       if ((symbol eq this.symbol) && (info eq this.info)) this
diff --git a/compiler/src/dotty/tools/dotc/core/Names.scala b/compiler/src/dotty/tools/dotc/core/Names.scala
index a72a028449af..9fb583373b77 100644
--- a/compiler/src/dotty/tools/dotc/core/Names.scala
+++ b/compiler/src/dotty/tools/dotc/core/Names.scala
@@ -14,6 +14,7 @@ import collection.mutable.{ Builder, StringBuilder, AnyRefMap }
 import collection.immutable.WrappedString
 import collection.generic.CanBuildFrom
 import util.{DotClass, SimpleMap}
+import config.Config
 import java.util.HashMap
 
 //import annotation.volatile
@@ -67,6 +68,7 @@ object Names {
     def asSimpleName: SimpleTermName
     def toSimpleName: SimpleTermName
     def mangled: Name
+    def mangledString: String = mangled.toString
 
     def rewrite(f: PartialFunction[Name, Name]): ThisName
     def collect[T](f: PartialFunction[Name, T]): Option[T]
@@ -287,7 +289,40 @@ object Names {
     override def hashCode: Int = start
 
     override def toString =
-      if (length == 0) "" else new String(chrs, start, length)
+      if (length == 0) ""
+      else {
+        if (Config.checkBackendNames) {
+          if (!toStringOK) {
+            // We print the stacktrace instead of doing an assert directly,
+            // because asserts are caught in exception handlers which might
+            // cause other failures. In that case the first, important failure
+            // is lost.
+            println("Backend should not call Name#toString, Name#mangledString should be used instead.")
+            new Error().printStackTrace()
+            assert(false)
+          }
+        }
+        new String(chrs, start, length)
+      }
+
+    /** It's OK to take a toString if the stacktrace does not occur a method
+     *  in GenBCode or it also contains one of the whitelisted methods below.
+     */
+    private def toStringOK = {
+      val trace = Thread.currentThread.getStackTrace
+      !trace.exists(_.getClassName.endsWith("GenBCode")) ||
+      trace.exists(elem =>
+          List(
+              "mangledString",
+              "toSimpleName",
+              "decode",
+              "unmangle",
+              "dotty$tools$dotc$core$NameOps$NameDecorator$$functionArityFor$extension",
+              "dotty$tools$dotc$typer$Checking$CheckNonCyclicMap$$apply",
+              "$plus$plus",
+              "readConstant")
+            .contains(elem.getMethodName))
+    }
 
     def debugString: String = toString
   }
diff --git a/compiler/src/dotty/tools/dotc/core/StdNames.scala b/compiler/src/dotty/tools/dotc/core/StdNames.scala
index 92befdacbd9f..e364fe28d057 100644
--- a/compiler/src/dotty/tools/dotc/core/StdNames.scala
+++ b/compiler/src/dotty/tools/dotc/core/StdNames.scala
@@ -33,6 +33,7 @@ object StdNames {
     final val INTERPRETER_LINE_PREFIX    = "line"
     final val INTERPRETER_VAR_PREFIX     = "res"
     final val INTERPRETER_WRAPPER_SUFFIX = "$object"
+    final val MODULE_INSTANCE_FIELD      = NameTransformer.MODULE_INSTANCE_NAME  // "MODULE$"
 
     final val Function                   = "Function"
     final val ImplicitFunction           = "ImplicitFunction"
diff --git a/compiler/src/dotty/tools/dotc/core/SymbolLoaders.scala b/compiler/src/dotty/tools/dotc/core/SymbolLoaders.scala
index 63c2817a647e..9e19c5f05eec 100644
--- a/compiler/src/dotty/tools/dotc/core/SymbolLoaders.scala
+++ b/compiler/src/dotty/tools/dotc/core/SymbolLoaders.scala
@@ -220,7 +220,7 @@ class SymbolLoaders {
       if (!sourceModule.isCompleted)
         sourceModule.completer.complete(sourceModule)
 
-      val packageName = if (root.isEffectiveRoot) "" else root.fullName.toString
+      val packageName = if (root.isEffectiveRoot) "" else root.fullName.mangledString
 
       enterFlatClasses = Some { ctx =>
         enterFlatClasses = None
diff --git a/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala b/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala
index 9415c047fda8..eef8faf8a233 100644
--- a/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala
+++ b/compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala
@@ -677,7 +677,7 @@ class ClassfileParser(
     for (entry <- innerClasses.values) {
       // create a new class member for immediate inner classes
       if (entry.outerName == currentClassName) {
-        val file = ctx.platform.classPath.findClassFile(entry.externalName.toString) getOrElse {
+        val file = ctx.platform.classPath.findClassFile(entry.externalName.mangledString) getOrElse {
           throw new AssertionError(entry.externalName)
         }
         enterClassAndModule(entry, file, entry.jflags)
diff --git a/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala b/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
index eb7773ef3869..79de2cd88741 100644
--- a/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
+++ b/compiler/src/dotty/tools/dotc/transform/TreeChecker.scala
@@ -63,7 +63,7 @@ class TreeChecker extends Phase with SymTransformer {
   val NoSuperClass = Trait | Package
 
   def testDuplicate(sym: Symbol, registry: mutable.Map[String, Symbol], typ: String)(implicit ctx: Context) = {
-    val name = sym.fullName.toString
+    val name = sym.fullName.mangledString
     if (this.flatClasses && registry.contains(name))
         printError(s"$typ defined twice $sym ${sym.id} ${registry(name).id}")
     registry(name) = sym
diff --git a/scala-backend b/scala-backend
index ed7050296c5c..253ae6c0155a 160000
--- a/scala-backend
+++ b/scala-backend
@@ -1 +1 @@
-Subproject commit ed7050296c5c5107ae69d7a330a5706268ab1b6e
+Subproject commit 253ae6c0155a553bef4b1a312e25fc5f18f4c5e4