sdk/lib/_internal/compiler/implementation/types/inferrer_visitor.dart

// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

part of simple_types_inferrer;

/**
 * The interface [InferrerVisitor] will use when working on types.
 */
abstract class TypeSystem<T> {
  T get dynamicType;
  T get nullType;
  T get intType;
  T get doubleType;
  T get numType;
  T get boolType;
  T get functionType;
  T get listType;
  T get constListType;
  T get fixedListType;
  T get growableListType;
  T get mapType;
  T get constMapType;
  T get stringType;
  T get typeType;

  T nonNullSubtype(DartType type);
  T nonNullSubclass(DartType type);
  T nonNullExact(DartType type);
  T nonNullEmpty();
  T nullable(T type);
  Selector newTypedSelector(T receiver, Selector selector);

  T allocateContainer(T type,
                      Node node,
                      Element enclosing,
                      [T elementType, int length]);

  /**
   * Returns the least upper bound between [firstType] and
   * [secondType].
   */
  T computeLUB(T firstType, T secondType);

  /**
   * Returns the intersection between [T] and [annotation].
   * [isNullable] indicates whether the annotation implies a null
   * type.
   */
  T narrowType(T type, DartType annotation, {bool isNullable: true});

  /**
   * Returns a new type that unions [firstInput] and [secondInput].
   */
  T allocateDiamondPhi(T firstInput, T secondInput);

  /**
   * Returns a new type for holding the potential types of [element].
   * [inputType] is the first incoming type of the phi.
   */
  T allocatePhi(Node node, Element element, T inputType);

  /**
   * Simplies the phi representing [element] and of the type
   * [phiType]. For example, if this phi has one incoming input, an
   * implementation of this method could just return that incoming
   * input type.
   */
  T simplifyPhi(Node node, Element element, T phiType);

  /**
   * Adds [newType] as an input of [phiType].
   */
  T addPhiInput(Element element, T phiType, T newType);
}

/**
 * An implementation of [TypeSystem] for [TypeMask].
 */
class TypeMaskSystem implements TypeSystem<TypeMask> {
  final Compiler compiler;
  TypeMaskSystem(this.compiler);

  TypeMask narrowType(TypeMask type,
                      DartType annotation,
                      {bool isNullable: true}) {
    if (annotation.treatAsDynamic) return type;
    if (annotation.isVoid) return nullType;
    if (annotation.element == compiler.objectClass) return type;
    TypeMask otherType;
    if (annotation.kind == TypeKind.TYPEDEF
        || annotation.kind == TypeKind.FUNCTION) {
      otherType = functionType;
    } else if (annotation.kind == TypeKind.TYPE_VARIABLE) {
      // TODO(ngeoffray): Narrow to bound.
      return type;
    } else {
      assert(annotation.kind == TypeKind.INTERFACE);
      otherType = new TypeMask.nonNullSubtype(annotation);
    }
    if (isNullable) otherType = otherType.nullable();
    if (type == null) return otherType;
    return type.intersection(otherType, compiler);
  }

  TypeMask computeLUB(TypeMask firstType, TypeMask secondType) {
    if (firstType == null) {
      return secondType;
    } else if (secondType == dynamicType || firstType == dynamicType) {
      return dynamicType;
    } else if (firstType == secondType) {
      return firstType;
    } else {
      TypeMask union = firstType.union(secondType, compiler);
      // TODO(kasperl): If the union isn't nullable it seems wasteful
      // to use dynamic. Fix that.
      return union.containsAll(compiler) ? dynamicType : union;
    }
  }

  TypeMask allocateDiamondPhi(TypeMask firstType, TypeMask secondType) {
    return computeLUB(firstType, secondType);
  }

  TypeMask get dynamicType => compiler.typesTask.dynamicType;
  TypeMask get nullType => compiler.typesTask.nullType;
  TypeMask get intType => compiler.typesTask.intType;
  TypeMask get doubleType => compiler.typesTask.doubleType;
  TypeMask get numType => compiler.typesTask.numType;
  TypeMask get boolType => compiler.typesTask.boolType;
  TypeMask get functionType => compiler.typesTask.functionType;
  TypeMask get listType => compiler.typesTask.listType;
  TypeMask get constListType => compiler.typesTask.constListType;
  TypeMask get fixedListType => compiler.typesTask.fixedListType;
  TypeMask get growableListType => compiler.typesTask.growableListType;
  TypeMask get mapType => compiler.typesTask.mapType;
  TypeMask get constMapType => compiler.typesTask.constMapType;
  TypeMask get stringType => compiler.typesTask.stringType;
  TypeMask get typeType => compiler.typesTask.typeType;

  TypeMask nonNullSubtype(DartType type) => new TypeMask.nonNullSubtype(type);
  TypeMask nonNullSubclass(DartType type) => new TypeMask.nonNullSubclass(type);
  TypeMask nonNullExact(DartType type) => new TypeMask.nonNullExact(type);
  TypeMask nonNullEmpty() => new TypeMask.nonNullEmpty();

  TypeMask nullable(TypeMask type) {
    return type.nullable();
  }

  TypeMask allocateContainer(TypeMask type,
                             Node node,
                             Element enclosing,
                             [TypeMask elementType, int length]) {
    ContainerTypeMask mask = new ContainerTypeMask(type, node, enclosing);
    mask.elementType = elementType;
    mask.length = length;
    return mask;
  }

  Selector newTypedSelector(TypeMask receiver, Selector selector) {
    return new TypedSelector(receiver, selector);
  }

  TypeMask addPhiInput(Element element, TypeMask phiType, TypeMask newType) {
    return computeLUB(phiType, newType);
  }

  TypeMask allocatePhi(Node node, Element element, TypeMask inputType) {
    return inputType;
  }

  TypeMask simplifyPhi(Node node, Element element, TypeMask phiType) {
    return phiType;
  }
}

/**
 * A variable scope holds types for variables. It has a link to a
 * parent scope, but never changes the types in that parent. Instead,
 * updates to locals of a parent scope are put in the current scope.
 * The inferrer makes sure updates get merged into the parent scope,
 * once the control flow block has been visited.
 */
class VariableScope<T> {
  Map<Element, T> variables;

  /// The parent of this scope. Null for the root scope.
  final VariableScope<T> parent;

  /// The [Node] that created this scope.
  final Node block;

  VariableScope(this.block, [parent])
      : this.variables = null,
        this.parent = parent;

  VariableScope.deepCopyOf(VariableScope<T> other)
      : variables = other.variables == null
            ? null
            : new Map<Element, T>.from(other.variables),
        block = other.block,
        parent = other.parent == null
            ? null
            : new VariableScope<T>.deepCopyOf(other.parent);

  T operator [](Element variable) {
    T result;
    if (variables == null || (result = variables[variable]) == null) {
      return parent == null ? null : parent[variable];
    }
    return result;
  }

  void operator []=(Element variable, T mask) {
    assert(mask != null);
    if (variables == null) {
      variables = new Map<Element, T>();
    }
    variables[variable] = mask;
  }

  void forEachOwnLocal(void f(Element element, T type)) {
    if (variables == null) return;
    variables.forEach(f);
  }

  void forEachLocalUntil(Node node, void f(Element, T type)) {
    forEachOwnLocal(f);
    if (block == node) return;
    if (parent != null) parent.forEachLocalUntil(node, f);
  }

  void forEachLocal(void f(Element, T type)) {
    forEachLocalUntil(null, f);
  }

  void remove(Element element) {
    variables.remove(element);
  }

  String toString() {
    String rest = parent == null ? "null" : parent.toString();
    return '$variables $rest';
  }
}

class FieldInitializationScope<T> {
  final TypeSystem<T> types;
  Map<Element, T> fields;
  bool isThisExposed;

  FieldInitializationScope(this.types) : isThisExposed = false;

  FieldInitializationScope.internalFrom(FieldInitializationScope<T> other)
      : types = other.types,
        isThisExposed = other.isThisExposed;

  factory FieldInitializationScope.from(FieldInitializationScope<T> other) {
    if (other == null) return null;
    return new FieldInitializationScope<T>.internalFrom(other);
  }

  void updateField(Element field, T type) {
    if (isThisExposed) return;
    if (fields == null) fields = new Map<Element, T>();
    fields[field] = type;
  }

  T readField(Element field) {
    return fields == null ? null : fields[field];
  }

  void forEach(void f(Element element, T type)) {
    if (fields == null) return;
    fields.forEach(f);
  }

  void mergeDiamondFlow(FieldInitializationScope<T> thenScope,
                        FieldInitializationScope<T> elseScope) {
    // Quick bailout check. If [isThisExposed] is true, we know the
    // code following won't do anything.
    if (isThisExposed) return;
    if (elseScope == null || elseScope.fields == null) {
      elseScope = this;
    }

    thenScope.forEach((Element field, T type) {
      T otherType = elseScope.readField(field);
      if (otherType == null) return;
      updateField(field, types.allocateDiamondPhi(type, otherType));
    });
    isThisExposed = thenScope.isThisExposed || elseScope.isThisExposed;
  }
}

/**
 * Placeholder for inferred types of local variables.
 */
class LocalsHandler<T> {
  final Compiler compiler;
  final TypeSystem<T> types;
  final InferrerEngine<T> inferrer;
  final VariableScope<T> locals;
  final Map<Element, Element> capturedAndBoxed;
  final FieldInitializationScope<T> fieldScope;
  LocalsHandler<T> tryBlock;
  bool seenReturnOrThrow = false;
  bool seenBreakOrContinue = false;

  bool get aborts {
    return seenReturnOrThrow || seenBreakOrContinue;
  }
  bool get inTryBlock => tryBlock != null;

  LocalsHandler(this.inferrer,
                this.types,
                this.compiler,
                Node block,
                [this.fieldScope])
      : locals = new VariableScope<T>(block),
        capturedAndBoxed = new Map<Element, Element>(),
        tryBlock = null;

  LocalsHandler.from(LocalsHandler<T> other,
                     Node block,
                     {bool useOtherTryBlock: true})
      : locals = new VariableScope<T>(block, other.locals),
        fieldScope = new FieldInitializationScope<T>.from(other.fieldScope),
        capturedAndBoxed = other.capturedAndBoxed,
        types = other.types,
        inferrer = other.inferrer,
        compiler = other.compiler {
    tryBlock = useOtherTryBlock ? other.tryBlock : this;
  }

  LocalsHandler.deepCopyOf(LocalsHandler<T> other)
      : locals = new VariableScope<T>.deepCopyOf(other.locals),
        fieldScope = new FieldInitializationScope<T>.from(other.fieldScope),
        capturedAndBoxed = other.capturedAndBoxed,
        tryBlock = other.tryBlock,
        types = other.types,
        inferrer = other.inferrer,
        compiler = other.compiler;

  T use(Element local) {
    return capturedAndBoxed.containsKey(local)
        ? inferrer.typeOfElement(capturedAndBoxed[local])
        : locals[local];
  }

  void update(Element local, T type, Node node) {
    assert(type != null);
    if (compiler.trustTypeAnnotations || compiler.enableTypeAssertions) {
      type = types.narrowType(type, local.computeType(compiler));
    }
    if (capturedAndBoxed.containsKey(local)) {
      inferrer.recordTypeOfNonFinalField(
          node, capturedAndBoxed[local], type, null);
    } else if (inTryBlock) {
      // We don't know if an assignment in a try block
      // will be executed, so all assigments in that block are
      // potential types after we have left it. We update the parent
      // of the try block so that, at exit of the try block, we get
      // the right phi for it.
      T existing = tryBlock.locals.parent[local];
      T phiType = types.allocatePhi(tryBlock.locals.block, local, existing);
      T inputType = types.addPhiInput(local, phiType, type);
      tryBlock.locals.parent[local] = inputType;
      // Update the current handler unconditionnally with the new
      // type.
      locals[local] = type;
    } else {
      locals[local] = type;
    }
  }

  void setCapturedAndBoxed(Element local, Element field) {
    capturedAndBoxed[local] = field;
  }

  void mergeDiamondFlow(LocalsHandler<T> thenBranch,
                        LocalsHandler<T> elseBranch) {
    if (fieldScope != null && elseBranch != null) {
      fieldScope.mergeDiamondFlow(thenBranch.fieldScope, elseBranch.fieldScope);
    }
    seenReturnOrThrow = thenBranch.seenReturnOrThrow
        && elseBranch != null
        && elseBranch.seenReturnOrThrow;
    seenBreakOrContinue = thenBranch.seenBreakOrContinue
        && elseBranch != null
        && elseBranch.seenBreakOrContinue;
    if (aborts) return;
    if (thenBranch.aborts) {
      thenBranch = this;
      if (elseBranch == null) return;
    } else if (elseBranch == null || elseBranch.aborts) {
      elseBranch = this;
    }

    thenBranch.locals.forEachOwnLocal((Element local, T type) {
      T otherType = elseBranch.locals[local];
      if (otherType == null) return;
      T existing = locals[local];
      if (type == existing && otherType == existing) return;
      locals[local] = types.allocateDiamondPhi(type, otherType);
    });

  }

  /**
   * Merge all [LocalsHandler] in [handlers] into [:this:]. Returns
   * whether a local in [:this:] has changed.
   */
  bool mergeAll(List<LocalsHandler<T>> handlers) {
    bool changed = false;
    handlers.forEach((LocalsHandler<T> handler) {
      if (handler.seenReturnOrThrow) return;
      Node level = locals.block;
      handler.locals.forEachLocalUntil(level, (Element local, T otherType) {
        T myType = locals[local];
        if (myType == null) return;
        T newType = types.addPhiInput(local, myType, otherType);
        if (newType != myType) {
          changed = true;
          locals[local] = newType;
        }
      });
    });
    return changed;
  }

  void startLoop(Node loop) {
    locals.forEachLocal((Element element, T type) {
      T newType = types.allocatePhi(loop, element, type);
      if (newType != type) {
        locals[element] = type;
      }
    });
  }

  void endLoop(Node loop) {
    locals.forEachLocal((Element element, T type) {
      T newType = types.simplifyPhi(loop, element, type);
      if (newType != type) {
        locals[element] = type;
      }
    });
  }

  void updateField(Element element, T type) {
    fieldScope.updateField(element, type);
  }
}

abstract class InferrerVisitor<T> extends ResolvedVisitor<T> {
  final Element analyzedElement;
  final TypeSystem<T> types;
  final InferrerEngine<T> inferrer;
  final Compiler compiler;
  final Map<TargetElement, List<LocalsHandler<T>>> breaksFor =
      new Map<TargetElement, List<LocalsHandler<T>>>();
  final Map<TargetElement, List<LocalsHandler>> continuesFor =
      new Map<TargetElement, List<LocalsHandler<T>>>();
  LocalsHandler<T> locals;

  bool accumulateIsChecks = false;
  bool conditionIsSimple = false;
  List<Send> isChecks;
  int loopLevel = 0;

  bool get inLoop => loopLevel > 0;
  bool get isThisExposed {
    return analyzedElement.isGenerativeConstructor()
        ? locals.fieldScope.isThisExposed
        : true;
  }
  void set isThisExposed(value) {
    if (analyzedElement.isGenerativeConstructor()) {
      locals.fieldScope.isThisExposed = value;
    }
  }

  InferrerVisitor(Element analyzedElement,
                  this.inferrer,
                  this.types,
                  Compiler compiler,
                  [LocalsHandler<T> handler])
    : this.compiler = compiler,
      this.analyzedElement = analyzedElement,
      this.locals = handler,
      super(compiler.enqueuer.resolution.getCachedElements(analyzedElement)) {
    if (handler != null) return;
    Node node = analyzedElement.parseNode(compiler);
    FieldInitializationScope<T> fieldScope =
        analyzedElement.isGenerativeConstructor()
            ? new FieldInitializationScope<T>(types)
            : null;
    locals = new LocalsHandler<T>(inferrer, types, compiler, node, fieldScope);
  }

  T visitSendSet(SendSet node);

  T visitSuperSend(Send node);

  T visitStaticSend(Send node);

  T visitGetterSend(Send node);

  T visitClosureSend(Send node);

  T visitDynamicSend(Send node);

  T visitForIn(ForIn node);

  T visitReturn(Return node);

  T visitFunctionExpression(FunctionExpression node);

  T visitNode(Node node) {
    node.visitChildren(this);
  }

  T visitNewExpression(NewExpression node) {
    return node.send.accept(this);
  }

  T visit(Node node) {
    return node == null ? null : node.accept(this);
  }

  T visitFunctionDeclaration(FunctionDeclaration node) {
    locals.update(elements[node], types.functionType, node);
    return visit(node.function);
  }

  T visitLiteralString(LiteralString node) {
    return types.stringType;
  }

  T visitStringInterpolation(StringInterpolation node) {
    node.visitChildren(this);
    return types.stringType;
  }

  T visitStringJuxtaposition(StringJuxtaposition node) {
    node.visitChildren(this);
    return types.stringType;
  }

  T visitLiteralBool(LiteralBool node) {
    return types.boolType;
  }

  T visitLiteralDouble(LiteralDouble node) {
    return types.doubleType;
  }

  T visitLiteralInt(LiteralInt node) {
    ConstantSystem constantSystem = compiler.backend.constantSystem;
    Constant constant = constantSystem.createInt(node.value);
    // The JavaScript backend may turn this literal into a double at
    // runtime.
    return constantSystem.isDouble(constant)
        ? types.doubleType
        : types.intType;
  }

  T visitLiteralList(LiteralList node) {
    node.visitChildren(this);
    return node.isConst() ? types.constListType : types.growableListType;
  }

  T visitLiteralMap(LiteralMap node) {
    node.visitChildren(this);
    return node.isConst() ? types.constMapType : types.mapType;
  }

  T visitLiteralNull(LiteralNull node) {
    return types.nullType;
  }

  T visitTypeReferenceSend(Send node) {
    return types.typeType;
  }

  bool isThisOrSuper(Node node) => node.isThis() || node.isSuper();

  Element get outermostElement {
    return
        analyzedElement.getOutermostEnclosingMemberOrTopLevel().implementation;
  }

  T _thisType;
  T get thisType {
    if (_thisType != null) return _thisType;
    ClassElement cls = outermostElement.getEnclosingClass();
    if (compiler.world.isUsedAsMixin(cls)) {
      return _thisType = types.nonNullSubtype(cls.rawType);
    } else if (compiler.world.hasAnySubclass(cls)) {
      return _thisType = types.nonNullSubclass(cls.rawType);
    } else {
      return _thisType = types.nonNullExact(cls.rawType);
    }
  }

  T _superType;
  T get superType {
    if (_superType != null) return _superType;
    return _superType = types.nonNullExact(
        outermostElement.getEnclosingClass().superclass.rawType);
  }

  T visitIdentifier(Identifier node) {
    if (node.isThis()) {
      return thisType;
    } else if (node.isSuper()) {
      return superType;
    }
  }

  void potentiallyAddIsCheck(Send node) {
    if (!accumulateIsChecks) return;
    if (!Elements.isLocal(elements[node.receiver])) return;
    isChecks.add(node);
  }

  void updateIsChecks(List<Node> tests, {bool usePositive}) {
    if (tests == null) return;
    for (Send node in tests) {
      if (node.isIsNotCheck) {
        if (usePositive) continue;
      } else {
        if (!usePositive) continue;
      }
      DartType type = elements.getType(node.typeAnnotationFromIsCheckOrCast);
      Element element = elements[node.receiver];
      T existing = locals.use(element);
      T newType = types.narrowType(existing, type, isNullable: false);
      locals.update(element, newType, node);
    }
  }

  T visitOperatorSend(Send node) {
    Operator op = node.selector;
    if (const SourceString("[]") == op.source) {
      return visitDynamicSend(node);
    } else if (const SourceString("&&") == op.source) {
      conditionIsSimple = false;
      bool oldAccumulateIsChecks = accumulateIsChecks;
      accumulateIsChecks = true;
      if (isChecks == null) isChecks = <Send>[];
      visit(node.receiver);
      accumulateIsChecks = oldAccumulateIsChecks;
      if (!accumulateIsChecks) isChecks = null;
      LocalsHandler<T> saved = locals;
      locals = new LocalsHandler<T>.from(locals, node);
      updateIsChecks(isChecks, usePositive: true);
      visit(node.arguments.head);
      saved.mergeDiamondFlow(locals, null);
      locals = saved;
      return types.boolType;
    } else if (const SourceString("||") == op.source) {
      conditionIsSimple = false;
      visit(node.receiver);
      LocalsHandler<T> saved = locals;
      locals = new LocalsHandler<T>.from(locals, node);
      updateIsChecks(isChecks, usePositive: false);
      bool oldAccumulateIsChecks = accumulateIsChecks;
      accumulateIsChecks = false;
      visit(node.arguments.head);
      accumulateIsChecks = oldAccumulateIsChecks;
      saved.mergeDiamondFlow(locals, null);
      locals = saved;
      return types.boolType;
    } else if (const SourceString("!") == op.source) {
      bool oldAccumulateIsChecks = accumulateIsChecks;
      accumulateIsChecks = false;
      node.visitChildren(this);
      accumulateIsChecks = oldAccumulateIsChecks;
      return types.boolType;
    } else if (const SourceString("is") == op.source) {
      potentiallyAddIsCheck(node);
      node.visitChildren(this);
      return types.boolType;
    } else if (const SourceString("as") == op.source) {
      T receiverType = visit(node.receiver);
      DartType type = elements.getType(node.arguments.head);
      return types.narrowType(receiverType, type);
    } else if (node.argumentsNode is Prefix) {
      // Unary operator.
      return visitDynamicSend(node);
    } else if (const SourceString('===') == op.source
               || const SourceString('!==') == op.source) {
      node.visitChildren(this);
      return types.boolType;
    } else {
      // Binary operator.
      return visitDynamicSend(node);
    }
  }

  // Because some nodes just visit their children, we may end up
  // visiting a type annotation, that may contain a send in case of a
  // prefixed type. Therefore we explicitly visit the type annotation
  // to avoid confusing the [ResolvedVisitor].
  visitTypeAnnotation(TypeAnnotation node) {}

  T visitConditional(Conditional node) {
    List<Send> tests = <Send>[];
    bool simpleCondition = handleCondition(node.condition, tests);
    LocalsHandler<T> saved = locals;
    locals = new LocalsHandler<T>.from(locals, node);
    updateIsChecks(tests, usePositive: true);
    T firstType = visit(node.thenExpression);
    LocalsHandler<T> thenLocals = locals;
    locals = new LocalsHandler<T>.from(saved, node);
    if (simpleCondition) updateIsChecks(tests, usePositive: false);
    T secondType = visit(node.elseExpression);
    saved.mergeDiamondFlow(thenLocals, locals);
    locals = saved;
    T type = types.allocateDiamondPhi(firstType, secondType);
    return type;
  }

  T visitVariableDefinitions(VariableDefinitions node) {
    for (Link<Node> link = node.definitions.nodes;
         !link.isEmpty;
         link = link.tail) {
      Node definition = link.head;
      if (definition is Identifier) {
        locals.update(elements[definition], types.nullType, node);
      } else {
        assert(definition.asSendSet() != null);
        visit(definition);
      }
    }
  }

  bool handleCondition(Node node, List<Send> tests) {
    bool oldConditionIsSimple = conditionIsSimple;
    bool oldAccumulateIsChecks = accumulateIsChecks;
    List<Send> oldIsChecks = isChecks;
    accumulateIsChecks = true;
    conditionIsSimple = true;
    isChecks = tests;
    visit(node);
    bool simpleCondition = conditionIsSimple;
    accumulateIsChecks = oldAccumulateIsChecks;
    isChecks = oldIsChecks;
    conditionIsSimple = oldConditionIsSimple;
    return simpleCondition;
  }

  T visitIf(If node) {
    List<Send> tests = <Send>[];
    bool simpleCondition = handleCondition(node.condition, tests);
    LocalsHandler<T> saved = locals;
    locals = new LocalsHandler<T>.from(locals, node);
    updateIsChecks(tests, usePositive: true);
    visit(node.thenPart);
    LocalsHandler<T> thenLocals = locals;
    locals = new LocalsHandler<T>.from(saved, node);
    if (simpleCondition) updateIsChecks(tests, usePositive: false);
    visit(node.elsePart);
    saved.mergeDiamondFlow(thenLocals, locals);
    locals = saved;
  }

  void setupBreaksAndContinues(TargetElement element) {
    if (element == null) return;
    if (element.isContinueTarget) continuesFor[element] = <LocalsHandler>[];
    if (element.isBreakTarget) breaksFor[element] = <LocalsHandler>[];
  }

  void clearBreaksAndContinues(TargetElement element) {
    continuesFor.remove(element);
    breaksFor.remove(element);
  }

  List<LocalsHandler<T>> getBreaks(TargetElement element) {
    List<LocalsHandler<T>> list = <LocalsHandler<T>>[locals];
    if (element == null) return list;
    if (!element.isBreakTarget) return list;
    return list..addAll(breaksFor[element]);
  }

  List<LocalsHandler<T>> getLoopBackEdges(TargetElement element) {
    List<LocalsHandler<T>> list = <LocalsHandler<T>>[locals];
    if (element == null) return list;
    if (!element.isContinueTarget) return list;
    return list..addAll(continuesFor[element]);
  }

  T handleLoop(Node node, void logic()) {
    loopLevel++;
    bool changed = false;
    TargetElement target = elements[node];
    setupBreaksAndContinues(target);
    locals.startLoop(node);
    LocalsHandler<T> saved;
    do {
      saved = locals;
      locals = new LocalsHandler<T>.from(locals, node);
      logic();
      changed = saved.mergeAll(getLoopBackEdges(target));
      locals = saved;
    } while (changed);
    loopLevel--;
    saved.mergeAll(getBreaks(target));
    locals = saved;
    locals.endLoop(node);
    clearBreaksAndContinues(target);
  }

  T visitWhile(While node) {
    return handleLoop(node, () {
      List<Send> tests = <Send>[];
      handleCondition(node.condition, tests);
      updateIsChecks(tests, usePositive: true);
      visit(node.body);
    });
  }

  T visitDoWhile(DoWhile node) {
    return handleLoop(node, () {
      visit(node.body);
      List<Send> tests = <Send>[];
      handleCondition(node.condition, tests);
      updateIsChecks(tests, usePositive: true);
    });
  }

  T visitFor(For node) {
    visit(node.initializer);
    return handleLoop(node, () {
      List<Send> tests = <Send>[];
      handleCondition(node.condition, tests);
      updateIsChecks(tests, usePositive: true);
      visit(node.body);
      visit(node.update);
    });
  }

  T visitTryStatement(TryStatement node) {
    LocalsHandler<T> saved = locals;
    locals = new LocalsHandler<T>.from(
        locals, node, useOtherTryBlock: false);
    visit(node.tryBlock);
    saved.mergeDiamondFlow(locals, null);
    locals = saved;
    for (Node catchBlock in node.catchBlocks) {
      saved = locals;
      locals = new LocalsHandler<T>.from(locals, catchBlock);
      visit(catchBlock);
      saved.mergeDiamondFlow(locals, null);
      locals = saved;
    }
    visit(node.finallyBlock);
  }

  T visitThrow(Throw node) {
    node.visitChildren(this);
    locals.seenReturnOrThrow = true;
    return types.dynamicType;
  }

  T visitCatchBlock(CatchBlock node) {
    Node exception = node.exception;
    if (exception != null) {
      DartType type = elements.getType(node.type);
      T mask = type == null || type.treatAsDynamic
          ? types.dynamicType
          : types.nonNullSubtype(type.asRaw());
      locals.update(elements[exception], mask, node);
    }
    Node trace = node.trace;
    if (trace != null) {
      locals.update(elements[trace], types.dynamicType, node);
    }
    visit(node.block);
  }

  T visitParenthesizedExpression(ParenthesizedExpression node) {
    return visit(node.expression);
  }

  T visitBlock(Block node) {
    if (node.statements != null) {
      for (Node statement in node.statements) {
        visit(statement);
        if (locals.aborts) break;
      }
    }
  }

  T visitLabeledStatement(LabeledStatement node) {
    Statement body = node.statement;
    if (body is Loop
        || body is SwitchStatement
        || Elements.isUnusedLabel(node, elements)) {
      // Loops and switches handle their own labels.
      visit(body);
    } else {
      TargetElement targetElement = elements[body];
      setupBreaksAndContinues(targetElement);
      visit(body);
      locals.mergeAll(getBreaks(targetElement));
      clearBreaksAndContinues(targetElement);
    }
  }

  T visitBreakStatement(BreakStatement node) {
    TargetElement target = elements[node];
    locals.seenBreakOrContinue = true;
    // Do a deep-copy of the locals, because the code following the
    // break will change them.
    breaksFor[target].add(new LocalsHandler<T>.deepCopyOf(locals));
  }

  T visitContinueStatement(ContinueStatement node) {
    TargetElement target = elements[node];
    locals.seenBreakOrContinue = true;
    // Do a deep-copy of the locals, because the code following the
    // continue will change them.
    continuesFor[target].add(new LocalsHandler<T>.deepCopyOf(locals));
  }

  void internalError(String reason, {Node node}) {
    compiler.internalError(reason, node: node);
  }

  T visitSwitchStatement(SwitchStatement node) {
    visit(node.parenthesizedExpression);

    setupBreaksAndContinues(elements[node]);
    if (Elements.switchStatementHasContinue(node, elements)) {
      void forEachLabeledCase(void action(TargetElement target)) {
        for (SwitchCase switchCase in node.cases) {
          for (Node labelOrCase in switchCase.labelsAndCases) {
            if (labelOrCase.asLabel() == null) continue;
            LabelElement labelElement = elements[labelOrCase];
            if (labelElement != null) {
              action(labelElement.target);
            }
          }
        }
      }

      forEachLabeledCase((TargetElement target) {
        setupBreaksAndContinues(target);
      });

      // If the switch statement has a continue, we conservatively
      // visit all cases and update [locals] until we have reached a
      // fixed point.
      bool changed;
      locals.startLoop(node);
      do {
        changed = false;
        for (Node switchCase in node.cases) {
          LocalsHandler<T> saved = locals;
          locals = new LocalsHandler<T>.from(locals, switchCase);
          visit(switchCase);
          changed = saved.mergeAll([locals]) || changed;
          locals = saved;
        }
      } while (changed);
      locals.endLoop(node);

      forEachLabeledCase((TargetElement target) {
        clearBreaksAndContinues(target);
      });
    } else {
      LocalsHandler<T> saved = locals;
      List<LocalsHandler<T>> localsToMerge = <LocalsHandler<T>>[];

      for (SwitchCase switchCase in node.cases) {
        locals = new LocalsHandler<T>.from(saved, switchCase);
        visit(switchCase);
        localsToMerge.add(locals);
      }
      saved.mergeAll(localsToMerge);
      locals = saved;
    }
    clearBreaksAndContinues(elements[node]);
    // In case there is a default in the switch we discard the
    // incoming localsHandler, because the types it holds do not need
    // to be merged after the switch statement. This means that, if all
    // cases, including the default, break or continue, the [result]
    // handler may think it just aborts the current block. Therefore
    // we set the current locals to not have any break or continue, so
    // that the [visitBlock] method does not assume the code after the
    // switch is dead code.
    locals.seenBreakOrContinue = false;
  }

  T visitCascadeReceiver(CascadeReceiver node) {
    // TODO(ngeoffray): Implement.
    node.visitChildren(this);
    return types.dynamicType;
  }

  T visitCascade(Cascade node) {
    // TODO(ngeoffray): Implement.
    node.visitChildren(this);
    return types.dynamicType;
  }
}