sdk/pkg/kernel/lib/transformations/continuation.dart

// Copyright (c) 2016, 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.

library kernel.transformations.continuation;

import 'dart:math' as math;

import '../ast.dart';
import '../core_types.dart';
import '../visitor.dart';

import 'async.dart';

class ContinuationVariables {
  static const awaitJumpVar = ':await_jump_var';
  static const awaitContextVar = ':await_ctx_var';
  static const exceptionParam = ':exception';
  static const stackTraceParam = ':stack_trace';

  static String savedTryContextVar(int depth) => ':saved_try_context_var$depth';
  static String exceptionVar(int depth) => ':exception$depth';
  static String stackTraceVar(int depth) => ':stack_trace$depth';
}

void transformLibraries(CoreTypes coreTypes, List<Library> libraries) {
  var helper = new HelperNodes.fromCoreTypes(coreTypes);
  var rewriter = new RecursiveContinuationRewriter(helper);
  for (var library in libraries) {
    rewriter.rewriteLibrary(library);
  }
}

Component transformComponent(CoreTypes coreTypes, Component component) {
  var helper = new HelperNodes.fromCoreTypes(coreTypes);
  var rewriter = new RecursiveContinuationRewriter(helper);
  return rewriter.rewriteComponent(component);
}

Procedure transformProcedure(CoreTypes coreTypes, Procedure procedure) {
  var helper = new HelperNodes.fromCoreTypes(coreTypes);
  var rewriter = new RecursiveContinuationRewriter(helper);
  return rewriter.visitProcedure(procedure);
}

class RecursiveContinuationRewriter extends Transformer {
  final HelperNodes helper;

  final VariableDeclaration asyncJumpVariable = new VariableDeclaration(
      ContinuationVariables.awaitJumpVar,
      initializer: new IntLiteral(0));
  final VariableDeclaration asyncContextVariable =
      new VariableDeclaration(ContinuationVariables.awaitContextVar);

  RecursiveContinuationRewriter(this.helper);

  Component rewriteComponent(Component node) {
    return node.accept(this);
  }

  Library rewriteLibrary(Library node) {
    return node.accept(this);
  }

  visitProcedure(Procedure node) {
    return node.isAbstract ? node : super.visitProcedure(node);
  }

  visitFunctionNode(FunctionNode node) {
    switch (node.asyncMarker) {
      case AsyncMarker.Sync:
      case AsyncMarker.SyncYielding:
        node.transformChildren(new RecursiveContinuationRewriter(helper));
        return node;
      case AsyncMarker.SyncStar:
        return new SyncStarFunctionRewriter(helper, node).rewrite();
      case AsyncMarker.Async:
        return new AsyncFunctionRewriter(helper, node).rewrite();
      case AsyncMarker.AsyncStar:
        return new AsyncStarFunctionRewriter(helper, node).rewrite();
      default:
        return null;
    }
  }
}

abstract class ContinuationRewriterBase extends RecursiveContinuationRewriter {
  final FunctionNode enclosingFunction;

  int currentTryDepth = 0; // Nesting depth for try-blocks.
  int currentCatchDepth = 0; // Nesting depth for catch-blocks.
  int capturedTryDepth = 0; // Deepest yield point within a try-block.
  int capturedCatchDepth = 0; // Deepest yield point within a catch-block.

  ContinuationRewriterBase(HelperNodes helper, this.enclosingFunction)
      : super(helper);

  /// Given a container [type], which is an instantiation of the given
  /// [containerClass] extract its element type.
  ///
  /// This is used to extract element type from Future<T>, Iterable<T> and
  /// Stream<T> instantiations.
  ///
  /// If instantiation is not valid (has more than 1 type argument) then
  /// this function returns [InvalidType].
  static DartType elementTypeFrom(Class containerClass, DartType type) {
    if (type is InterfaceType) {
      if (type.classNode == containerClass) {
        if (type.typeArguments.length == 0) {
          return const DynamicType();
        } else if (type.typeArguments.length == 1) {
          return type.typeArguments[0];
        } else {
          return const InvalidType();
        }
      }
    }
    return const DynamicType();
  }

  DartType elementTypeFromReturnType(Class expected) =>
      elementTypeFrom(expected, enclosingFunction.returnType);

  Statement createContinuationPoint([Expression value]) {
    if (value == null) value = new NullLiteral();
    capturedTryDepth = math.max(capturedTryDepth, currentTryDepth);
    capturedCatchDepth = math.max(capturedCatchDepth, currentCatchDepth);
    return new YieldStatement(value, isNative: true);
  }

  TreeNode visitTryCatch(TryCatch node) {
    if (node.body != null) {
      ++currentTryDepth;
      node.body = node.body.accept(this);
      node.body?.parent = node;
      --currentTryDepth;
    }

    ++currentCatchDepth;
    transformList(node.catches, this, node);
    --currentCatchDepth;
    return node;
  }

  TreeNode visitTryFinally(TryFinally node) {
    if (node.body != null) {
      ++currentTryDepth;
      node.body = node.body.accept(this);
      node.body?.parent = node;
      --currentTryDepth;
    }
    if (node.finalizer != null) {
      ++currentCatchDepth;
      node.finalizer = node.finalizer.accept(this);
      node.finalizer?.parent = node;
      --currentCatchDepth;
    }
    return node;
  }

  Iterable<VariableDeclaration> createCapturedTryVariables() =>
      new Iterable.generate(
          capturedTryDepth,
          (depth) => new VariableDeclaration(
              ContinuationVariables.savedTryContextVar(depth)));

  Iterable<VariableDeclaration> createCapturedCatchVariables() =>
      new Iterable.generate(capturedCatchDepth).expand((depth) => [
            new VariableDeclaration(ContinuationVariables.exceptionVar(depth)),
            new VariableDeclaration(ContinuationVariables.stackTraceVar(depth)),
          ]);

  List<VariableDeclaration> variableDeclarations() =>
      [asyncJumpVariable, asyncContextVariable]
        ..addAll(createCapturedTryVariables())
        ..addAll(createCapturedCatchVariables());
}

class SyncStarFunctionRewriter extends ContinuationRewriterBase {
  final VariableDeclaration iteratorVariable;

  SyncStarFunctionRewriter(HelperNodes helper, FunctionNode enclosingFunction)
      : iteratorVariable = new VariableDeclaration(':iterator')
          ..type = new InterfaceType(helper.syncIteratorClass, [
            ContinuationRewriterBase.elementTypeFrom(
                helper.iterableClass, enclosingFunction.returnType)
          ]),
        super(helper, enclosingFunction);

  FunctionNode rewrite() {
    // :sync_op(:iterator) {
    //     modified <node.body>;
    // }

    // Note: SyncYielding functions have no Dart equivalent. Since they are
    // synchronous, we use Sync. (Note also that the Dart VM backend uses the
    // Dart async marker to decide if functions are debuggable.)
    final nestedClosureVariable = new VariableDeclaration(":sync_op");
    final function = new FunctionNode(buildClosureBody(),
        positionalParameters: [iteratorVariable],
        requiredParameterCount: 1,
        asyncMarker: AsyncMarker.SyncYielding,
        dartAsyncMarker: AsyncMarker.Sync)
      ..fileOffset = enclosingFunction.fileOffset
      ..fileEndOffset = enclosingFunction.fileEndOffset
      ..returnType = helper.coreTypes.boolClass.rawType;

    final closureFunction =
        new FunctionDeclaration(nestedClosureVariable, function)
          ..fileOffset = enclosingFunction.parent.fileOffset;

    // return new _SyncIterable<T>(:sync_body);
    final arguments = new Arguments([new VariableGet(nestedClosureVariable)],
        types: [elementTypeFromReturnType(helper.iterableClass)]);
    final returnStatement = new ReturnStatement(
        new ConstructorInvocation(helper.syncIterableConstructor, arguments));

    enclosingFunction.body = new Block([]
      ..addAll(variableDeclarations())
      ..addAll([closureFunction, returnStatement]));
    enclosingFunction.body.parent = enclosingFunction;
    enclosingFunction.asyncMarker = AsyncMarker.Sync;
    return enclosingFunction;
  }

  Statement buildClosureBody() {
    // The body will insert calls to
    //    :iterator.current_=
    //    :iterator.isYieldEach=
    // and return `true` as long as it did something and `false` when it's done.
    return new Block(<Statement>[
      enclosingFunction.body.accept(this),
      new ReturnStatement(new BoolLiteral(false))
        ..fileOffset = enclosingFunction.fileEndOffset
    ]);
  }

  visitYieldStatement(YieldStatement node) {
    Expression transformedExpression = node.expression.accept(this);

    var statements = <Statement>[];
    if (node.isYieldStar) {
      statements.add(new ExpressionStatement(new PropertySet(
          new VariableGet(iteratorVariable),
          new Name("_yieldEachIterable", helper.coreLibrary),
          transformedExpression,
          helper.syncIteratorYieldEachIterable)));
    } else {
      statements.add(new ExpressionStatement(new PropertySet(
          new VariableGet(iteratorVariable),
          new Name("_current", helper.coreLibrary),
          transformedExpression,
          helper.syncIteratorCurrent)));
    }

    statements.add(createContinuationPoint(new BoolLiteral(true))
      ..fileOffset = node.fileOffset);
    return new Block(statements);
  }

  TreeNode visitReturnStatement(ReturnStatement node) {
    // sync* functions cannot return a value.
    assert(node.expression == null || node.expression is NullLiteral);
    node.expression = new BoolLiteral(false)..parent = node;
    return node;
  }
}

abstract class AsyncRewriterBase extends ContinuationRewriterBase {
  final VariableDeclaration nestedClosureVariable =
      new VariableDeclaration(":async_op");
  final VariableDeclaration stackTraceVariable =
      new VariableDeclaration(":async_stack_trace");
  final VariableDeclaration thenContinuationVariable =
      new VariableDeclaration(":async_op_then");
  final VariableDeclaration catchErrorContinuationVariable =
      new VariableDeclaration(":async_op_error");

  LabeledStatement labeledBody;

  ExpressionLifter expressionRewriter;

  AsyncRewriterBase(HelperNodes helper, FunctionNode enclosingFunction)
      : super(helper, enclosingFunction) {}

  void setupAsyncContinuations(List<Statement> statements) {
    expressionRewriter = new ExpressionLifter(this);

    // var :async_stack_trace;
    statements.add(stackTraceVariable);

    // var :async_op_then;
    statements.add(thenContinuationVariable);

    // var :async_op_error;
    statements.add(catchErrorContinuationVariable);

    // :async_op([:result, :exception, :stack_trace]) {
    //     modified <node.body>;
    // }
    final parameters = <VariableDeclaration>[
      expressionRewriter.asyncResult,
      new VariableDeclaration(ContinuationVariables.exceptionParam),
      new VariableDeclaration(ContinuationVariables.stackTraceParam),
    ];

    // Note: SyncYielding functions have no Dart equivalent. Since they are
    // synchronous, we use Sync. (Note also that the Dart VM backend uses the
    // Dart async marker to decide if functions are debuggable.)
    final function = new FunctionNode(buildWrappedBody(),
        positionalParameters: parameters,
        requiredParameterCount: 0,
        asyncMarker: AsyncMarker.SyncYielding,
        dartAsyncMarker: AsyncMarker.Sync)
      ..fileOffset = enclosingFunction.fileOffset
      ..fileEndOffset = enclosingFunction.fileEndOffset;

    // The await expression lifter might have created a number of
    // [VariableDeclarations].
    // TODO(kustermann): If we didn't need any variables we should not emit
    // these.
    statements.addAll(variableDeclarations());
    statements.addAll(expressionRewriter.variables);

    // Now add the closure function itself.
    final closureFunction =
        new FunctionDeclaration(nestedClosureVariable, function)
          ..fileOffset = enclosingFunction.parent.fileOffset;
    statements.add(closureFunction);

    // :async_stack_trace = _asyncStackTraceHelper(asyncBody);
    final stackTrace = new StaticInvocation(helper.asyncStackTraceHelper,
        new Arguments(<Expression>[new VariableGet(nestedClosureVariable)]));
    final stackTraceAssign = new ExpressionStatement(
        new VariableSet(stackTraceVariable, stackTrace));
    statements.add(stackTraceAssign);

    // :async_op_then = _asyncThenWrapperHelper(asyncBody);
    final boundThenClosure = new StaticInvocation(helper.asyncThenWrapper,
        new Arguments(<Expression>[new VariableGet(nestedClosureVariable)]));
    final thenClosureVariableAssign = new ExpressionStatement(
        new VariableSet(thenContinuationVariable, boundThenClosure));
    statements.add(thenClosureVariableAssign);

    // :async_op_error = _asyncErrorWrapperHelper(asyncBody);
    final boundCatchErrorClosure = new StaticInvocation(
        helper.asyncErrorWrapper,
        new Arguments(<Expression>[new VariableGet(nestedClosureVariable)]));
    final catchErrorClosureVariableAssign = new ExpressionStatement(
        new VariableSet(
            catchErrorContinuationVariable, boundCatchErrorClosure));
    statements.add(catchErrorClosureVariableAssign);
  }

  Statement buildWrappedBody() {
    ++currentTryDepth;
    labeledBody = new LabeledStatement(null);
    labeledBody.body = visitDelimited(enclosingFunction.body)
      ..parent = labeledBody;
    --currentTryDepth;

    var exceptionVariable = new VariableDeclaration(":exception");
    var stackTraceVariable = new VariableDeclaration(":stack_trace");

    return new TryCatch(
      buildReturn(labeledBody),
      <Catch>[
        new Catch(
            exceptionVariable,
            new Block(<Statement>[
              buildCatchBody(exceptionVariable, stackTraceVariable)
            ]),
            stackTrace: stackTraceVariable)
      ],
      isSynthetic: true,
    );
  }

  Statement buildCatchBody(
      Statement exceptionVariable, Statement stackTraceVariable);

  Statement buildReturn(Statement body);

  List<Statement> statements = <Statement>[];

  TreeNode visitExpressionStatement(ExpressionStatement stmt) {
    stmt.expression = expressionRewriter.rewrite(stmt.expression, statements)
      ..parent = stmt;
    statements.add(stmt);
    return null;
  }

  TreeNode visitBlock(Block stmt) {
    var saved = statements;
    statements = <Statement>[];
    for (var statement in stmt.statements) {
      statement.accept(this);
    }
    saved.add(new Block(statements));
    statements = saved;
    return null;
  }

  TreeNode visitEmptyStatement(EmptyStatement stmt) {
    statements.add(stmt);
    return null;
  }

  TreeNode visitAssertBlock(AssertBlock stmt) {
    var saved = statements;
    statements = <Statement>[];
    for (var statement in stmt.statements) {
      statement.accept(this);
    }
    saved.add(new Block(statements));
    statements = saved;
    return null;
  }

  TreeNode visitAssertStatement(AssertStatement stmt) {
    var condEffects = <Statement>[];
    var cond = expressionRewriter.rewrite(stmt.condition, condEffects);
    if (stmt.message == null) {
      stmt.condition = cond..parent = stmt;
      // If the translation of the condition produced a non-empty list of
      // statements, ensure they are guarded by whether asserts are enabled.
      statements.add(
          condEffects.isEmpty ? stmt : new AssertBlock(condEffects..add(stmt)));
      return null;
    }

    // The translation depends on the translation of the message, by cases.
    Statement result;
    var msgEffects = <Statement>[];
    stmt.message = expressionRewriter.rewrite(stmt.message, msgEffects)
      ..parent = stmt;
    if (condEffects.isEmpty) {
      if (msgEffects.isEmpty) {
        // The condition rewrote to ([], C) and the message rewrote to ([], M).
        // The result is
        //
        // assert(C, M)
        stmt.condition = cond..parent = stmt;
        result = stmt;
      } else {
        // The condition rewrote to ([], C) and the message rewrote to (S*, M)
        // where S* is non-empty.  The result is
        //
        // assert { if (C) {} else { S*; assert(false, M); }}
        stmt.condition = new BoolLiteral(false)..parent = stmt;
        result = new AssertBlock([
          new IfStatement(
              cond, new EmptyStatement(), new Block(msgEffects..add(stmt)))
        ]);
      }
    } else {
      if (msgEffects.isEmpty) {
        // The condition rewrote to (S*, C) where S* is non-empty and the
        // message rewrote to ([], M).  The result is
        //
        // assert { S*; assert(C, M); }
        stmt.condition = cond..parent = stmt;
        condEffects.add(stmt);
      } else {
        // The condition rewrote to (S0*, C) and the message rewrote to (S1*, M)
        // where both S0* and S1* are non-empty.  The result is
        //
        // assert { S0*; if (C) {} else { S1*; assert(false, M); }}
        stmt.condition = new BoolLiteral(false)..parent = stmt;
        condEffects.add(new IfStatement(
            cond, new EmptyStatement(), new Block(msgEffects..add(stmt))));
      }
      result = new AssertBlock(condEffects);
    }
    statements.add(result);
    return null;
  }

  Statement visitDelimited(Statement stmt) {
    var saved = statements;
    statements = <Statement>[];
    stmt.accept(this);
    Statement result =
        statements.length == 1 ? statements.first : new Block(statements);
    statements = saved;
    return result;
  }

  Statement visitLabeledStatement(LabeledStatement stmt) {
    stmt.body = visitDelimited(stmt.body)..parent = stmt;
    statements.add(stmt);
    return null;
  }

  Statement visitBreakStatement(BreakStatement stmt) {
    statements.add(stmt);
    return null;
  }

  TreeNode visitWhileStatement(WhileStatement stmt) {
    Statement body = visitDelimited(stmt.body);
    List<Statement> effects = <Statement>[];
    Expression cond = expressionRewriter.rewrite(stmt.condition, effects);
    if (effects.isEmpty) {
      stmt.condition = cond..parent = stmt;
      stmt.body = body..parent = stmt;
      statements.add(stmt);
    } else {
      // The condition rewrote to a non-empty sequence of statements S* and
      // value V.  Rewrite the loop to:
      //
      // L: while (true) {
      //   S*
      //   if (V) {
      //     [body]
      //   else {
      //     break L;
      //   }
      // }
      LabeledStatement labeled = new LabeledStatement(stmt);
      stmt.condition = new BoolLiteral(true)..parent = stmt;
      effects.add(new IfStatement(cond, body, new BreakStatement(labeled)));
      stmt.body = new Block(effects)..parent = stmt;
      statements.add(labeled);
    }
    return null;
  }

  TreeNode visitDoStatement(DoStatement stmt) {
    Statement body = visitDelimited(stmt.body);
    List<Statement> effects = <Statement>[];
    stmt.condition = expressionRewriter.rewrite(stmt.condition, effects)
      ..parent = stmt;
    if (effects.isNotEmpty) {
      // The condition rewrote to a non-empty sequence of statements S* and
      // value V.  Add the statements to the end of the loop body.
      Block block = body is Block ? body : body = new Block(<Statement>[body]);
      for (var effect in effects) {
        block.statements.add(effect);
        effect.parent = body;
      }
    }
    stmt.body = body..parent = stmt;
    statements.add(stmt);
    return null;
  }

  TreeNode visitForStatement(ForStatement stmt) {
    // Because of for-loop scoping and variable capture, it is tricky to deal
    // with await in the loop's variable initializers or update expressions.
    bool isSimple = true;
    int length = stmt.variables.length;
    List<List<Statement>> initEffects = new List<List<Statement>>(length);
    for (int i = 0; i < length; ++i) {
      VariableDeclaration decl = stmt.variables[i];
      initEffects[i] = <Statement>[];
      if (decl.initializer != null) {
        decl.initializer = expressionRewriter.rewrite(
            decl.initializer, initEffects[i])
          ..parent = decl;
      }
      isSimple = isSimple && initEffects[i].isEmpty;
    }

    length = stmt.updates.length;
    List<List<Statement>> updateEffects = new List<List<Statement>>(length);
    for (int i = 0; i < length; ++i) {
      updateEffects[i] = <Statement>[];
      stmt.updates[i] = expressionRewriter.rewrite(
          stmt.updates[i], updateEffects[i])
        ..parent = stmt;
      isSimple = isSimple && updateEffects[i].isEmpty;
    }

    Statement body = visitDelimited(stmt.body);
    Expression cond = stmt.condition;
    List<Statement> condEffects;
    if (cond != null) {
      condEffects = <Statement>[];
      cond = expressionRewriter.rewrite(stmt.condition, condEffects);
    }

    if (isSimple) {
      // If the condition contains await, we use a translation like the one for
      // while loops, but leaving the variable declarations and the update
      // expressions in place.
      if (condEffects == null || condEffects.isEmpty) {
        if (cond != null) stmt.condition = cond..parent = stmt;
        stmt.body = body..parent = stmt;
        statements.add(stmt);
      } else {
        LabeledStatement labeled = new LabeledStatement(stmt);
        // No condition in a for loop is the same as true.
        stmt.condition = null;
        condEffects
            .add(new IfStatement(cond, body, new BreakStatement(labeled)));
        stmt.body = new Block(condEffects)..parent = stmt;
        statements.add(labeled);
      }
      return null;
    }

    // If the rewrite of the initializer or update expressions produces a
    // non-empty sequence of statements then the loop is desugared.  If the loop
    // has the form:
    //
    // label: for (Type x = init; cond; update) body
    //
    // it is translated as if it were:
    //
    // {
    //   bool first = true;
    //   Type temp;
    //   label: while (true) {
    //     Type x;
    //     if (first) {
    //       first = false;
    //       x = init;
    //     } else {
    //       x = temp;
    //       update;
    //     }
    //     if (cond) {
    //       body;
    //       temp = x;
    //     } else {
    //       break;
    //     }
    //   }
    // }

    // Place the loop variable declarations at the beginning of the body
    // statements and move their initializers to a guarded list of statements.
    // Add assignments to the loop variables from the previous iteration's temp
    // variables before the updates.
    //
    // temps.first is the flag 'first'.
    // TODO(kmillikin) bool type for first.
    List<VariableDeclaration> temps = <VariableDeclaration>[
      new VariableDeclaration.forValue(new BoolLiteral(true), isFinal: false)
    ];
    List<Statement> loopBody = <Statement>[];
    List<Statement> initializers = <Statement>[
      new ExpressionStatement(
          new VariableSet(temps.first, new BoolLiteral(false)))
    ];
    List<Statement> updates = <Statement>[];
    List<Statement> newBody = <Statement>[body];
    for (int i = 0; i < stmt.variables.length; ++i) {
      VariableDeclaration decl = stmt.variables[i];
      temps.add(new VariableDeclaration(null, type: decl.type));
      loopBody.add(decl);
      if (decl.initializer != null) {
        initializers.addAll(initEffects[i]);
        initializers.add(
            new ExpressionStatement(new VariableSet(decl, decl.initializer)));
        decl.initializer = null;
      }
      updates.add(new ExpressionStatement(
          new VariableSet(decl, new VariableGet(temps.last))));
      newBody.add(new ExpressionStatement(
          new VariableSet(temps.last, new VariableGet(decl))));
    }
    // Add the updates to their guarded list of statements.
    for (int i = 0; i < stmt.updates.length; ++i) {
      updates.addAll(updateEffects[i]);
      updates.add(new ExpressionStatement(stmt.updates[i]));
    }
    // Initializers or updates could be empty.
    loopBody.add(new IfStatement(new VariableGet(temps.first),
        new Block(initializers), new Block(updates)));

    LabeledStatement labeled = new LabeledStatement(null);
    if (cond != null) {
      loopBody.addAll(condEffects);
    } else {
      cond = new BoolLiteral(true);
    }
    loopBody.add(
        new IfStatement(cond, new Block(newBody), new BreakStatement(labeled)));
    labeled.body =
        new WhileStatement(new BoolLiteral(true), new Block(loopBody))
          ..parent = labeled;
    statements.add(new Block(<Statement>[]
      ..addAll(temps)
      ..add(labeled)));
    return null;
  }

  TreeNode visitForInStatement(ForInStatement stmt) {
    if (stmt.isAsync) {
      // Transform
      //
      //   await for (T variable in <stream-expression>) { ... }
      //
      // To:
      //
      //   {
      //     :stream = <stream-expression>;
      //     _asyncStarListenHelper(:stream, :async_op);
      //     _StreamIterator<T> :for-iterator = new _StreamIterator<T>(:stream);
      //     const bool :product-mode =
      //         const bool.fromEnvironment("dart.vm.product");
      //     try {
      //       while (let _ = :product-mode ?
      //           null : _asyncStarMoveNextHelper(:stream) in
      //           await :for-iterator.moveNext()) {
      //         T <variable> = :for-iterator.current;
      //         ...
      //       }
      //     } finally {
      //       if (:for-iterator._subscription != null)
      //           await :for-iterator.cancel();
      //     }
      //   }
      var valueVariable = stmt.variable;

      var streamVariable =
          new VariableDeclaration(':stream', initializer: stmt.iterable);

      var asyncStarListenHelper = new ExpressionStatement(new StaticInvocation(
          helper.asyncStarListenHelper,
          new Arguments([
            new VariableGet(streamVariable),
            new VariableGet(nestedClosureVariable)
          ])));

      var iteratorVariable = new VariableDeclaration(':for-iterator',
          initializer: new ConstructorInvocation(
              helper.streamIteratorConstructor,
              new Arguments(<Expression>[new VariableGet(streamVariable)],
                  types: [valueVariable.type])),
          type: new InterfaceType(
              helper.streamIteratorClass, [valueVariable.type]));

      // const bool :product-mode =
      //    const bool.fromEnvironment("dart.vm.product");
      var productMode = new VariableDeclaration(':product-mode',
          isConst: true,
          type: new InterfaceType(helper.boolClass),
          initializer: new StaticInvocation(helper.boolFromEnvironment,
              new Arguments([new StringLiteral("dart.vm.product")]),
              isConst: true));

      // await :for-iterator.moveNext()
      var condition = new AwaitExpression(new MethodInvocation(
          new VariableGet(iteratorVariable),
          new Name('moveNext'),
          new Arguments(<Expression>[]),
          helper.streamIteratorMoveNext));

      // _asyncStarMoveNextHelper(:stream)
      var asyncStarMoveNextCall = new StaticInvocation(
          helper.asyncStarMoveNextHelper,
          new Arguments([new VariableGet(streamVariable)]))
        ..fileOffset = stmt.fileOffset;

      // let _ = :product-mode ? null : asyncStarMoveNextCall in (condition)
      var whileCondition = new Let(
          new VariableDeclaration(null,
              initializer: new ConditionalExpression(
                  new VariableGet(productMode),
                  new NullLiteral(),
                  asyncStarMoveNextCall,
                  new DynamicType())),
          condition);

      // T <variable> = :for-iterator.current;
      valueVariable.initializer = new PropertyGet(
          new VariableGet(iteratorVariable),
          new Name('current'),
          helper.streamIteratorCurrent)
        ..fileOffset = stmt.bodyOffset;
      valueVariable.initializer.parent = valueVariable;

      var whileBody = new Block(<Statement>[valueVariable, stmt.body]);
      var tryBody = new WhileStatement(whileCondition, whileBody);

      // if (:for-iterator._subscription != null) await :for-iterator.cancel();
      var tryFinalizer = new IfStatement(
          new Not(new MethodInvocation(
              new PropertyGet(
                  new VariableGet(iteratorVariable),
                  new Name("_subscription", helper.asyncLibrary),
                  helper.coreTypes.streamIteratorSubscription),
              new Name("=="),
              new Arguments([new NullLiteral()]),
              helper.coreTypes.objectEquals)),
          new ExpressionStatement(new AwaitExpression(new MethodInvocation(
              new VariableGet(iteratorVariable),
              new Name('cancel'),
              new Arguments(<Expression>[]),
              helper.streamIteratorCancel))),
          null);

      var tryFinally = new TryFinally(tryBody, tryFinalizer);

      var block = new Block(<Statement>[
        streamVariable,
        asyncStarListenHelper,
        iteratorVariable,
        productMode,
        tryFinally
      ]);
      block.accept(this);
    } else {
      stmt.iterable = expressionRewriter.rewrite(stmt.iterable, statements)
        ..parent = stmt;
      stmt.body = visitDelimited(stmt.body)..parent = stmt;
      statements.add(stmt);
    }
    return null;
  }

  TreeNode visitSwitchStatement(SwitchStatement stmt) {
    stmt.expression = expressionRewriter.rewrite(stmt.expression, statements)
      ..parent = stmt;
    for (var switchCase in stmt.cases) {
      // Expressions in switch cases cannot contain await so they do not need to
      // be translated.
      switchCase.body = visitDelimited(switchCase.body)..parent = switchCase;
    }
    statements.add(stmt);
    return null;
  }

  TreeNode visitContinueSwitchStatement(ContinueSwitchStatement stmt) {
    statements.add(stmt);
    return null;
  }

  TreeNode visitIfStatement(IfStatement stmt) {
    stmt.condition = expressionRewriter.rewrite(stmt.condition, statements)
      ..parent = stmt;
    stmt.then = visitDelimited(stmt.then)..parent = stmt;
    if (stmt.otherwise != null) {
      stmt.otherwise = visitDelimited(stmt.otherwise)..parent = stmt;
    }
    statements.add(stmt);
    return null;
  }

  TreeNode visitTryCatch(TryCatch stmt) {
    ++currentTryDepth;
    stmt.body = visitDelimited(stmt.body)..parent = stmt;
    --currentTryDepth;

    ++currentCatchDepth;
    for (var clause in stmt.catches) {
      clause.body = visitDelimited(clause.body)..parent = clause;
    }
    --currentCatchDepth;
    statements.add(stmt);
    return null;
  }

  TreeNode visitTryFinally(TryFinally stmt) {
    ++currentTryDepth;
    stmt.body = visitDelimited(stmt.body)..parent = stmt;
    --currentTryDepth;
    ++currentCatchDepth;
    stmt.finalizer = visitDelimited(stmt.finalizer)..parent = stmt;
    --currentCatchDepth;
    statements.add(stmt);
    return null;
  }

  TreeNode visitYieldStatement(YieldStatement stmt) {
    stmt.expression = expressionRewriter.rewrite(stmt.expression, statements)
      ..parent = stmt;
    statements.add(stmt);
    return null;
  }

  TreeNode visitVariableDeclaration(VariableDeclaration stmt) {
    if (stmt.initializer != null) {
      stmt.initializer = expressionRewriter.rewrite(
          stmt.initializer, statements)
        ..parent = stmt;
    }
    statements.add(stmt);
    return null;
  }

  TreeNode visitFunctionDeclaration(FunctionDeclaration stmt) {
    stmt.function = stmt.function.accept(this)..parent = stmt;
    statements.add(stmt);
    return null;
  }

  defaultExpression(TreeNode node) => throw 'unreachable';
}

class AsyncStarFunctionRewriter extends AsyncRewriterBase {
  VariableDeclaration controllerVariable;

  AsyncStarFunctionRewriter(HelperNodes helper, FunctionNode enclosingFunction)
      : super(helper, enclosingFunction);

  FunctionNode rewrite() {
    var statements = <Statement>[];

    final elementType = elementTypeFromReturnType(helper.streamClass);

    // _AsyncStarStreamController<T> :controller;
    controllerVariable = new VariableDeclaration(":controller",
        type: new InterfaceType(
            helper.asyncStarStreamControllerClass, [elementType]));
    statements.add(controllerVariable);

    // dynamic :controller_stream;
    VariableDeclaration controllerStreamVariable =
        new VariableDeclaration(":controller_stream");
    statements.add(controllerStreamVariable);

    setupAsyncContinuations(statements);

    // :controller = new _AsyncStarStreamController<T>(:async_op);
    var arguments = new Arguments(
        <Expression>[new VariableGet(nestedClosureVariable)],
        types: [elementType]);
    var buildController = new ConstructorInvocation(
        helper.asyncStarStreamControllerConstructor, arguments)
      ..fileOffset = enclosingFunction.fileOffset;
    var setController = new ExpressionStatement(
        new VariableSet(controllerVariable, buildController));
    statements.add(setController);

    // :controller_stream = :controller.stream;
    var completerGet = new VariableGet(controllerVariable);
    statements.add(new ExpressionStatement(new VariableSet(
        controllerStreamVariable,
        new PropertyGet(completerGet, new Name('stream', helper.asyncLibrary),
            helper.asyncStarStreamControllerStream))));

    // return :controller_stream;
    var returnStatement =
        new ReturnStatement(new VariableGet(controllerStreamVariable));
    statements.add(returnStatement);

    enclosingFunction.body = new Block(statements);
    enclosingFunction.body.parent = enclosingFunction;
    enclosingFunction.asyncMarker = AsyncMarker.Sync;
    return enclosingFunction;
  }

  Statement buildWrappedBody() {
    ++currentTryDepth;
    Statement body = super.buildWrappedBody();
    --currentTryDepth;

    var finallyBody = new ExpressionStatement(new MethodInvocation(
        new VariableGet(controllerVariable),
        new Name("close"),
        new Arguments(<Expression>[]),
        helper.asyncStarStreamControllerClose));

    var tryFinally = new TryFinally(body, new Block(<Statement>[finallyBody]));
    return tryFinally;
  }

  Statement buildCatchBody(exceptionVariable, stackTraceVariable) {
    return new ExpressionStatement(new MethodInvocation(
        new VariableGet(controllerVariable),
        new Name("addError"),
        new Arguments(<Expression>[
          new VariableGet(exceptionVariable),
          new VariableGet(stackTraceVariable)
        ]),
        helper.asyncStarStreamControllerAddError));
  }

  Statement buildReturn(Statement body) {
    // Async* functions cannot return a value.  The returns from the function
    // have been translated into breaks from the labeled body.
    return new Block(<Statement>[
      body,
      new ReturnStatement()..fileOffset = enclosingFunction.fileEndOffset,
    ]);
  }

  TreeNode visitYieldStatement(YieldStatement stmt) {
    Expression expr = expressionRewriter.rewrite(stmt.expression, statements);

    var addExpression = new MethodInvocation(
        new VariableGet(controllerVariable),
        new Name(stmt.isYieldStar ? 'addStream' : 'add', helper.asyncLibrary),
        new Arguments(<Expression>[expr]),
        stmt.isYieldStar
            ? helper.asyncStarStreamControllerAddStream
            : helper.asyncStarStreamControllerAdd)
      ..fileOffset = stmt.fileOffset;

    statements.add(new IfStatement(
        addExpression,
        new ReturnStatement(new NullLiteral()),
        createContinuationPoint()..fileOffset = stmt.fileOffset));
    return null;
  }

  TreeNode visitReturnStatement(ReturnStatement node) {
    // Async* functions cannot return a value.
    assert(node.expression == null || node.expression is NullLiteral);
    statements
        .add(new BreakStatement(labeledBody)..fileOffset = node.fileOffset);
    return null;
  }
}

class AsyncFunctionRewriter extends AsyncRewriterBase {
  VariableDeclaration completerVariable;
  VariableDeclaration returnVariable;

  AsyncFunctionRewriter(HelperNodes helper, FunctionNode enclosingFunction)
      : super(helper, enclosingFunction);

  FunctionNode rewrite() {
    var statements = <Statement>[];

    // The original function return type should be Future<T> or FutureOr<T>
    // because the function is async. If it was, we make a Completer<T>,
    // otherwise we make a malformed type.  In a "Future<T> foo() async {}"
    // function the body can either return a "T" or a "Future<T>" => a
    // "FutureOr<T>".
    DartType valueType = elementTypeFromReturnType(helper.futureClass);
    if (valueType == const DynamicType()) {
      valueType = elementTypeFromReturnType(helper.futureOrClass);
    }
    final DartType returnType =
        new InterfaceType(helper.futureOrClass, <DartType>[valueType]);
    var completerTypeArguments = <DartType>[valueType];

    final completerType = new InterfaceType(
        helper.asyncAwaitCompleterClass, completerTypeArguments);
    // final Completer<T> :async_completer = new _AsyncAwaitCompleter<T>();
    completerVariable = new VariableDeclaration(":async_completer",
        initializer: new ConstructorInvocation(
            helper.asyncAwaitCompleterConstructor,
            new Arguments([], types: completerTypeArguments))
          ..fileOffset = enclosingFunction.body?.fileOffset ?? -1,
        isFinal: true,
        type: completerType);
    statements.add(completerVariable);

    returnVariable = new VariableDeclaration(":return_value", type: returnType);
    statements.add(returnVariable);

    setupAsyncContinuations(statements);

    // :async_completer.start(:async_op);
    var startStatement = new ExpressionStatement(new MethodInvocation(
        new VariableGet(completerVariable),
        new Name('start'),
        new Arguments([new VariableGet(nestedClosureVariable)]))
      ..fileOffset = enclosingFunction.fileOffset);
    statements.add(startStatement);
    // return :async_completer.future;
    var completerGet = new VariableGet(completerVariable);
    var returnStatement = new ReturnStatement(new PropertyGet(completerGet,
        new Name('future', helper.asyncLibrary), helper.completerFuture));
    statements.add(returnStatement);

    enclosingFunction.body = new Block(statements);
    enclosingFunction.body.parent = enclosingFunction;
    enclosingFunction.asyncMarker = AsyncMarker.Sync;
    return enclosingFunction;
  }

  Statement buildCatchBody(exceptionVariable, stackTraceVariable) {
    return new ExpressionStatement(new MethodInvocation(
        new VariableGet(completerVariable),
        new Name("completeError"),
        new Arguments([
          new VariableGet(exceptionVariable),
          new VariableGet(stackTraceVariable)
        ]),
        helper.completerCompleteError));
  }

  Statement buildReturn(Statement body) {
    // Returns from the body have all been translated into assignments to the
    // return value variable followed by a break from the labeled body.
    return new Block(<Statement>[
      body,
      new ExpressionStatement(new StaticInvocation(
          helper.completeOnAsyncReturn,
          new Arguments([
            new VariableGet(completerVariable),
            new VariableGet(returnVariable)
          ]))),
      new ReturnStatement()..fileOffset = enclosingFunction.fileEndOffset
    ]);
  }

  visitReturnStatement(ReturnStatement node) {
    var expr = node.expression == null
        ? new NullLiteral()
        : expressionRewriter.rewrite(node.expression, statements);
    statements.add(new ExpressionStatement(
        new VariableSet(returnVariable, expr)..fileOffset = node.fileOffset));
    statements.add(new BreakStatement(labeledBody));
    return null;
  }
}

class HelperNodes {
  final Procedure asyncErrorWrapper;
  final Library asyncLibrary;
  final Procedure asyncStackTraceHelper;
  final Member asyncStarStreamControllerAdd;
  final Member asyncStarStreamControllerAddError;
  final Member asyncStarStreamControllerAddStream;
  final Class asyncStarStreamControllerClass;
  final Member asyncStarStreamControllerClose;
  final Constructor asyncStarStreamControllerConstructor;
  final Member asyncStarStreamControllerStream;
  final Member asyncStarListenHelper;
  final Member asyncStarMoveNextHelper;
  final Procedure asyncThenWrapper;
  final Procedure awaitHelper;
  final Class asyncAwaitCompleterClass;
  final Member completerCompleteError;
  final Member asyncAwaitCompleterConstructor;
  final Member completeOnAsyncReturn;
  final Member completerFuture;
  final Library coreLibrary;
  final CoreTypes coreTypes;
  final Class futureClass;
  final Class futureOrClass;
  final Class iterableClass;
  final Class streamClass;
  final Member streamIteratorCancel;
  final Class streamIteratorClass;
  final Constructor streamIteratorConstructor;
  final Member streamIteratorCurrent;
  final Member streamIteratorMoveNext;
  final Constructor syncIterableConstructor;
  final Class syncIteratorClass;
  final Member syncIteratorCurrent;
  final Member syncIteratorYieldEachIterable;
  final Class boolClass;
  final Member boolFromEnvironment;

  HelperNodes._(
      this.asyncErrorWrapper,
      this.asyncLibrary,
      this.asyncStackTraceHelper,
      this.asyncStarStreamControllerAdd,
      this.asyncStarStreamControllerAddError,
      this.asyncStarStreamControllerAddStream,
      this.asyncStarStreamControllerClass,
      this.asyncStarStreamControllerClose,
      this.asyncStarStreamControllerConstructor,
      this.asyncStarStreamControllerStream,
      this.asyncStarListenHelper,
      this.asyncStarMoveNextHelper,
      this.asyncThenWrapper,
      this.awaitHelper,
      this.asyncAwaitCompleterClass,
      this.completerCompleteError,
      this.asyncAwaitCompleterConstructor,
      this.completeOnAsyncReturn,
      this.completerFuture,
      this.coreLibrary,
      this.coreTypes,
      this.futureClass,
      this.futureOrClass,
      this.iterableClass,
      this.streamClass,
      this.streamIteratorCancel,
      this.streamIteratorClass,
      this.streamIteratorConstructor,
      this.streamIteratorCurrent,
      this.streamIteratorMoveNext,
      this.syncIterableConstructor,
      this.syncIteratorClass,
      this.syncIteratorCurrent,
      this.syncIteratorYieldEachIterable,
      this.boolClass,
      this.boolFromEnvironment);

  factory HelperNodes.fromCoreTypes(CoreTypes coreTypes) {
    return new HelperNodes._(
        coreTypes.asyncErrorWrapperHelperProcedure,
        coreTypes.asyncLibrary,
        coreTypes.asyncStackTraceHelperProcedure,
        coreTypes.asyncStarStreamControllerAdd,
        coreTypes.asyncStarStreamControllerAddError,
        coreTypes.asyncStarStreamControllerAddStream,
        coreTypes.asyncStarStreamControllerClass,
        coreTypes.asyncStarStreamControllerClose,
        coreTypes.asyncStarStreamControllerDefaultConstructor,
        coreTypes.asyncStarStreamControllerStream,
        coreTypes.asyncStarListenHelper,
        coreTypes.asyncStarMoveNextHelper,
        coreTypes.asyncThenWrapperHelperProcedure,
        coreTypes.awaitHelperProcedure,
        coreTypes.asyncAwaitCompleterClass,
        coreTypes.completerCompleteError,
        coreTypes.asyncAwaitCompleterConstructor,
        coreTypes.completeOnAsyncReturn,
        coreTypes.completerFuture,
        coreTypes.coreLibrary,
        coreTypes,
        coreTypes.futureClass,
        coreTypes.futureOrClass,
        coreTypes.iterableClass,
        coreTypes.streamClass,
        coreTypes.streamIteratorCancel,
        coreTypes.streamIteratorClass,
        coreTypes.streamIteratorDefaultConstructor,
        coreTypes.streamIteratorCurrent,
        coreTypes.streamIteratorMoveNext,
        coreTypes.syncIterableDefaultConstructor,
        coreTypes.syncIteratorClass,
        coreTypes.syncIteratorCurrent,
        coreTypes.syncIteratorYieldEachIterable,
        coreTypes.boolClass,
        coreTypes.boolFromEnvironment);
  }
}