Implement modular compilation

lib/src/compiler/element_loader.dart

+// Copyright (c) 2015, 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.
+
+import 'dart:collection' show HashMap, HashSet;
+
+import 'package:analyzer/dart/ast/ast.dart';
+import 'package:analyzer/dart/element/element.dart';
+import 'package:func/func.dart';
+import '../js_ast/js_ast.dart' as JS;
+
+typedef void ModuleItemEmitter(AstNode item);
+
+/// Helper that tracks order of elements visited by the compiler, detecting
+/// if the top level item can be loaded eagerly or not.
+class ElementLoader {
+  /// Whether an item has been loaded (emitted) already.
+  final _loaded = new HashSet<Element>();
+
+  final HashMap<Element, AstNode> _declarationNodes;
+
+  /// The stack of currently emitting elements, if generating top-level code
+  /// for them. This is not used when inside method bodies, because order does
+  /// not matter for those.
+  final _topLevelElements = new List<Element>();
+
+  /// The current element being loaded.
+  /// We can use this to determine if we're loading top-level code or not:
+  ///
+  ///     _currentElements.last == _topLevelElements.last
+  final _currentElements = new List<Element>();
+
+  bool _checkReferences;
+
+  final ModuleItemEmitter _emitModuleItem;
+
+  ElementLoader(this._emitModuleItem, this._declarationNodes) {
+    assert(!_declarationNodes.containsKey(null));
+  }
+
+  Element get currentElement => _currentElements.last;
+
+  bool isLoaded(Element e) =>
+      !_declarationNodes.containsKey(e) || _loaded.contains(e);
+
+  /// True if the element is currently being loaded.
+  bool _isLoading(Element e) => _currentElements.contains(e);
+
+  /// Start generating top-level code for the element [e].
+  ///
+  /// Subsequent [emitDeclaration] calls will cause those elements to be
+  /// generated before this one, until [finishTopLevel] is called.
+  void startTopLevel(Element e) {
+    assert(identical(e, currentElement));
+    _topLevelElements.add(e);
+  }
+
+  /// Finishes the top-level code for the element [e].
+  void finishTopLevel(Element e) {
+    var last = _topLevelElements.removeLast();
+    assert(identical(e, last));
+  }
+
+  /// Starts recording calls to [declareBeforeUse], until
+  /// [finishCheckingReferences] is called.
+  void startCheckingReferences() {
+    // This function should not be reentrant, and we should not current be
+    // emitting top-level code.
+    assert(_checkReferences == null);
+    assert(_topLevelElements.isEmpty ||
+        !identical(currentElement, _topLevelElements.last));
+    // Assume true until proven otherwise
+    _checkReferences = true;
+  }
+
+  /// Finishes recording references, and returns `true` if all referenced
+  /// items were loaded (or if no items were referenced).
+  bool finishCheckingReferences() {
+    var result = _checkReferences;
+    _checkReferences = null;
+    return result;
+  }
+
+  /// Ensures a top-level declaration is generated, and returns `true` if it
+  /// is part of the current module.
+  void emitDeclaration(Element e) {
+    var node = _declarationNodes[e];
+    if (node == null) return; // not from this module.
+
+    // If we already tried to load it, see if we succeeded or not.
+    // Otherwise try and load now.
+    if (_loaded.contains(e)) return;
+
+    _loaded.add(e);
+    _currentElements.add(e);
+
+    _emitModuleItem(node);
+
+    var last = _currentElements.removeLast();
+    assert(identical(e, last));
+  }
+
+  /// Used to immediately emit a declaration and return the result.
+  ///
+  /// This will operate regardless of whether the node is currently loaded.
+  /// We use this when we encounter a getter/setter, to immediately emit the
+  /// pair and combine them.
+  /*=T*/ customEmitDeclaration/*<T extends JS.Node>*/(
+      Element e, Func1<AstNode, JS.Node/*=T*/ > visit) {
+    _loaded.add(e);
+    _currentElements.add(e);
+    var result = visit(_declarationNodes[e]);
+    var last = _currentElements.removeLast();
+    assert(identical(e, last));
+    return result;
+  }
+
+  /// To emit top-level module items, we sometimes need to reorder them.
+  ///
+  /// This function takes care of that, and also detects cases where reordering
+  /// failed, and we need to resort to lazy loading, by marking the element as
+  /// lazy. All elements need to be aware of this possibility and generate code
+  /// accordingly.
+  ///
+  /// If we are not emitting top-level code, this does nothing, because all
+  /// declarations are assumed to be available before we start execution.
+  /// See [startTopLevel].
+  void declareBeforeUse(Element e) {
+    if (e == null) return;
+
+    if (_checkReferences != null) {
+      _checkReferences = _checkReferences && isLoaded(e) && !_isLoading(e);
+      return;
+    }
+
+    var topLevel = _topLevelElements;
+    if (topLevel.isNotEmpty && identical(currentElement, topLevel.last)) {
+      // If the item is from our library, try to emit it now.
+      emitDeclaration(e);
+    }
+  }
+}