First step for modular output in fasta.

pkg/front_end/lib/src/fasta/kernel/kernel_outline_shaker.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.
+
+/// A transformation to create a self-contained modular kernel without
+/// unnecessary references to other libraries.
+library fasta.kernel.kernel_outline_shaker;
+
+import 'package:kernel/ast.dart';
+import 'package:kernel/core_types.dart';
+
+import '../errors.dart' show internalError;
+
+/// Removes from [program] unnecessary libraries, classes, and members.
+///
+/// This applies a simple "tree-shaking" technique: the full body of libraries
+/// whose URI match [isIncluded] is preserved, and so is the outline of the
+/// members and classes which are indicated by [data] (which should
+/// practically include all members and classes transitively visible from the
+/// included libraries).
+///
+/// The intent is that the resulting program has the entire code that is meant
+/// to be included and the minimum required to prevent dangling references and
+/// allow modular program transformations.
+///
+/// Note that the resulting program may include libraries not in [isIncluded],
+/// but those will be marked as external. There should be no method bodies for
+/// any members of those libraries.
+void trimProgram(Program program, RetainedData data, bool isIncluded(Uri uri)) {
+  new KernelOutlineShaker(data, isIncluded).transform(program);
+}
+
+/// Informs about which libraries, classes, and members should be retained by
+/// the [KernelOutlineShaker] when tree-shaking.
+abstract class RetainedData {
+  /// Whether a library should be preserved and mark as external.
+  bool isLibraryUsed(Library library);
+
+  /// Whether a class should be preserved. If a class is preserved, its
+  /// supertypes will be preserved too, but some of it members may not be
+  /// included.
+  bool isClassUsed(Class cls);
+
+  /// Whether a member should be preserved. If so, its enclosing class/library
+  /// will be preserved too.
+  bool isMemberUsed(Member member);
+}
+
+/// A builder of [RetainedData] that recursively marks transitive dependencies.
+///
+/// This builder contains APIs to mark the roots that are needed (e.g.
+/// [markClass] and [markMember]). Note this builder does not determine what
+/// roots to keep, that is done either directly by fasta while it is parsing, or
+/// by using a visitor like the [RootsMarker] below.
+class RetainedDataBuilder extends RetainedData {
+  /// Libraries that contained code that is transitively reachable from the
+  /// included libraries.
+  final Set<Library> libraries = new Set<Library>();
+
+  /// Classes that are transitively reachable from the included libraries.
+  final Set<Class> classes = new Set<Class>();
+
+  /// Members that are transitively reachable from the included libraries.
+  final Set<Member> members = new Set<Member>();
+
+  TypeMarker typeMarker;
+
+  @override
+  bool isLibraryUsed(Library library) => libraries.contains(library);
+
+  @override
+  bool isClassUsed(Class cls) => classes.contains(cls);
+
+  @override
+  bool isMemberUsed(Member m) => members.contains(m);
+
+  RetainedDataBuilder() {
+    typeMarker = new TypeMarker(this);
+  }
+
+  /// Mark a library as used.
+  void markLibrary(Library lib) {
+    libraries.add(lib);
+  }
+
+  /// Mark a class and it's supertypes as used.
+  void markClass(Class cls) {
+    if (cls == null || !classes.add(cls)) return;
+    markLibrary(cls.parent);
+    // TODO(sigmund): retain annotations?
+    // visitList(cls.annotations, this);
+    markSupertype(cls.supertype);
+    markSupertype(cls.mixedInType);
+    cls.implementedTypes.forEach(markSupertype);
+    cls.typeParameters.forEach((t) => t.bound.accept(typeMarker));
+  }
+
+  /// Mark the class and type arguments of [node].
+  void markSupertype(Supertype node) {
+    if (node == null) return;
+    markClass(node.classNode);
+    node.typeArguments.forEach((t) => t.accept(typeMarker));
+  }
+
+  /// Mark a member and types mentioned on its interface.
+  void markMember(Member m) {
+    if (m == null || !members.add(m)) return;
+    markMemberInterface(m);
+    var parent = m.parent;
+    if (parent is Library) {
+      markLibrary(parent);
+    } else if (parent is Class) {
+      markClass(parent);
+    }
+  }
+
+  void markMemberInterface(Member node) {
+    if (node is Field) {
+      node.type.accept(typeMarker);
+    } else if (node is Procedure) {
+      var function = node.function;
+      function.typeParameters.forEach((p) => p.bound.accept(typeMarker));
+      function.positionalParameters.forEach((p) => p.type.accept(typeMarker));
+      function.namedParameters.forEach((p) => p.type.accept(typeMarker));
+      function.returnType.accept(typeMarker);
+    }
+  }
+}
+
+/// A helper visitor used to mark transitive types by the [RetainedDataBuilder].
+class TypeMarker extends DartTypeVisitor {
+  RetainedDataBuilder data;
+
+  TypeMarker(this.data);
+
+  visitInterfaceType(InterfaceType node) {
+    data.markClass(node.classNode);
+    node.typeArguments.forEach((t) => t.accept(this));
+  }
+
+  visitFunctionType(FunctionType node) {
+    node.typeParameters.forEach((t) => t.bound.accept(this));
+    node.positionalParameters.forEach((t) => t.accept(this));
+    node.namedParameters.forEach((t) => t.type.accept(this));
+    node.returnType.accept(this);
+  }
+
+  visitTypeParameterType(TypeParameterType node) {
+    // Note: node.parameter is marked by marking the enclosing element.
+  }
+
+  visitTypedefType(TypedefType node) {
+    node.typeArguments.forEach((t) => t.accept(this));
+  }
+}
+
+/// Determines the root APIs that need to be retained before running the
+/// tree-shaker.
+///
+/// This is implemented using a visitor that walks through the sources that are
+/// intended to be part of the kernel output.
+// TODO(sigmund): delete. We should collect this information while
+// building kernel without having to run a visitor afterwards.
+class RootsMarker extends RecursiveVisitor {
+  final RetainedDataBuilder data;
+  RootsMarker(this.data);
+
+  void run(Program program, bool isIncluded(Uri uri)) {
+    markRequired(program);
+    data.markMember(program.mainMethod);
+    for (var library in program.libraries) {
+      if (isIncluded(library.importUri)) {
+        library.accept(this);
+      }
+    }
+  }
+
+  /// Marks classes and members that are assumed to exist by fasta or by
+  /// transformers.
+  // TODO(sigmund): consider being more fine-grained and only marking what is
+  // seen and used.
+  void markRequired(Program program) {
+    var coreTypes = new CoreTypes(program);
+    coreTypes.objectClass.members.forEach(data.markMember);
+
+    // These are assumed to be available by fasta:
+    data.markClass(coreTypes.objectClass);
+    data.markClass(coreTypes.nullClass);
+    data.markClass(coreTypes.boolClass);
+    data.markClass(coreTypes.intClass);
+    data.markClass(coreTypes.numClass);
+    data.markClass(coreTypes.doubleClass);
+    data.markClass(coreTypes.stringClass);
+    data.markClass(coreTypes.listClass);
+    data.markClass(coreTypes.mapClass);
+    data.markClass(coreTypes.iterableClass);
+    data.markClass(coreTypes.iteratorClass);
+    data.markClass(coreTypes.futureClass);
+    data.markClass(coreTypes.streamClass);
+    data.markClass(coreTypes.symbolClass);
+    data.markClass(coreTypes.internalSymbolClass);
+    data.markClass(coreTypes.typeClass);
+    data.markClass(coreTypes.functionClass);
+    data.markClass(coreTypes.invocationClass);
+    data.markMember(coreTypes.getMember("dart:_internal", "ExternalName", ""));
+
+    // These are needed by the continuation (async/await) transformer:
+    data.markClass(coreTypes.getClass('dart:core', 'Iterator'));
+    data.markClass(coreTypes.getClass('dart:async', 'Future'));
+    data.markClass(coreTypes.getClass('dart:async', 'FutureOr'));
+    data.markClass(coreTypes.getClass('dart:async', 'Completer'));
+    data.markMember(coreTypes.getMember('dart:async', 'Completer', 'sync'));
+    data.markMember(coreTypes.getMember('dart:core', '_SyncIterable', ''));
+    data.markMember(coreTypes.getMember('dart:async', '_StreamIterator', ''));
+    data.markMember(coreTypes.getMember('dart:async', 'Future', 'microtask'));
+    data.markMember(
+        coreTypes.getMember('dart:async', '_AsyncStarStreamController', ''));
+    data.markMember(coreTypes.getTopLevelMember('dart:core', 'print'));
+    data.markMember(
+        coreTypes.getTopLevelMember('dart:async', '_asyncThenWrapperHelper'));
+    data.markMember(
+        coreTypes.getTopLevelMember('dart:async', '_asyncErrorWrapperHelper'));
+    data.markMember(coreTypes.getTopLevelMember('dart:async', '_awaitHelper'));
+
+    // These are needed by the mixin transformer
+    data.markMember(coreTypes.getMember('dart:core', '_InvocationMirror', ''));
+    data.markMember(coreTypes.getMember('dart:core', 'List', 'from'));
+  }
+
+  visitConstructor(Constructor node) {
+    if (!node.initializers.any((i) => i is SuperInitializer)) {
+      // super() is currently implicit.
+      for (var ctor in node.enclosingClass.supertype.classNode.constructors) {
+        if (ctor.name.name == '') data.markMember(ctor);
+      }
+    }
+    node.visitChildren(this);
+  }
+
+  @override
+  visitSuperInitializer(SuperInitializer node) {
+    data.markMember(node.target);
+    node.visitChildren(this);
+  }
+
+  @override
+  visitRedirectingInitializer(RedirectingInitializer node) {
+    data.markMember(node.target);
+    node.visitChildren(this);
+  }
+
+  @override
+  visitConstructorInvocation(ConstructorInvocation node) {
+    data.markMember(node.target);
+    node.visitChildren(this);
+  }
+
+  @override
+  visitStaticInvocation(StaticInvocation node) {
+    data.markMember(node.target);
+    node.visitChildren(this);
+  }
+
+  @override
+  visitDirectMethodInvocation(DirectMethodInvocation node) {
+    if (node.receiver is! ThisExpression) {
+      return internalError('Direct calls are only supported on "this"');
+    }
+    data.markMember(node.target);
+    node.visitChildren(this);
+  }
+
+  @override
+  visitMethodInvocation(MethodInvocation node) {
+    data.markMember(node.interfaceTarget);
+    node.visitChildren(this);
+  }
+
+  @override
+  visitStaticGet(StaticGet node) {
+    data.markMember(node.target);
+    node.visitChildren(this);
+  }
+
+  @override
+  visitStaticSet(StaticSet node) {
+    data.markMember(node.target);
+    node.visitChildren(this);
+  }
+
+  @override
+  visitDirectPropertyGet(DirectPropertyGet node) {
+    data.markMember(node.target);
+    node.visitChildren(this);
+  }
+
+  @override
+  visitDirectPropertySet(DirectPropertySet node) {
+    data.markMember(node.target);
+    node.visitChildren(this);
+  }
+
+  @override
+  visitSuperPropertyGet(SuperPropertyGet node) {
+    data.markMember(node.interfaceTarget);
+    node.visitChildren(this);
+  }
+
+  @override
+  visitSuperPropertySet(SuperPropertySet node) {
+    data.markMember(node.interfaceTarget);
+    node.visitChildren(this);
+  }
+
+  @override
+  visitPropertyGet(PropertyGet node) {
+    data.markMember(node.interfaceTarget);
+    node.visitChildren(this);
+  }
+
+  @override
+  visitPropertySet(PropertySet node) {
+    data.markMember(node.interfaceTarget);
+    node.visitChildren(this);
+  }
+
+  @override
+  visitInterfaceType(InterfaceType node) {
+    data.markClass(node.classNode);
+    node.visitChildren(this);
+  }
+
+  @override
+  visitSupertype(Supertype node) {
+    data.markClass(node.classNode);
+    node.visitChildren(this);
+  }
+
+  @override
+  visitTypedefReference(Typedef node) {
+    return internalError('not implemented');
+  }
+}
+
+/// Transformer that trims everything in the excluded libraries that is not
+/// marked as preserved by the given [RetainedData]. For every member in these
+/// excluded libraries, this transformer also removes function bodies and
+/// initializers.
+class KernelOutlineShaker extends Transformer {
+  final RetainedData data;
+  final Filter isIncluded;
+
+  KernelOutlineShaker(this.data, this.isIncluded);
+
+  void transform(Program program) {
+    var toRemove = new Set<Library>();
+    for (var library in program.libraries) {
+      if (!isIncluded(library.importUri)) {
+        if (!data.isLibraryUsed(library)) {
+          toRemove.add(library);
+        } else {
+          library.isExternal = true;
+          library.transformChildren(this);
+        }
+      }
+    }
+    program.libraries.removeWhere(toRemove.contains);
+  }
+
+  Class visitClass(Class node) {
+    if (!data.isClassUsed(node)) {
+      node.canonicalName?.unbind();
+      return null; // Remove the class.
+    } else {
+      node.transformChildren(this);
+      return node;
+    }
+  }
+
+  Member defaultMember(Member node) {
+    if (!data.isMemberUsed(node)) {
+      node.canonicalName?.unbind();
+      return null;
+    } else {
+      if (node is Procedure) {
+        node.function.body = null;
+      } else if (node is Field) {
+        node.initializer = null;
+      } else if (node is Constructor) {
+        node.initializers.clear();
+        node.function.body = null;
+      }
+      return node;
+    }
+  }
+
+  /// Types appear to be encoded directly, so we have no need to preserve
+  /// typedefs.
+  // TODO(sigmund): revisit if this is not the case, the `inputError` in
+  // [RootsMarker] is meant to detect this.
+  Typedef visitTypedef(Typedef node) => null;
+
+  TreeNode defaultTreeNode(TreeNode node) => node;
+}
+
+typedef bool Filter(Uri uri);