[analyzer] Make two derived classes for FunctionElementImpl.

Derived class `LocalFunctionFragmentImpl` is used for local functions
and `TopLevelFunctionFragmentImpl` is used for top level
functions. Only `LocalFunctionFragmentImpl` implements
`LocalFunctionFragment` and only `TopLevelFunctionFragmentImpl`
implements `TopLevelFunctionFragment`. This ensures that clients
performing `is LocalFunctionFragment` and `is
TopLevelFunctionFragment` checks get the expected
behavior. (Previously, any instance of `FunctionElementImpl` would
satisfy *both* `is LocalFunctionFragment` and `is
TopLevelFunctionFragment` tests).

This made it possible for the types of getters defined in
`LocalFunctionFragmentImpl` and `TopLevelFunctionFragmentImpl` to be
more precise, which in turn allowed some casts to be removed from the
analyzer.

Change-Id: I5637366936683f43d0f0818d3d8a28191086dad5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/414162
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

Author: stereotype441

pkg/analyzer/lib/src/dart/element/element.dart

@@ -816,7 +816,7 @@ class CompilationUnitElementImpl extends UriReferencedElementImpl
 
   /// A list containing all of the top-level functions contained in this
   /// compilation unit.
-  List<FunctionElementImpl> _functions = const [];
+  List<TopLevelFunctionFragmentImpl> _functions = const [];
 
   List<MixinElementImpl> _mixins = const [];
 
@@ -973,13 +973,13 @@ class CompilationUnitElementImpl extends UriReferencedElementImpl
       extensionTypes.cast<ExtensionTypeFragment>();
 
   @override
-  List<FunctionElementImpl> get functions {
+  List<TopLevelFunctionFragmentImpl> get functions {
     return _functions;
   }
 
   /// Set the top-level functions contained in this compilation unit to the
   ///  given[functions].
-  set functions(List<FunctionElementImpl> functions) {
+  set functions(List<TopLevelFunctionFragmentImpl> functions) {
     for (var function in functions) {
       function.enclosingElement3 = this;
     }
@@ -5272,26 +5272,17 @@ mixin FragmentedTypeParameterizedElementMixin<
 }
 
 /// A concrete implementation of a [FunctionElement].
-class FunctionElementImpl extends ExecutableElementImpl
-    with AugmentableElement<FunctionElementImpl>
+sealed class FunctionElementImpl extends ExecutableElementImpl
     implements
         FunctionElement,
         FunctionTypedElementImpl,
-        LocalFunctionFragment,
-        TopLevelFunctionFragment,
         ExecutableElementOrMember {
   @override
   String? name2;
 
   @override
   int? nameOffset2;
 
-  /// The element corresponding to this fragment.
-  // TODO(brianwilkerson): Use either `LocalFunctionElement` or
-  //  `TopLevelFunctionElement` when this class is split.
-  @override
-  late ExecutableElementImpl2 element;
-
   /// Initialize a newly created function element to have the given [name] and
   /// [offset].
   FunctionElementImpl(super.name, super.offset);
@@ -5300,14 +5291,6 @@ class FunctionElementImpl extends ExecutableElementImpl
   /// [nameOffset]. This is used for function expressions, that have no name.
   FunctionElementImpl.forOffset(int nameOffset) : super("", nameOffset);
 
-  @override
-  List<Element2> get children2 {
-    if (enclosingElement3 is LibraryFragment) {
-      return element.children2;
-    }
-    throw StateError('Not an Element2');
-  }
-
   @override
   ExecutableElementImpl get declaration => this;
 
@@ -5332,37 +5315,9 @@ class FunctionElementImpl extends ExecutableElementImpl
     throw UnsupportedError('This is not a fragment');
   }
 
-  @override
-  bool get isDartCoreIdentical {
-    return isStatic && name == 'identical' && library.isDartCore;
-  }
-
-  @override
-  bool get isEntryPoint {
-    return isStatic && displayName == FunctionElement.MAIN_FUNCTION_NAME;
-  }
-
-  /// Whether this function element represents a top-level function, and is
-  /// therefore safe to treat as a fragment.
-  bool get isTopLevelFunction =>
-      enclosingElement3 is CompilationUnitElementImpl;
-
   @override
   ElementKind get kind => ElementKind.FUNCTION;
 
-  @override
-  FunctionElementImpl? get nextFragment => augmentation;
-
-  @override
-  FunctionElementImpl? get previousFragment => augmentationTarget;
-
-  @override
-  bool get _includeNameOffsetInIdentifier {
-    return super._includeNameOffsetInIdentifier ||
-        enclosingElement3 is ExecutableElement ||
-        enclosingElement3 is VariableElement;
-  }
-
   @Deprecated('Use Element2 and accept2() instead')
   @override
   T? accept<T>(ElementVisitor<T> visitor) => visitor.visitFunctionElement(this);
@@ -7444,7 +7399,7 @@ class LibraryElementImpl extends ElementImpl
   }
 
   @override
-  FunctionElementImpl get loadLibraryFunction {
+  TopLevelFunctionFragmentImpl get loadLibraryFunction {
     return loadLibraryFunction2.firstFragment;
   }
 
@@ -7837,7 +7792,7 @@ final class LoadLibraryFunctionProvider {
   TopLevelFunctionElementImpl _create(LibraryElementImpl library) {
     var name = FunctionElement.LOAD_LIBRARY_NAME;
 
-    var fragment = FunctionElementImpl(name, -1);
+    var fragment = TopLevelFunctionFragmentImpl(name, -1);
     fragment.name2 = name;
     fragment.isSynthetic = true;
     fragment.isStatic = true;
@@ -7855,7 +7810,7 @@ class LocalFunctionElementImpl extends ExecutableElementImpl2
     with WrappedElementMixin
     implements LocalFunctionElement {
   @override
-  final FunctionElementImpl _wrappedElement;
+  final LocalFunctionFragmentImpl _wrappedElement;
 
   LocalFunctionElementImpl(this._wrappedElement) {
     _wrappedElement.element = this;
@@ -7869,7 +7824,7 @@ class LocalFunctionElementImpl extends ExecutableElementImpl2
   Element2? get enclosingElement2 => null;
 
   @override
-  FunctionElementImpl get firstFragment => _wrappedElement;
+  LocalFunctionFragmentImpl get firstFragment => _wrappedElement;
 
   @override
   List<FormalParameterElementMixin> get formalParameters =>
@@ -7878,9 +7833,9 @@ class LocalFunctionElementImpl extends ExecutableElementImpl2
           .toList();
 
   @override
-  List<FunctionElementImpl> get fragments {
+  List<LocalFunctionFragmentImpl> get fragments {
     return [
-      for (FunctionElementImpl? fragment = firstFragment;
+      for (LocalFunctionFragmentImpl? fragment = firstFragment;
           fragment != null;
           fragment = fragment.nextFragment)
         fragment,
@@ -7930,6 +7885,43 @@ class LocalFunctionElementImpl extends ExecutableElementImpl2
   }
 }
 
+/// A concrete implementation of a [LocalFunctionFragment].
+class LocalFunctionFragmentImpl extends FunctionElementImpl
+    with AugmentableElement<LocalFunctionFragmentImpl>
+    implements LocalFunctionFragment {
+  /// The element corresponding to this fragment.
+  @override
+  late LocalFunctionElementImpl element;
+
+  LocalFunctionFragmentImpl(super.name, super.offset);
+
+  LocalFunctionFragmentImpl.forOffset(super.nameOffset) : super.forOffset();
+
+  @override
+  Never get children2 {
+    throw StateError('Not an Element2');
+  }
+
+  @override
+  bool get isDartCoreIdentical => false;
+
+  @override
+  bool get isEntryPoint => false;
+
+  @override
+  LocalFunctionFragmentImpl? get nextFragment => augmentation;
+
+  @override
+  LocalFunctionFragmentImpl? get previousFragment => augmentationTarget;
+
+  @override
+  bool get _includeNameOffsetInIdentifier {
+    return super._includeNameOffsetInIdentifier ||
+        enclosingElement3 is ExecutableElement ||
+        enclosingElement3 is VariableElement;
+  }
+}
+
 /// A concrete implementation of a [LocalVariableElement].
 class LocalVariableElementImpl extends NonParameterVariableElementImpl
     implements
@@ -10932,7 +10924,7 @@ class TopLevelFunctionElementImpl extends ExecutableElementImpl2
   final Reference reference;
 
   @override
-  final FunctionElementImpl firstFragment;
+  final TopLevelFunctionFragmentImpl firstFragment;
 
   TopLevelFunctionElementImpl(this.reference, this.firstFragment) {
     reference.element2 = this;
@@ -10946,9 +10938,9 @@ class TopLevelFunctionElementImpl extends ExecutableElementImpl2
   Element2? get enclosingElement2 => firstFragment._enclosingElement3?.library2;
 
   @override
-  List<FunctionElementImpl> get fragments {
+  List<TopLevelFunctionFragmentImpl> get fragments {
     return [
-      for (FunctionElementImpl? fragment = firstFragment;
+      for (TopLevelFunctionFragmentImpl? fragment = firstFragment;
           fragment != null;
           fragment = fragment.nextFragment)
         fragment,
@@ -10965,8 +10957,8 @@ class TopLevelFunctionElementImpl extends ExecutableElementImpl2
   ElementKind get kind => ElementKind.FUNCTION;
 
   @override
-  FunctionElementImpl get lastFragment {
-    return super.lastFragment as FunctionElementImpl;
+  TopLevelFunctionFragmentImpl get lastFragment {
+    return super.lastFragment as TopLevelFunctionFragmentImpl;
   }
 
   @override
@@ -10983,6 +10975,43 @@ class TopLevelFunctionElementImpl extends ExecutableElementImpl2
   }
 }
 
+/// A concrete implementation of a [TopLevelFunctionFragment].
+class TopLevelFunctionFragmentImpl extends FunctionElementImpl
+    with AugmentableElement<TopLevelFunctionFragmentImpl>
+    implements TopLevelFunctionFragment {
+  /// The element corresponding to this fragment.
+  @override
+  late TopLevelFunctionElementImpl element;
+
+  TopLevelFunctionFragmentImpl(super.name, super.offset);
+
+  @override
+  List<Element2> get children2 => element.children2;
+
+  @override
+  CompilationUnitElementImpl get enclosingElement3 =>
+      super.enclosingElement3 as CompilationUnitElementImpl;
+
+  @override
+  set enclosingElement3(covariant CompilationUnitElementImpl element);
+
+  @override
+  bool get isDartCoreIdentical {
+    return name == 'identical' && library.isDartCore;
+  }
+
+  @override
+  bool get isEntryPoint {
+    return displayName == FunctionElement.MAIN_FUNCTION_NAME;
+  }
+
+  @override
+  TopLevelFunctionFragmentImpl? get nextFragment => augmentation;
+
+  @override
+  TopLevelFunctionFragmentImpl? get previousFragment => augmentationTarget;
+}
+
 /// A concrete implementation of a [TopLevelVariableElement].
 class TopLevelVariableElementImpl extends PropertyInducingElementImpl
     with AugmentableElement<TopLevelVariableElementImpl>

pkg/analyzer/lib/src/dart/resolver/resolution_visitor.dart

@@ -712,7 +712,7 @@ class ResolutionVisitor extends RecursiveAstVisitor<void> {
       node.declaredFragment = fragment;
       expression.declaredFragment = fragment;
     } else {
-      var functionFragment = node.declaredFragment as FunctionElementImpl;
+      var functionFragment = node.declaredFragment as LocalFunctionFragmentImpl;
       functionFragment.element = LocalFunctionElementImpl(functionFragment);
 
       fragment = functionFragment;
@@ -779,7 +779,7 @@ class ResolutionVisitor extends RecursiveAstVisitor<void> {
 
   @override
   void visitFunctionExpression(covariant FunctionExpressionImpl node) {
-    var fragment = FunctionElementImpl.forOffset(node.offset);
+    var fragment = LocalFunctionFragmentImpl.forOffset(node.offset);
     fragment.element = LocalFunctionElementImpl(fragment);
 
     _elementHolder.enclose(fragment);
@@ -1495,7 +1495,8 @@ class ResolutionVisitor extends RecursiveAstVisitor<void> {
     var node = statement.functionDeclaration;
     var nameToken = node.name;
 
-    var fragment = FunctionElementImpl(nameToken.lexeme, nameToken.offset);
+    var fragment =
+        LocalFunctionFragmentImpl(nameToken.lexeme, nameToken.offset);
     fragment.name2 = nameToken.nameIfNotEmpty;
     fragment.nameOffset2 = nameToken.offsetIfNotEmpty;
     node.declaredFragment = fragment;

pkg/analyzer/lib/src/summary2/augmentation.dart

@@ -384,13 +384,13 @@ class SetterElementBuilder
 }
 
 class TopLevelFunctionElementBuilder extends FragmentedElementBuilder<
-    TopLevelFunctionElementImpl, FunctionElementImpl> {
+    TopLevelFunctionElementImpl, TopLevelFunctionFragmentImpl> {
   TopLevelFunctionElementBuilder({
     required super.element,
     required super.firstFragment,
   });
 
-  void addFragment(FunctionElementImpl fragment) {
+  void addFragment(TopLevelFunctionFragmentImpl fragment) {
     if (!identical(fragment, firstFragment)) {
       lastFragment.augmentation = fragment;
       lastFragment = fragment;

pkg/analyzer/lib/src/summary2/bundle_reader.dart

@@ -1207,7 +1207,7 @@ class LibraryReader {
       var fragmentName = _readFragmentName();
       var name = reference.elementName;
 
-      var fragment = FunctionElementImpl(name, -1);
+      var fragment = TopLevelFunctionFragmentImpl(name, -1);
       fragment.name2 = fragmentName;
 
       if (reference2.element2 case TopLevelFunctionElementImpl element?) {

pkg/analyzer/lib/src/summary2/bundle_writer.dart

@@ -401,10 +401,10 @@ class BundleWriter {
     _sink._writeOptionalStringReference(fragment.name2);
   }
 
-  void _writeFunctionElement(FunctionElementImpl fragment) {
+  void _writeFunctionElement(TopLevelFunctionFragmentImpl fragment) {
     _sink.writeUInt30(_resolutionSink.offset);
 
-    var element = fragment.element as TopLevelFunctionElementImpl;
+    var element = fragment.element;
 
     _writeReference(fragment);
     _writeReference2(element.reference);

pkg/analyzer/lib/src/summary2/element_builder.dart

@@ -881,7 +881,7 @@ class ElementBuilder extends ThrowingAstVisitor<void> {
         _libraryBuilder.elementBuilderSetters[name] = elementBuilder;
       }
     } else {
-      var fragment = FunctionElementImpl(name, nameOffset);
+      var fragment = TopLevelFunctionFragmentImpl(name, nameOffset);
       fragment.name2 = _getFragmentName(nameToken);
       fragment.nameOffset2 = _getFragmentNameOffset(nameToken);
       fragment.isAugmentation = node.augmentKeyword != null;
@@ -1920,7 +1920,7 @@ class _EnclosingContext {
   final List<ExtensionElementImpl> _extensions = [];
   final List<ExtensionTypeElementImpl> _extensionTypes = [];
   final List<FieldElementImpl> _fields = [];
-  final List<FunctionElementImpl> _functions = [];
+  final List<TopLevelFunctionFragmentImpl> _functions = [];
   final List<MethodElementImpl> _methods = [];
   final List<MixinElementImpl> _mixins = [];
   final List<ParameterElementImpl> _parameters = [];
@@ -1974,7 +1974,7 @@ class _EnclosingContext {
     return fragment.reference!;
   }
 
-  List<FunctionElementImpl> get functions {
+  List<TopLevelFunctionFragmentImpl> get functions {
     return _functions.toFixedList();
   }
 
@@ -2060,7 +2060,7 @@ class _EnclosingContext {
     _bindReference(reference, element);
   }
 
-  Reference addFunction(String name, FunctionElementImpl element) {
+  Reference addFunction(String name, TopLevelFunctionFragmentImpl element) {
     _functions.add(element);
     var containerName =
         element.isAugmentation ? '@functionAugmentation' : '@function';

pkg/analyzer/lib/src/summary2/element_flags.dart

@@ -220,7 +220,8 @@ class FunctionElementFlags {
   static const int _isGenerator = 1 << 4;
   static const int _isStatic = 1 << 5;
 
-  static void read(SummaryDataReader reader, FunctionElementImpl element) {
+  static void read(
+      SummaryDataReader reader, TopLevelFunctionFragmentImpl element) {
     var byte = reader.readByte();
     element.hasImplicitReturnType = (byte & _hasImplicitReturnType) != 0;
     element.isAsynchronous = (byte & _isAsynchronous) != 0;

pkg/analyzer/lib/src/summary2/informative_data.dart

@@ -568,7 +568,7 @@ class InformativeDataApplier {
     FunctionElement element,
     _InfoFunctionDeclaration info,
   ) {
-    element as FunctionElementImpl;
+    element as TopLevelFunctionFragmentImpl;
     element.setCodeRange(info.codeOffset, info.codeLength);
     element.nameOffset = info.nameOffset;
     element.nameOffset2 = info.nameOffset2;