Elements. APIs for constant initializers.

I found that we have ConstTopLevelVariableElementImpl,
ConstFieldElementImpl, DefaultSuperFormalParameterElementImpl, etc.

But we don't have corresponding Element2.
And this is probably good.

Having so many classes might be too much.
Why not turn the constant initializer into a property?

Change-Id: I1fe7c2709e2bee4d8868e2916a3b3e8f7a6fb1f6
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/406880
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: scheglov

pkg/analyzer/lib/dart/element/element2.dart

@@ -50,6 +50,7 @@ library;
 
 import 'package:analyzer/dart/analysis/features.dart';
 import 'package:analyzer/dart/analysis/session.dart';
+import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/constant/value.dart';
 import 'package:analyzer/dart/element/element.dart'
     show
@@ -253,6 +254,22 @@ abstract class ClassFragment implements InterfaceFragment {
   ClassFragment? get previousFragment;
 }
 
+/// The initializer of a constant variable, or the default value for a formal
+/// parameter.
+abstract class ConstantInitializer {
+  /// The expression of the initializer.
+  ///
+  /// For variables that have multiple fragments, this is the expression from
+  /// the last fragment. For formal parameters, only the first fragment can
+  /// have the default value.
+  Expression get expression;
+
+  /// Returns the result of evaluating [expression], computes it if necessary.
+  ///
+  /// Returns `null` if the value could not be computed because of errors.
+  DartObject? evaluate();
+}
+
 /// An element representing a constructor defined by a class, enum, or extension
 /// type.
 ///
@@ -2806,6 +2823,13 @@ abstract class TypeParameterizedFragment implements Fragment, Annotatable {
 ///
 /// Clients may not extend, implement or mix-in this class.
 abstract class VariableElement2 implements Element2 {
+  /// The constant initializer for this constant variable, or the default
+  /// value for this formal parameter.
+  ///
+  /// Is `null` if this variable is not a constant, or does not have the
+  /// initializer or the default value specified.
+  ConstantInitializer? get constantInitializer2;
+
   @override
   VariableFragment get firstFragment;
 
@@ -2857,6 +2881,13 @@ abstract class VariableElement2 implements Element2 {
 ///
 /// Clients may not extend, implement or mix-in this class.
 abstract class VariableFragment implements Fragment {
+  /// The constant initializer for this constant variable fragment, or the
+  /// default value for this formal parameter fragment.
+  ///
+  /// Is `null` if this variable fragment is not a constant, or does not have
+  /// the initializer or the default value specified.
+  ConstantInitializer? get constantInitializer2;
+
   @override
   VariableElement2 get element;
 

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

@@ -11671,6 +11671,12 @@ abstract class VariableElementImpl extends ElementImpl
   /// initializers.
   Expression? get constantInitializer => null;
 
+  @override
+  ConstantInitializer? get constantInitializer2 {
+    // TODO(scheglov): implement it
+    throw UnimplementedError();
+  }
+
   @override
   VariableElement get declaration => this;
 
@@ -11768,6 +11774,12 @@ abstract class VariableElementImpl extends ElementImpl
 
 abstract class VariableElementImpl2 extends ElementImpl2
     implements VariableElement2OrMember {
+  @override
+  ConstantInitializer? get constantInitializer2 {
+    // TODO(scheglov): implement it
+    throw UnimplementedError();
+  }
+
   @override
   void visitChildren2<T>(ElementVisitor2<T> visitor) {
     for (var child in children2) {

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

@@ -561,6 +561,12 @@ class FieldMember extends VariableMember
   List<Element2> get children2 =>
       children.map((fragment) => fragment.asElement2).nonNulls.toList();
 
+  @override
+  ConstantInitializer? get constantInitializer2 {
+    // TODO(scheglov): implement it
+    throw UnimplementedError();
+  }
+
   @override
   FieldElement get declaration => super.declaration as FieldElement;
 
@@ -1211,6 +1217,12 @@ class ParameterMember extends VariableMember
   List<Element2> get children2 =>
       children.map((fragment) => fragment.asElement2).nonNulls.toList();
 
+  @override
+  ConstantInitializer? get constantInitializer2 {
+    // TODO(scheglov): implement it
+    throw UnimplementedError();
+  }
+
   @override
   ParameterElementImpl get declaration =>
       super.declaration as ParameterElementImpl;