[element model] migrate avoid_null_checks_in_equality_operators

This includes an incomplete migration of `canonicalElement`. I *think* it's sufficient for this lint but please check me. (It handles parens minimally.)

Looking ahead, some of the complexity in `canonicalElement` may go away in the new world but I'm still wrapping my head around it.

(Some relevant context: #35343.)

Bug: https://github.com/dart-lang/linter/issues/5099
Change-Id: Ie2ea18f173ae2a95cab0d3b3d5b82442fce6f026
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/391201
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>

Author: pq

pkg/linter/lib/src/extensions.dart

@@ -100,6 +100,21 @@ extension AstNodeNullableExtension on AstNode? {
     return null;
   }
 
+  Element2? get canonicalElement2 {
+    // TODO(pq): can this be replaced w/ a use of an `ElementLocator2`?
+    var self = this;
+    if (self is Expression) {
+      var node = self.unParenthesized;
+      if (node is Identifier) {
+        return node.element;
+      } else if (node is PropertyAccess) {
+        // TODO(pq): implement.
+        return null;
+      }
+    }
+    return null;
+  }
+
   /// Whether the expression is null-aware, or if one of its recursive targets
   /// is null-aware.
   bool get containsNullAwareInvocationInChain {

pkg/linter/lib/src/rules/avoid_null_checks_in_equality_operators.dart

@@ -5,7 +5,7 @@
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
-import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/dart/element/element2.dart';
 import 'package:analyzer/dart/element/nullability_suffix.dart';
 
 import '../analyzer.dart';
@@ -16,18 +16,19 @@ const _desc = r"Don't check for `null` in custom `==` operators.";
 bool _isComparingEquality(TokenType tokenType) =>
     tokenType == TokenType.BANG_EQ || tokenType == TokenType.EQ_EQ;
 
-bool _isComparingParameterWithNull(BinaryExpression node, Element? parameter) =>
+bool _isComparingParameterWithNull(
+        BinaryExpression node, Element2? parameter) =>
     _isComparingEquality(node.operator.type) &&
     ((node.leftOperand.isNullLiteral &&
             _isParameter(node.rightOperand, parameter)) ||
         (node.rightOperand.isNullLiteral &&
             _isParameter(node.leftOperand, parameter)));
 
-bool _isParameter(Expression expression, Element? parameter) =>
-    expression.canonicalElement == parameter;
+bool _isParameter(Expression expression, Element2? parameter) =>
+    expression.canonicalElement2 == parameter;
 
 bool _isParameterWithQuestionQuestion(
-        BinaryExpression node, Element? parameter) =>
+        BinaryExpression node, Element2? parameter) =>
     node.operator.type == TokenType.QUESTION_QUESTION &&
     _isParameter(node.leftOperand, parameter);
 
@@ -51,7 +52,7 @@ class AvoidNullChecksInEqualityOperators extends LintRule {
 }
 
 class _BodyVisitor extends RecursiveAstVisitor<void> {
-  final Element? parameter;
+  final Element2? parameter;
   final LintRule rule;
 
   _BodyVisitor(this.parameter, this.rule);
@@ -68,7 +69,7 @@ class _BodyVisitor extends RecursiveAstVisitor<void> {
   @override
   visitMethodInvocation(MethodInvocation node) {
     if (node.operator?.type == TokenType.QUESTION_PERIOD &&
-        node.target.canonicalElement == parameter) {
+        node.target.canonicalElement2 == parameter) {
       rule.reportLint(node);
     }
     super.visitMethodInvocation(node);
@@ -77,7 +78,7 @@ class _BodyVisitor extends RecursiveAstVisitor<void> {
   @override
   visitPropertyAccess(PropertyAccess node) {
     if (node.operator.type == TokenType.QUESTION_PERIOD &&
-        node.target.canonicalElement == parameter) {
+        node.target.canonicalElement2 == parameter) {
       rule.reportLint(node);
     }
     super.visitPropertyAccess(node);
@@ -100,11 +101,11 @@ class _Visitor extends SimpleAstVisitor<void> {
       return;
     }
 
-    var parameter = parameters.first.declaredElement?.canonicalElement;
+    var parameter = parameters.first.declaredFragment?.element;
 
     // Analyzer will produce UNNECESSARY_NULL_COMPARISON_FALSE|TRUE
     // See: https://github.com/dart-lang/linter/issues/2864
-    if (parameter is VariableElement &&
+    if (parameter is FormalParameterElement &&
         parameter.type.nullabilitySuffix != NullabilitySuffix.question) {
       return;
     }

pkg/linter/test/rules/avoid_null_checks_in_equality_operators_test.dart

@@ -73,6 +73,20 @@ class C {
     ]);
   }
 
+  test_nullableParameter_eqeqNull_not_parens() async {
+    await assertDiagnostics(r'''
+class C {
+  String foo = '';
+  @override
+  operator ==(Object? other) =>
+          !((other) == null) && (other) is C && foo == (other.foo);
+}
+''', [
+      error(WarningCode.NON_NULLABLE_EQUALS_PARAMETER, 52, 2),
+      lint(85, 15),
+    ]);
+  }
+
   test_nullableParameter_fieldComparisonOnLocal() async {
     await assertDiagnostics(r'''
 class C {