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

// Copyright (c) 2017, 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 'package:analyzer/dart/ast/ast.dart';
import 'package:analyzer/dart/ast/token.dart';
import 'package:analyzer/dart/ast/visitor.dart';

import '../analyzer.dart';
import '../extensions.dart';
import '../util/dart_type_utilities.dart' as type_utils;

const _desc = r'Prefer using `??=` over testing for `null`.';

bool _checkExpression(Expression expression, Expression condition) =>
    expression is AssignmentExpression &&
    type_utils.canonicalElementsFromIdentifiersAreEqual(
      expression.leftHandSide,
      condition,
    );

bool _checkStatement(Statement statement, Expression condition) {
  if (statement is ExpressionStatement) {
    return _checkExpression(statement.expression, condition);
  }
  if (statement is Block && statement.statements.length == 1) {
    return _checkStatement(statement.statements.first, condition);
  }
  return false;
}

Expression? _getExpressionCondition(Expression rawExpression) {
  var expression = rawExpression.unParenthesized;
  if (expression is BinaryExpression &&
      expression.operator.type == TokenType.EQ_EQ) {
    if (expression.rightOperand.isNullLiteral) {
      return expression.leftOperand;
    }
    if (expression.leftOperand.isNullLiteral) {
      return expression.rightOperand;
    }
  }
  return null;
}

class PreferConditionalAssignment extends LintRule {
  PreferConditionalAssignment()
    : super(name: LintNames.prefer_conditional_assignment, description: _desc);

  @override
  LintCode get lintCode => LinterLintCode.prefer_conditional_assignment;

  @override
  void registerNodeProcessors(
    NodeLintRegistry registry,
    LinterContext context,
  ) {
    var visitor = _Visitor(this);
    registry.addIfStatement(this, visitor);
  }
}

class _Visitor extends SimpleAstVisitor<void> {
  final LintRule rule;

  _Visitor(this.rule);

  @override
  void visitIfStatement(IfStatement node) {
    if (node.elseStatement != null) {
      return;
    }
    var expressionInCondition = _getExpressionCondition(node.expression);
    if (expressionInCondition != null &&
        _checkStatement(node.thenStatement, expressionInCondition)) {
      rule.reportAtNode(node);
    }
  }
}