lib/src/rules/prefer_final_in_for_each.dart

// Copyright (c) 2019, 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/visitor.dart';
import 'package:linter/src/analyzer.dart';

const _desc =
    r'Prefer final in for-each loop variable if reference is not reassigned.';

const _details = r'''

**DO** prefer declaring for-each loop variables as final if they are not
reassigned later in the code.

Declaring for-each loop variables as final when possible is a good practice
because it helps avoid accidental reassignments and allows the compiler to do
optimizations.

**BAD:**
```
for (var element in elements) { // LINT
  print('Element: $element');
}
```

**GOOD:**
```
for (final element in elements) {
  print('Element: $element');
}
```

**GOOD:**
```
for (var element in elements) {
  element = element + element;
  print('Element: $element');
}
```

''';

class PreferFinalInForEach extends LintRule implements NodeLintRule {
  PreferFinalInForEach()
      : super(
            name: 'prefer_final_in_for_each',
            description: _desc,
            details: _details,
            group: Group.style);

  @override
  void registerNodeProcessors(NodeLintRegistry registry,
      [LinterContext context]) {
    final visitor = new _Visitor(this);
    registry.addForEachStatement(this, visitor);
  }
}

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

  _Visitor(this.rule);

  @override
  void visitForEachStatement(ForEachStatement node) {
    final loopVariable = node.loopVariable;
    if (loopVariable == null) {
      // This statement is something like `for(a in b) { ... }`. Notice `a` is
      // not actually declared from within the loop. `a` is a variable declared
      // outside the loop.
      return;
    }

    if (loopVariable.isFinal) {
      return;
    }

    FunctionBody function = node.thisOrAncestorOfType<FunctionBody>();
    if (function != null &&
        !function.isPotentiallyMutatedInScope(loopVariable.declaredElement)) {
      rule.reportLint(loopVariable.identifier);
    }
  }
}