lib/src/rules/prefer_void_to_null.dart

// Copyright (c) 2018, 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 '../analyzer.dart';

const _desc =
    r"Don't use the Null type, unless you are positive that you don't want void.";

const _details = r'''

**DO NOT** use the type Null where void would work.

**BAD:**
```
Null f() {}
Future<Null> f() {}
Stream<Null> f() {}
f(Null x) {}
```

**GOOD:**
```
void f() {}
Future<void> f() {}
Stream<void> f() {}
f(void x) {}
```

Some exceptions include formulating special function types:

```
Null Function(Null, Null);
```

and for making empty literals which are safe to pass into read-only locations
for any type of map or list:

```
<Null>[];
<int, Null>{};
```
''';

class PreferVoidToNull extends LintRule implements NodeLintRule {
  PreferVoidToNull()
      : super(
            name: 'prefer_void_to_null',
            description: _desc,
            details: _details,
            group: Group.errors);

  @override
  void registerNodeProcessors(
      NodeLintRegistry registry, LinterContext context) {
    final visitor = _Visitor(this);
    registry.addSimpleIdentifier(this, visitor);
    registry.addTypeName(this, visitor);
  }
}

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

  _Visitor(this.rule);

  @override
  void visitTypeName(TypeName node) {
    if (!node.type.isDartCoreNull) {
      return;
    }

    final parent = node.parent;

    // Null Function()
    if (parent is GenericFunctionType) {
      return;
    }

    // Function(Null)
    if (parent is SimpleFormalParameter &&
        parent.parent is FormalParameterList &&
        parent.parent.parent is GenericFunctionType) {
      return;
    }

    // <Null>[] or <Null, Null>{}
    if (parent is TypeArgumentList) {
      final literal = parent.parent;
      if (literal is ListLiteral && literal.elements.isEmpty) {
        return;
      } else if (literal is SetOrMapLiteral && literal.elements.isEmpty) {
        return;
      }
    }

    rule.reportLint(node.name);
  }
}