lib/src/rules/avoid_void_async.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 'package:analyzer/dart/element/element.dart';

import '../analyzer.dart';

const _desc = r'Avoid async functions that return void.';

const _details = r'''
**DO** mark async functions as returning Future<void>.

When declaring an async method or function which does not return a value,
declare that it returns `Future<void>` and not just `void`.

**BAD:**
```dart
void f() async {}
void f2() async => null;
```

**GOOD:**
```dart
Future<void> f() async {}
Future<void> f2() async => null;
```

**EXCEPTION:**

An exception is made for top-level `main` functions, where the `Future`
annotation *can* (and generally should) be dropped in favor of `void`.

**GOOD:**
```dart
Future<void> f() async {}

void main() async {
  await f();
}
```
''';

class AvoidVoidAsync extends LintRule {
  static const LintCode code = LintCode('avoid_void_async',
      "The return type should be '{0}' when nothing is returned from an '{1}' function.",
      correctionMessage: 'Try changing the return type.');

  AvoidVoidAsync()
      : super(
            name: 'avoid_void_async',
            description: _desc,
            details: _details,
            group: Group.style);

  @override
  LintCode get lintCode => code;

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

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

  _Visitor(this.rule);

  @override
  void visitFunctionDeclaration(FunctionDeclaration node) {
    var arguments = _arguments(node.declaredElement);
    if (arguments != null &&
        _isVoid(node.returnType) &&
        node.name.lexeme != 'main') {
      rule.reportLintForToken(node.name, arguments: arguments);
    }
  }

  @override
  void visitMethodDeclaration(MethodDeclaration node) {
    var arguments = _arguments(node.declaredElement);
    if (arguments != null && _isVoid(node.returnType)) {
      rule.reportLintForToken(node.name, arguments: arguments);
    }
  }

  List<String>? _arguments(ExecutableElement? element) {
    if (element == null) {
      return null;
    } else if (element.isAsynchronous) {
      return ['Future', 'async'];
    } else if (element.isGenerator) {
      return ['Stream', 'async*'];
    }
    return null;
  }

  bool _isVoid(TypeAnnotation? typeAnnotation) =>
      typeAnnotation?.type?.isVoid ?? false;
}