pkg/linter/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/token.dart';
import 'package:analyzer/dart/ast/visitor.dart';
import 'package:analyzer/dart/element/element.dart';
import 'package:analyzer/dart/element/type.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',
    "An 'async' function should have a 'Future' return type when it doesn't "
        'return a value.',
    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) {
    if (node.name.lexeme == 'main') return;
    _check(
      declaredElement: node.declaredElement,
      returnType: node.returnType,
      errorNode: node.name,
    );
  }

  @override
  void visitMethodDeclaration(MethodDeclaration node) {
    _check(
      declaredElement: node.declaredElement,
      returnType: node.returnType,
      errorNode: node.name,
    );
  }

  void _check({
    required ExecutableElement? declaredElement,
    required TypeAnnotation? returnType,
    required Token errorNode,
  }) {
    if (declaredElement == null) return;
    if (declaredElement.isGenerator) return;
    if (!declaredElement.isAsynchronous) return;
    if (returnType == null) return;
    if (returnType.type is VoidType) {
      rule.reportLintForToken(errorNode);
    }
  }
}