| // Copyright (c) 2020, 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/analysis/features.dart'; |
| 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/type.dart'; |
| |
| import '../analyzer.dart'; |
| import 'unnecessary_null_checks.dart'; |
| |
| const _desc = r"Don't use null check on a potentially nullable type parameter."; |
| |
| const _details = r''' |
| |
| Don't use null check on a potentially nullable type parameter. |
| |
| Given a generic type parameter `T` which has a nullable bound (e.g. the default |
| bound of `Object?`), it is very easy to introduce erroneous null checks when |
| working with a variable of type `T?`. Specifically, it is not uncommon to have |
| `T? x;` and want to assert that `x` has been set to a valid value of type `T`. |
| A common mistake is to do so using `x!`. This is almost always incorrect, since |
| if `T` is a nullable type, `x` may validly hold `null` as a value of type `T`. |
| |
| **BAD:** |
| ```dart |
| T run<T>(T callback()) { |
| T? result; |
| (() { result = callback(); })(); |
| return result!; |
| } |
| ``` |
| |
| **GOOD:** |
| ```dart |
| T run<T>(T callback()) { |
| T? result; |
| (() { result = callback(); })(); |
| return result as T; |
| } |
| ``` |
| |
| '''; |
| |
| class NullCheckOnNullableTypeParameter extends LintRule |
| implements NodeLintRule { |
| NullCheckOnNullableTypeParameter() |
| : super( |
| name: 'null_check_on_nullable_type_parameter', |
| description: _desc, |
| details: _details, |
| maturity: Maturity.experimental, |
| group: Group.style, |
| ); |
| |
| @override |
| void registerNodeProcessors( |
| NodeLintRegistry registry, LinterContext context) { |
| if (!context.isEnabled(Feature.non_nullable)) { |
| return; |
| } |
| |
| var visitor = _Visitor(this, context); |
| registry.addCompilationUnit(this, visitor); |
| registry.addPostfixExpression(this, visitor); |
| } |
| } |
| |
| class _Visitor extends SimpleAstVisitor<void> { |
| _Visitor(this.rule, this.context); |
| |
| final LintRule rule; |
| final LinterContext context; |
| |
| @override |
| void visitPostfixExpression(PostfixExpression node) { |
| if (node.operator.type != TokenType.BANG) return; |
| |
| var expectedType = getExpectedType(node); |
| var type = node.operand.staticType; |
| if (type is TypeParameterType && |
| context.typeSystem.isNullable(type) && |
| expectedType != null && |
| context.typeSystem.isPotentiallyNullable(expectedType) && |
| context.typeSystem.promoteToNonNull(type) == |
| context.typeSystem.promoteToNonNull(expectedType)) { |
| rule.reportLintForToken(node.operator); |
| } |
| } |
| } |