[linter] Adds new switch_on_type lint

Fixes: #59546
Change-Id: Ib78714e03c1487376c961214910d10c24ec9f8ff
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/413600
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>
Auto-Submit: Felipe Morschel <[email protected]>

Author: FMorschel

pkg/analysis_server/lib/src/services/correction/error_fix_status.yaml

@@ -2394,6 +2394,8 @@ LintCode.strict_top_level_inference_split_to_types:
     Conceivably for variable declarations we can search some space (like the
     library or the compilation unit) for what values are assigned, and suggest
     the LUB of those value(s)' type(s).
+LintCode.switch_on_type:
+  status: noFix
 LintCode.test_types_in_equals:
   status: noFix
 LintCode.throw_in_finally:

pkg/linter/example/all.yaml

@@ -172,6 +172,7 @@ linter:
     - specify_nonobvious_local_variable_types
     - specify_nonobvious_property_types
     - strict_top_level_inference
+    - switch_on_type
     - test_types_in_equals
     - throw_in_finally
     - tighten_type_of_initializing_formals

pkg/linter/lib/src/lint_codes.g.dart

@@ -1542,6 +1542,12 @@ class LinterLintCode extends LintCode {
         uniqueName: 'strict_top_level_inference_split_to_types',
       );
 
+  static const LintCode switch_on_type = LinterLintCode(
+    LintNames.switch_on_type,
+    "Avoid switch statements on a 'Type'.",
+    correctionMessage: "Try using pattern matching on a variable instead.",
+  );
+
   static const LintCode test_types_in_equals = LinterLintCode(
     LintNames.test_types_in_equals,
     "Missing type test for '{0}' in '=='.",

pkg/linter/lib/src/lint_names.g.dart

@@ -461,6 +461,8 @@ abstract final class LintNames {
 
   static const String super_goes_last = 'super_goes_last';
 
+  static const String switch_on_type = 'switch_on_type';
+
   static const String test_types_in_equals = 'test_types_in_equals';
 
   static const String throw_in_finally = 'throw_in_finally';

pkg/linter/lib/src/rules.dart

@@ -190,6 +190,7 @@ import 'rules/specify_nonobvious_local_variable_types.dart';
 import 'rules/specify_nonobvious_property_types.dart';
 import 'rules/strict_top_level_inference.dart';
 import 'rules/super_goes_last.dart';
+import 'rules/switch_on_type.dart';
 import 'rules/test_types_in_equals.dart';
 import 'rules/throw_in_finally.dart';
 import 'rules/tighten_type_of_initializing_formals.dart';
@@ -442,6 +443,7 @@ void registerLintRules() {
     ..registerLintRule(SpecifyNonObviousLocalVariableTypes())
     ..registerLintRule(SpecifyNonObviousPropertyTypes())
     ..registerLintRule(StrictTopLevelInference())
+    ..registerLintRule(SwitchOnType())
     ..registerLintRule(TestTypesInEquals())
     ..registerLintRule(ThrowInFinally())
     ..registerLintRule(TightenTypeOfInitializingFormals())

pkg/linter/lib/src/rules/switch_on_type.dart

@@ -0,0 +1,158 @@
+// Copyright (c) 2025, 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/element.dart';
+import 'package:analyzer/dart/element/type.dart';
+import 'package:analyzer/error/error.dart';
+
+import '../analyzer.dart';
+import '../extensions.dart';
+
+const _desc = "Avoid switch statements on a 'Type'.";
+
+const _objectToStringName = 'toString';
+
+class SwitchOnType extends LintRule {
+  SwitchOnType() : super(name: LintNames.switch_on_type, description: _desc);
+
+  @override
+  DiagnosticCode get diagnosticCode => LinterLintCode.switch_on_type;
+
+  @override
+  LintCode get lintCode => LinterLintCode.switch_on_type;
+
+  @override
+  void registerNodeProcessors(
+    NodeLintRegistry registry,
+    LinterContext context,
+  ) {
+    if (!context.isEnabled(Feature.patterns)) return;
+    var visitor = _Visitor(this, context);
+    registry.addSwitchExpression(this, visitor);
+    registry.addSwitchStatement(this, visitor);
+  }
+}
+
+class _Visitor extends SimpleAstVisitor<void> {
+  final LintRule rule;
+
+  final LinterContext context;
+
+  /// The node where the lint will be reported.
+  late AstNode node;
+
+  _Visitor(this.rule, this.context);
+
+  /// A reference to the [Type] type.
+  ///
+  /// This is used to check if the type of the expression is assignable to
+  /// [Type].
+  ///
+  /// This shortens the code and avoids multiple calls to
+  /// `context.typeProvider.typeType`.
+  InterfaceType get _typeType => context.typeProvider.typeType;
+
+  @override
+  void visitSwitchExpression(SwitchExpression node) {
+    this.node = node.expression;
+    _processExpression(node.expression);
+  }
+
+  @override
+  void visitSwitchStatement(SwitchStatement node) {
+    this.node = node.expression;
+    _processExpression(node.expression);
+  }
+
+  /// Returns `true` if the [type] is assignable to [Type].
+  bool _isAssignableToType(DartType? type) {
+    if (type == null) return false;
+    return context.typeSystem.isAssignableTo(type, _typeType);
+  }
+
+  /// Processes the [expression] of a [SwitchStatement] or [SwitchExpression].
+  ///
+  /// Returns `true` if the lint was reportred and `false` otherwise.
+  bool _processExpression(Expression expression) {
+    if (expression case StringInterpolation(:var elements)) {
+      return _processInterpolation(elements);
+    }
+    if (expression case ConditionalExpression(
+      :var thenExpression,
+      :var elseExpression,
+    )) {
+      return _processExpression(thenExpression) ||
+          _processExpression(elseExpression);
+    }
+    if (expression case SwitchExpression(:var cases)) {
+      for (var caseClause in cases) {
+        if (_processExpression(caseClause.expression)) {
+          return true;
+        }
+      }
+      return false;
+    }
+    if (expression case BinaryExpression(
+      :var leftOperand,
+      :var rightOperand,
+      :var operator,
+    ) when operator.lexeme == TokenType.PLUS.lexeme) {
+      return _processExpression(leftOperand) ||
+          _processExpression(rightOperand);
+    }
+    var type = switch (expression) {
+      PrefixedIdentifier(:var identifier) => identifier.staticType,
+      PropertyAccess(:var propertyName) => propertyName.staticType,
+      SimpleIdentifier(:var staticType) => staticType,
+      MethodInvocation(:var methodName, :var realTarget?) =>
+        methodName.element.isToStringMethod ? realTarget.staticType : null,
+      _ => null,
+    };
+    return _reportIfAssignableToType(type);
+  }
+
+  /// Processes the [elements] of an [InterpolationExpression].
+  ///
+  /// Returns `true` if the lint was reported and `false` otherwise.
+  bool _processInterpolation(NodeList<InterpolationElement> elements) {
+    for (var element in elements) {
+      switch (element) {
+        case InterpolationExpression(:var expression):
+          var reported = _processExpression(expression);
+
+          // This return is necessary to avoid multiple reporting of the lint
+          if (reported) {
+            return true;
+          }
+        case InterpolationString():
+          break;
+      }
+    }
+    return false;
+  }
+
+  /// Reports the lint if the [type] is assignable to [Type].
+  ///
+  /// Returns `true` if the lint was reported and `false` otherwise.
+  bool _reportIfAssignableToType(DartType? type) {
+    var reported = false;
+    if (_isAssignableToType(type)) {
+      rule.reportAtNode(node);
+      reported = true;
+    }
+    return reported;
+  }
+}
+
+extension on Element? {
+  /// Returns `true` if this element is the `toString` method.
+  bool get isToStringMethod {
+    var self = this;
+    return self is MethodElement && self.name3 == _objectToStringName;
+  }
+}

pkg/linter/messages.yaml

@@ -11344,6 +11344,82 @@ LintCode:
           : _children = children,
             super(style) {
       ```
+  switch_on_type:
+    problemMessage: "Avoid switch statements on a 'Type'."
+    correctionMessage: "Try using pattern matching on a variable instead."
+    state:
+      stable: "3.0"
+    categories: [unintentional, style, languageFeatureUsage, errorProne]
+    hasPublishedDocs: false
+    documentation: |-
+      #### Description
+
+      The analyzer produces this diagnostic when a switch statement or switch
+      expression is used on either the value of a `Type` or a `toString` call
+      on a `Type`.
+
+      #### Example
+
+      The following code produces this diagnostic because the switch statement
+      is used on a `Type`:
+
+      ```dart
+      void f(Object o) {
+        switch ([!o.runtimeType!]) {
+          case const (int):
+            print('int');
+          case const (String):
+            print('String');
+        }
+      }
+      ```
+
+      #### Common fixes
+
+      Use pattern matching on the variable instead:
+
+      ```dart
+      void f(Object o) {
+        switch (o) {
+          case int():
+            print('int');
+          case String():
+            print('String');
+        }
+      }
+      ```
+    deprecatedDetails: |-
+      **AVOID** using switch on `Type`.
+
+      Switching on `Type` is not type-safe and can lead to bugs if the
+      class hierarchy changes. Prefer to use pattern matching on the variable
+      instead.
+
+      **BAD:**
+      ```dart
+      void f(Object o) {
+        switch (o.runtimeType) {
+          case int:
+            print('int');
+          case String:
+            print('String');
+        }
+      }
+      ```
+
+      **GOOD:**
+      ```dart
+      void f(Object o) {
+        switch(o) {
+          case int():
+            print('int');
+          case String _:
+            print('String');
+          default:
+            print('other');
+        }
+      }
+      ```
   test_types_in_equals:
     problemMessage: "Missing type test for '{0}' in '=='."
     correctionMessage: "Try testing the type of '{0}'."

pkg/linter/test/rules/all.dart

@@ -242,6 +242,7 @@ import 'specify_nonobvious_local_variable_types_test.dart'
 import 'specify_nonobvious_property_types_test.dart'
     as specify_nonobvious_property_types;
 import 'strict_top_level_inference_test.dart' as strict_top_level_inference;
+import 'switch_on_type_test.dart' as switch_on_type;
 import 'test_types_in_equals_test.dart' as test_types_in_equals;
 import 'throw_in_finally_test.dart' as throw_in_finally;
 import 'tighten_type_of_initializing_formals_test.dart'
@@ -502,6 +503,7 @@ void main() {
   specify_nonobvious_local_variable_types.main();
   specify_nonobvious_property_types.main();
   strict_top_level_inference.main();
+  switch_on_type.main();
   test_types_in_equals.main();
   throw_in_finally.main();
   tighten_type_of_initializing_formals.main();