Implement the never_discard_build_context lint

packages/leancode_lint/.fvmrc

@@ -1,3 +1,3 @@
 {
-  "flutter": "3.38.1"
+  "flutter": "3.41.2"
 }

packages/leancode_lint/README.md

@@ -219,7 +219,7 @@ None.
 </details>
 
 <details>
-<summary>`bloc_related_class_naming`</summary>
+<summary><code>bloc_related_class_naming</code></summary>
 
 ### `bloc_related_class_naming`
 
@@ -392,6 +392,58 @@ None.
 
 </details>
 
+<details>
+<summary><code>never_discard_build_context</code></summary>
+
+### `never_discard_build_context`
+
+**AVOID** discarding `BuildContext` parameters by naming them `_`.
+
+Discarding a `BuildContext` causes the body to use an ancestor context instead,
+risking incorrect theme lookups, navigation, or inherited widget reads.
+
+**BAD:**
+
+```dart
+Widget myBuilder(BuildContext _) {
+  return const SizedBox();
+}
+```
+
+**BAD:**
+
+```dart
+Widget build(BuildContext context) {
+  return Builder(
+    builder: (_) => const SizedBox(),
+  );
+}
+```
+
+**GOOD:**
+
+```dart
+Widget myBuilder(BuildContext context) {
+  return const SizedBox();
+}
+```
+
+**GOOD:**
+
+```dart
+Widget build(BuildContext context) {
+  return Builder(
+    builder: (context) => const SizedBox(),
+  );
+}
+```
+
+#### Configuration
+
+None.
+
+</details>
+
 <details>
 <summary><code>prefix_widgets_returning_slivers</code></summary>
 

packages/leancode_lint/lib/plugin.dart

@@ -11,6 +11,7 @@ import 'package:leancode_lint/src/lints/bloc_related_class_naming.dart';
 import 'package:leancode_lint/src/lints/catch_parameter_names.dart';
 import 'package:leancode_lint/src/lints/constructor_parameters_and_fields_should_have_the_same_order.dart';
 import 'package:leancode_lint/src/lints/hook_widget_does_not_use_hooks.dart';
+import 'package:leancode_lint/src/lints/never_discard_build_context.dart';
 import 'package:leancode_lint/src/lints/prefer_equatable_mixin.dart';
 import 'package:leancode_lint/src/lints/prefix_widgets_returning_slivers.dart';
 import 'package:leancode_lint/src/lints/start_comments_with_space.dart';
@@ -63,6 +64,11 @@ final class LeanCodeLintPlugin extends Plugin {
         HookWidgetDoesNotUseHooks.code,
         ConvertHookWidgetToStatelessWidget.new,
       )
+      ..registerWarningRule(NeverDiscardBuildContext())
+      ..registerFixForRule(
+        NeverDiscardBuildContext.code,
+        RenameDiscardedBuildContextFix.new,
+      )
       // TODO: disabled by default until stabilized. Add documentation.
       ..registerLintRule(ConstructorParametersAndFieldsShouldHaveTheSameOrder())
       ..registerWarningRule(AvoidSingleChildInMultiChildWidgets())

packages/leancode_lint/lib/src/helpers.dart

@@ -330,7 +330,7 @@ abstract class ChangeWidgetNameFix extends ResolvedCorrectionProducer {
   final String widgetName;
 
   @override
-  FixKind? get fixKind => .new(
+  FixKind get fixKind => .new(
     'leancode_lint.fix.replaceWith$widgetName',
     DartFixKindPriority.standard,
     'Replace with $widgetName',

packages/leancode_lint/lib/src/lints/hook_widget_does_not_use_hooks.dart

@@ -53,7 +53,7 @@ class ConvertHookWidgetToStatelessWidget extends ResolvedCorrectionProducer {
   ConvertHookWidgetToStatelessWidget({required super.context});
 
   @override
-  FixKind? get fixKind => const FixKind(
+  FixKind get fixKind => const .new(
     'leancode_lint.fix.convertHookWidgetToStatelessWidget',
     DartFixKindPriority.standard,
     'Convert HookWidget to StatelessWidget',

packages/leancode_lint/lib/src/lints/never_discard_build_context.dart

@@ -0,0 +1,97 @@
+import 'package:analysis_server_plugin/edit/dart/correction_producer.dart';
+import 'package:analysis_server_plugin/edit/dart/dart_fix_kind_priority.dart';
+import 'package:analyzer/analysis_rule/analysis_rule.dart';
+import 'package:analyzer/analysis_rule/rule_context.dart';
+import 'package:analyzer/analysis_rule/rule_visitor_registry.dart';
+import 'package:analyzer/dart/ast/ast.dart';
+import 'package:analyzer/dart/ast/visitor.dart';
+import 'package:analyzer/error/error.dart';
+import 'package:analyzer_plugin/utilities/change_builder/change_builder_core.dart';
+import 'package:analyzer_plugin/utilities/fixes/fixes.dart';
+import 'package:analyzer_plugin/utilities/range_factory.dart';
+import 'package:leancode_lint/src/type_checker.dart';
+
+/// Warns when a BuildContext parameter is explicitly discarded using `_`.
+///
+/// Discarding a context causes the body to use an ancestor context instead,
+/// risking incorrect theme lookups, navigation, or inherited widget reads.
+class NeverDiscardBuildContext extends AnalysisRule {
+  NeverDiscardBuildContext()
+    : super(name: code.lowerCaseName, description: code.problemMessage);
+
+  static const code = LintCode(
+    'never_discard_build_context',
+    "Don't discard BuildContext parameters.",
+    correctionMessage:
+        'Give the BuildContext parameter a name to avoid accidentally using an ancestor context.',
+    severity: .WARNING,
+  );
+
+  @override
+  LintCode get diagnosticCode => code;
+
+  @override
+  void registerNodeProcessors(
+    RuleVisitorRegistry registry,
+    RuleContext context,
+  ) {
+    registry.addFormalParameterList(this, _Visitor(this));
+  }
+}
+
+class _Visitor extends SimpleAstVisitor<void> {
+  _Visitor(this.rule);
+
+  final AnalysisRule rule;
+
+  static const _buildContextChecker = TypeChecker.fromName(
+    'BuildContext',
+    packageName: 'flutter',
+  );
+
+  @override
+  void visitFormalParameterList(FormalParameterList node) {
+    node.parameters.forEach(_checkParameter);
+  }
+
+  void _checkParameter(FormalParameter parameter) {
+    final name = parameter.name;
+    if (name == null || name.lexeme != '_') {
+      return;
+    }
+
+    final type = parameter.declaredFragment?.element.type;
+    if (type == null) {
+      return;
+    }
+
+    if (_buildContextChecker.isExactlyType(type)) {
+      rule.reportAtToken(name);
+    }
+  }
+}
+
+class RenameDiscardedBuildContextFix extends ResolvedCorrectionProducer {
+  RenameDiscardedBuildContextFix({required super.context});
+
+  @override
+  FixKind get fixKind => const .new(
+    'leancode_lint.fix.renameDiscardedBuildContext',
+    DartFixKindPriority.standard,
+    "Rename to 'context'",
+  );
+
+  @override
+  CorrectionApplicability get applicability => .automatically;
+
+  @override
+  Future<void> compute(ChangeBuilder builder) async {
+    await builder.addDartFileEdit(
+      file,
+      (builder) => builder.addSimpleReplacement(
+        range.diagnostic(diagnostic!),
+        'context',
+      ),
+    );
+  }
+}

packages/leancode_lint/lib/src/lints/prefer_equatable_mixin.dart

@@ -79,7 +79,7 @@ class ConvertToEquatableMixin extends ResolvedCorrectionProducer {
   ConvertToEquatableMixin({required super.context});
 
   @override
-  FixKind? get fixKind => const FixKind(
+  FixKind get fixKind => const .new(
     'leancode_lint.fix.convertToEquatableMixin',
     DartFixKindPriority.standard,
     'Convert to EquatableMixin',

packages/leancode_lint/lib/src/lints/start_comments_with_space.dart

@@ -91,7 +91,7 @@ class AddStartingSpaceToComment extends ResolvedCorrectionProducer {
   AddStartingSpaceToComment({required super.context});
 
   @override
-  FixKind? get fixKind => const .new(
+  FixKind get fixKind => const .new(
     'leancode_lint.fix.addStartingSpaceToComment',
     DartFixKindPriority.standard,
     'Add leading space to comment',

packages/leancode_lint/lib/src/lints/use_dedicated_media_query_methods.dart

@@ -147,7 +147,7 @@ class ReplaceMediaQueryOfWithDedicatedMethodFix
   ReplaceMediaQueryOfWithDedicatedMethodFix({required super.context});
 
   @override
-  FixKind? get fixKind => const .new(
+  FixKind get fixKind => const .new(
     'leancode_lint.fix.replaceMediaQueryOfWithDedicatedMethod',
     DartFixKindPriority.standard,
     'Replace with the dedicated MediaQuery method',

packages/leancode_lint/lib/src/lints/use_padding.dart

@@ -65,7 +65,7 @@ class UsePaddingFix extends ResolvedCorrectionProducer {
   UsePaddingFix({required super.context});
 
   @override
-  FixKind? get fixKind => const .new(
+  FixKind get fixKind => const .new(
     'leancode_lint.fix.usePadding',
     DartFixKindPriority.standard,
     'Replace with Padding',