Fix no_extension_access_modifier rule triggering on nonisolated modifier (#6174)

Co-authored-by: Danny Mösch <[email protected]>

Author: Copilot

CHANGELOG.md

@@ -95,6 +95,12 @@
 
 ### Bug Fixes
 
+* Fix `no_extension_access_modifier` rule incorrectly triggering for `nonisolated extension`.
+  The rule now only flags actual access control modifiers (`private`, `public`, `open`,
+  `internal`, `fileprivate`) and ignores isolation modifiers like `nonisolated`.  
+  [copilot](https://github.com/copilot)
+  [#6168](https://github.com/realm/SwiftLint/issues/6168)
+
 * Improved error reporting when SwiftLint exits, because of an invalid configuration file
   or other error.  
   [Martin Redington](https://github.com/mildm8nnered)

Source/SwiftLintBuiltInRules/Rules/Idiomatic/NoExtensionAccessModifierRule.swift

@@ -12,6 +12,7 @@ struct NoExtensionAccessModifierRule: Rule {
         nonTriggeringExamples: [
             Example("extension String {}"),
             Example("\n\n extension String {}"),
+            Example("nonisolated extension String {}"),
         ],
         triggeringExamples: [
             Example("↓private extension String {}"),
@@ -29,8 +30,8 @@ private extension NoExtensionAccessModifierRule {
 
         override func visitPost(_ node: ExtensionDeclSyntax) {
             let modifiers = node.modifiers
-            if modifiers.isNotEmpty {
-                violations.append(modifiers.positionAfterSkippingLeadingTrivia)
+            if let accessLevelModifier = modifiers.accessLevelModifier {
+                violations.append(accessLevelModifier.positionAfterSkippingLeadingTrivia)
             }
         }
     }