Fix redundantSendable incorrectly removing Sendable from types inside public extension (#2423)

Co-authored-by: calda <1811727+calda@users.noreply.github.com>

Sources/Rules/RedundantSendable.swift

@@ -21,8 +21,16 @@ public extension FormatRule {
             switch typeDeclaration.visibility() {
             case .public, .open:
                 return
-            case .internal, .package, .fileprivate, .private, nil:
+            case .internal, .package, .fileprivate, .private:
                 break
+            case nil:
+                // A type with no explicit access modifier inside a public extension is effectively public
+                let isInPublicExtension = typeDeclaration.parentDeclarations.last.map {
+                    $0.keyword == "extension" && $0.visibility() == .public
+                } ?? false
+                if isInPublicExtension {
+                    return
+                }
             }
 
             guard let sendableConformance = typeDeclaration.conformances.first(where: {

Tests/Rules/RedundantSendableTests.swift

@@ -93,6 +93,39 @@ final class RedundantSendableTests: XCTestCase {
         testFormatting(for: input, [output], rules: [.redundantSendable])
     }
 
+    func testDoesNotRemoveSendableFromTypeInsidePublicExtension() {
+        let input = """
+        public struct OuterStruct {}
+
+        public extension OuterStruct {
+            struct InnerStruct: Sendable {}
+            enum InnerEnum: Sendable {}
+        }
+        """
+
+        testFormatting(for: input, rules: [.redundantSendable])
+    }
+
+    func testRemovesSendableFromTypeInsideInternalExtension() {
+        let input = """
+        struct OuterStruct {}
+
+        extension OuterStruct {
+            struct InnerStruct: Sendable {}
+        }
+        """
+
+        let output = """
+        struct OuterStruct {}
+
+        extension OuterStruct {
+            struct InnerStruct {}
+        }
+        """
+
+        testFormatting(for: input, [output], rules: [.redundantSendable])
+    }
+
     func testRemovesSendableFromPackageValueTypes() {
         let input = """
         package struct PackageStruct: Sendable {