Add @_objcImplementation attribute

Part of swiftlang/swift#60630, but safe to merge before the compiler-side implementation.

Author: beccadax

Sources/SwiftParser/gyb_generated/DeclarationAttribute.swift

@@ -64,6 +64,7 @@ extension Parser {
     case _staticInitializeObjCMetadata = "_staticInitializeObjCMetadata"
     case _restatedObjCConformance = "_restatedObjCConformance"
     case _typeSequence = "_typeSequence"
+    case _objcImplementation = "_objcImplementation"
     case _optimize = "_optimize"
     case _clangImporterSynthesizedType = "_clangImporterSynthesizedType"
     case _weakLinked = "_weakLinked"

Sources/generate-swift-syntax-builder/gyb_syntax_support/AttributeKinds.py

@@ -398,7 +398,11 @@ def __init__(self, name, swift_name=None):
                   NotSerialized,
                   ABIBreakingToAdd,  ABIBreakingToRemove,  APIBreakingToAdd,  APIBreakingToRemove,
                   code=71),
-  # NOTE: 72 is unused
+    DeclAttribute('_objcImplementation', 'ObjCImplementation',
+                  OnExtension,
+                  UserInaccessible,
+                  ABIBreakingToAdd, ABIBreakingToRemove, APIBreakingToAdd, APIBreakingToRemove,
+                  code=72),
     DeclAttribute('_optimize', 'Optimize',
                   OnAbstractFunction,  OnSubscript,  OnVar,
                   UserInaccessible,

Tests/SwiftParserTest/Attributes.swift

@@ -207,4 +207,15 @@ final class AttributeTests: XCTestCase {
       }
       """)
   }
+    
+  func testObjcImplementationAttribute() throws {
+    AssertParse("""
+      @_objcImplementation extension MyClass {
+        func fn() {}
+      }
+      @_objcImplementation(Category) extension MyClass {
+        func fn2() {}
+      }
+      """)
+  }
 }