Pseudo-cherry-pick 9669120#r31799259 to the 5.0 branch, since it no longer merges cleanly over (#21807)

Author: Catfish-Man

stdlib/public/core/StringBridge.swift

@@ -302,12 +302,9 @@ extension String {
 // The @_swift_native_objc_runtime_base attribute
 // This allows us to subclass an Objective-C class and use the fast Swift
 // memory allocator.
-@_fixed_layout // FIXME(sil-serialize-all)
 @objc @_swift_native_objc_runtime_base(__SwiftNativeNSStringBase)
-public class __SwiftNativeNSString {
-  @usableFromInline // FIXME(sil-serialize-all)
-  @objc
-  internal init() {}
+class __SwiftNativeNSString {
+  @objc internal init() {}
   deinit {}
 }
 

stdlib/public/core/StringStorage.swift

@@ -76,9 +76,8 @@ extension _AbstractStringStorage {
       fallthrough
     case (_cocoaUTF8Encoding, _):
       guard maxLength >= count + 1 else { return 0 }
-      let buffer =
-        UnsafeMutableBufferPointer(start: outputPtr, count: maxLength)
-      buffer.initialize(from: UnsafeBufferPointer(start: start, count: count))
+      let buffer = UnsafeMutableBufferPointer(start: outputPtr, count: maxLength)
+      _ = buffer.initialize(from: UnsafeBufferPointer(start: start, count: count))
       buffer[count] = 0
       return 1
     default:

test/api-digester/Outputs/stability-stdlib-abi.swift.expected

@@ -1 +1 @@
-
+Class __SwiftNativeNSString has been removed