[stdlib] Avoid retaining storage in _StringGuts.updateNativeStorage

Author: lorentey

stdlib/public/core/StringGutsRangeReplaceable.swift

@@ -70,14 +70,14 @@ extension _StringGuts {
   internal mutating func updateNativeStorage<R>(
     _ body: (__StringStorage) -> R
   ) -> R {
-    let storage = self._object.nativeStorage
-    self = _StringGuts()
-    defer {
-      // We re-initialize from the modified storage to pick up new count, flags,
-      // etc.
-      self = _StringGuts(storage)
+    let (r, cf) = self._object.withNativeStorage {
+      let r = body($0)
+      let cf = $0._countAndFlags
+      return (r, cf)
     }
-    return body(storage)
+    // We need to pick up new count/flags from the modified storage.
+    self._object._setCountAndFlags(to: cf)
+    return r
   }
 
   @inlinable

stdlib/public/core/StringObject.swift

@@ -172,6 +172,17 @@ internal struct _StringObject {
     _internalInvariant(!isSmall)
     return CountAndFlags(rawUnchecked: _countAndFlagsBits)
   }
+
+  // FIXME: This ought to be the setter for property `_countAndFlags`.
+  @_alwaysEmitIntoClient
+  internal mutating func _setCountAndFlags(to value: CountAndFlags) {
+#if arch(i386) || arch(arm) || arch(arm64_32) || arch(wasm32)
+    self._count = value.count
+    self._flags = value.flags
+#else
+    self._countAndFlagsBits = value._storage
+#endif
+  }
 }
 
 // Raw