Fix potentially undefined behavior in StringObject.nativeStorage and document Builtin.unsafeBitCast

stdlib/public/core/Builtin.swift

@@ -72,14 +72,23 @@ func _canBeClass<T>(_: T.Type) -> Int8 {
 ///   `unsafeBitCast(_:to:)` with class or pointer types; doing so may
 ///   introduce undefined behavior.
 ///
-/// - Warning: Calling this function breaks the guarantees of the Swift type
-///   system; use with extreme care.
+/// Warning: Calling this function breaks the guarantees of the Swift type
+/// system; use with extreme care.
 ///
-/// - Parameters:
+/// Warning: Casting from an integer or a pointer type to a reference type
+/// is undefined behavior. It may result in incorrect code in any future
+/// compiler release. To convert a bit pattern to a reference type:
+/// 1. convert the bit pattern to an UnsafeRawPointer.
+/// 2. create an unmanaged reference using Unmanaged.fromOpaque()
+/// 3. obtain a managed reference using Unmanaged.takeUnretainedValue()
+/// The programmer must ensure that the resulting reference has already been
+/// manually retained.
+///
+/// Parameters:
 ///   - x: The instance to cast to `type`.
 ///   - type: The type to cast `x` to. `type` and the type of `x` must have the
 ///     same size of memory representation and compatible memory layout.
-/// - Returns: A new instance of type `U`, cast from `x`.
+/// Returns: A new instance of type `U`, cast from `x`.
 @inlinable // unsafe-performance
 @_transparent
 public func unsafeBitCast<T, U>(_ x: T, to type: U.Type) -> U {

stdlib/public/core/StringObject.swift

@@ -940,7 +940,10 @@ extension _StringObject {
     return _unsafeUncheckedDowncast(storage, to: __StringStorage.self)
 #else
     _internalInvariant(hasNativeStorage)
-    return Builtin.reinterpretCast(largeAddressBits)
+    let storageAddress =
+       UnsafeRawPointer(bitPattern:largeAddressBits)._unsafelyUnwrappedUnchecked
+    let unmanagedRef = Unmanaged<__StringStorage>.fromOpaque(storageAddress)
+    return unmanagedRef.takeUnretainedValue()
 #endif
   }