[stdlib] Add and use _identityCast

Author: Dave Abrahams

stdlib/public/core/ASCII.swift

@@ -49,12 +49,12 @@ extension Unicode.ASCII : Unicode.Encoding {
     _ content: FromEncoding.EncodedScalar, from _: FromEncoding.Type
   ) -> EncodedScalar? {
     if _fastPath(FromEncoding.self == UTF16.self) {
-      let c = unsafeBitCast(content, to: UTF16.EncodedScalar.self)
+      let c = _identityCast(content, to: UTF16.EncodedScalar.self)
       guard (c._storage & 0xFF80 == 0) else { return nil }
       return EncodedScalar(CodeUnit(c._storage & 0x7f))
     }
     else if _fastPath(FromEncoding.self == UTF8.self) {
-      let c = unsafeBitCast(content, to: UTF8.EncodedScalar.self)
+      let c = _identityCast(content, to: UTF8.EncodedScalar.self)
       guard (c._storage & 0x80 == 0) else { return nil }
       return EncodedScalar(CodeUnit(c._storage & 0x7f))
     }

stdlib/public/core/Builtin.swift

@@ -114,6 +114,18 @@ public func unsafeBitCast<T, U>(_ x: T, to type: U.Type) -> U {
   return Builtin.reinterpretCast(x)
 }
 
+/// Returns `x` as its concrete type `U`.
+///
+/// This cast can be useful for dispatching to specializations of generic
+/// functions.
+///
+/// - Requires: `x` has type `U`.
+@_transparent
+public func _identityCast<T, U>(_ x: T, to expectedType: U.Type) -> U {
+  _precondition(T.self == expectedType, "_identityCast to wrong type")
+  return Builtin.reinterpretCast(x)
+}
+
 /// `unsafeBitCast` something to `AnyObject`.
 @_transparent
 internal func _reinterpretCastToAnyObject<T>(_ x: T) -> AnyObject {