It would still be exposed to user code that checks its size with sizeof, but yeah the long would only have numerical values between 2^-63 and 2^63-1. And yes,

It would still be exposed to user code that checks its size with sizeof, but yeah the long would only have numerical values between 2^-63 and 2^63-1. And yes, there would still be some overhead for storing and propagating the metadata, and struct alignment would change and FFI wouldn't work with longs.