I guess that's exactly what I don't understand. If I add a 32-bit integer and a 64-bit integer, what other possible result could I be expecting besides a 64 bit integer?

> If I add a 32-bit integer and a 64-bit integer, what other possible result could I be expecting besides a 64 bit integer?

A 128-bit integer (if you are adding a 32-bit integer and a 64-bit integer, the smallest power-of-2-bits representation guaranteed not to have an overflow is 128-bits, so its the safest result. Though, I'd agree, not the most likely thing most programmers would intend.)

Good point. As painfully explicit as Rust is at times, I'm actually slightly surprised they didn't go that route. (At least for integer sizes less than 32 bits.)

An exception, a 32 bit integer, underflows, or overflows.

That's a problem with addition, period, not type promotion. Adding a 64-bit and 32-bit integer and promoting the 32-bit integer to 64-bit doesn't produce any problems that you won't have adding two 32-bit integers together or two 64-bit integers.