Also, sometimes you can arrange that the "check" is just the natural way to express what you meant in the language, so whether it's a runtime check or not, the

Also, sometimes you can arrange that the "check" is just the natural way to express what you meant in the language, so whether it's a runtime check or not, the ergonomics are improved enormously, it feels natural. In Rust NonZeroI8::new(n).unwrap() is either a NonZeroI8 or, if n was zero, it panics because we asked to unwrap the Option and that's None. What's inside NonZeroI8::new ? Literally a cast. When compiled this is nothing at all, no machine code is generated. It is relying on the fact that Option<NonZeroI8> has bit pattern 00000000 for None, and that's also the bit pattern for the integer zero. So if we ask at compile time, NonZeroI8::new(FOO).unwrap() for some constant FOO, the compiler will, at compile time, transform the FOO bit pattern, if it's all zeroes this code is just a runtime panic, and if it's on the clear through line (e.g. the only code in the main function) by default the compiler says well that's not going to work, here's a compiler error [if you actually want a program which just panics when run you can ask for that with a compiler setting, good luck to you]. If FOO isn't zero then, still at compile time, now we've got a NonZeroI8 with value FOO