[tialaramex]

Also, because Rust is intended for writing low level software where you might very well care deeply about how big this type is:

Rust has NonZero versions of the unsigned types, so NonZeroU32 is the same size as a u32, four bytes with an unsigned integer in it, except it is never zero.

Option<NonZeroU32> promises to be exactly the same size as u32 was. Rust calls the space left by the unused zero value a "niche" and that's the perfect size of niche for None.

As a result you get the same machine code you'd have for a "normal" 32-bit unsigned integer with zero used as a sentinel value, but because Rust knows None isn't an integer, when you mistakenly try to add None to sixteen in some code deep in the software having forgotten to check for the sentinel you get a compile error, not a mysterious bug report from a customer where somehow it got 16 which was supposed to be impossible.

When a maintenance programmer ten years later decides actually zero is a possible value for this parameter as well as "None", there's a NonZeroU32, they swap it for u32, and the program works just fine - but because there's no niche left in u32 the type is now bigger.