Should I use small datatypes when I know my numbers are small?

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It's really more of a gut feeling thing than a hard rule. Here's my heuristic:

  • If they're indices into a slice, you should probably use usize, unless the unnecessarily large size causes memory or performance issues. Numbers have to be sign-extended to index into memory anyway and if you have usizes already you don't have to do as many noisy conversions (i as usize) when you use them that way.
  • If they're not indices, but represent numbers without a hard limit that will in practice never be "big", you should probably just use u32 (or i32 for signed) unless the size causes memory or performance issues. On most modern architectures, 32-bit integers are usually faster than 64-bit integers, but it is not generally true that 16- and 8-bit integers are faster still; often they just use the same registers and instructions but ignore the higher bytes, so there's not necessarily any benefit to going smaller except the size itself. If integer size is a performance concern you should definitely be profiling your code so you'll know whether it matters or not.
  • If they are numbers with a hard, compile-time limit that is "small", feel free to use whatever type is big enough (but maybe give it a type alias so it's easy to change in the future).
    • Addendum: also if you want to impose a hard limit on the number in order to make the storage more efficient, that's fine too.
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