the "units of measure" crate uom would be a nice fit also, but it's not available here

oh sorry, will do! thanks

who voted best practice on this xD
Well i guess in some circumstances this would be the safest bet (system with ROM?)...

for testing i used this kind of method as well, i admit it

wow, this is something... took me like 3 times as long to understand this code than it took to write my own solution :D
Are people really writing this kind of code for production?

You could use a match on the comparison, this way you don't have "i if ..." (that would be clearer to read). But just a suggestion :)

use std::cmp::Ordering;

// ...

match i.cmp(&3) {
      Ordering::Less => arr.push(signature[i]),
      _ => arr.push(arr[i - 3] + arr[i - 2] + arr[i - 1]),
  }

Alternatively you could just use a simple if {} else {} of course, as you just want to distinguish exactly two cases.