with some help from @CobaltVelvet @george_ and @jeancf, I learned a bit about #Rust's fold and map methods this morning

https://sts10.github.io/2020/09/08/rust-map-fold.html

@schlink i'd have used that:

let slices_needed: usize = persons.iter().map(|p| p.slices_requested).sum();

with map to get the number we need, and sum to concisely sum them.
fold could work too but i find it more verbose and less elegant