What are Some and None?

The signature of get (for slices, not Vec, since you’re using an array/slice) is

fn get(&self, index: usize) -> Option<&T>

That is, it returns an Option, which is an enum defined like

pub enum Option<T> {
    None,
    Some(T),
}

None and Some are the variants of the enum, that is, a value with type Option<T> can either be a None, or it can be a Some containing a value of type T. You can create the Option enum using the variants as well:

let foo = Some(42);
let bar = None;

This is the same as the core data Maybe a = Nothing | Just a type in Haskell; both represent an optional value, it’s either there (Some/Just), or it’s not (None/Nothing).

These types are often used to represent failure when there’s only one possibility for why something failed, for example, .get uses Option to give type-safe bounds-checked array access: it returns None (i.e. no data) when the index is out of bounds, otherwise it returns a Some containing the requested pointer.

See also:

• Why don’t Option’s Some and None variants need to be qualified?
• What is the difference between Some and Option in Rust?