I mean you can index into Rust's strings, it's just that you probably don't want that:

    "Clown"[2..5]  // is "own"

Notice that's a range, Rust's string slice type doesn't consider itself just an array (as the Ada type is) and so we can't just provide an integer index, the index is a range of integers to specify where our sub-string should begin and end. If we specify the middle of a Unicode character then the code panics - don't do that.

Yes, since AoC always uses ASCII it will typically make sense to use &[u8] (the reference to a slice of bytes) and indeed the str::as_bytes method literally gives you that byte slice if you realise that's what you actually needed.