[pmulard]

There's more of an explanation further down:

   To better understand, notice that longer_of is not a function; it’s a subscript. A subscript does not return a value, it projects one, granting the caller temporary read and/or write access to it.

I'm not really sure how this doesn't fall under a move? Regarding the string, they could also place it on the heap to allow for the variable sizing, which is what Rust does.