Cow means "clone on write". It's a generic wrapper for types that have both a borrowed version and an owned version. For example, `Cow<'a, str>` can hold either an `&'a str` (a borrowed string) or a `String` (an owned string), and a `Cow<'a, [u8]>` can hold a `&'a [u8]` (a slice of bytes) or a `Vec<u8>` (an owned vector of bytes).

In that Rust program, Cow is being used here so if the user provides an argument it ends up with an owned vector that contains that argument plus a newline, otherwise it ends up with a borrowed byte slice that contains "y\n". That said, there's not really a whole lot of point to using Cow here since allocation is a one-time cost and nobody cares if yes starts up a few microseconds slower, but the usage of Cow is limited to just a couple of lines so it's not really complicating anything (the actual write() call just ends up taking a &[u8] byte slice, so the usage of Cow is strictly limited to main()).

tine, tiny note: it means Clone on write, not Copy, and it's String not Str.

Oops, typo. I've updated my comment accordingly, thanks.

Also, I didn't realize it was Clone-on-write. Interesting, and the documentation does confirm this. I say "interesting" because the actual operation involved is `to_owned()`, not `clone()`, seeing as how `clone()` on a `&str` just gives you the same `&str` back.

Yeah, conceptually it's closer to Clone even if it's not literally Clone.