The example at the end of that section:

        pub fn encrypted(&self) -> Vec<u8> {
            encrypt(self.payload.as_ref().unwrap_or(&vec![]))
        }

Needlessly allocates a `Vec` if `self.payload` is `Some`. Really you should do:

     encrypt(&self.payload.as_ref().unwrap_or_else(|| &vec![]))

But that isn't going to work because the `else` Vec will be dropped at the end of the closure.

In this situation (if the code was on a hot path where every bit of performance mattered) I would probably do something like:

    match self.payload.as_ref() {
        Some(payload) => encrypt(payload),
        None => encrypt(&vec![])
    }

There is a slight bit of duplicated code. I'd be curious if anyone could come up with a better way.