Tricky borrow checker problems in the rustls deframer

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6

According to @kornel in When is transmuting lifetimes useful? - #4 by kornel

Mutable iterators are a common case where this is necessary. There's no way to explain to the borrow checker that next() returns a different, non-overlapping mutable borrow each time if they all come from the same source. This can be reduced to implementation of slice.split_at_mut() , which also requires such fudge.

And indeed, this works:

impl Buffer {
    fn pop(&mut self) -> Option<Borrowed<'_>> {
        loop {
            let mutable = Mutable {
                buf: &mut self.buf[..10],
            };
            if mutable.buf[0] == 0 {
                let mutable: Mutable<'_> = unsafe { std::mem::transmute(mutable) };
                return Some(mutable.borrow());
            }
        }
    }
}

..but I certainly wouldn't feel good shipping that (even though miri seems happy with it)

@steffahn below (I can't reply anymore, Discourse hates me because I just signed up for it) mentioned polonius-the-crab, for the curious here's how it would look for the toy case. (This doesn't actually build on the playground, I guess it's not one of the top 100 crates)