Idea: Early returns and irrefutability

[scottmcm]

@djc

    let stream = if let Some(x) = self.streams.get_send_mut(id) {
        x
    } else {
        continue;
    };

While I do like something more pattern-general, I think these cases could all be done with something like let stream = self.streams.get_send_mut(id) || continue; using

Something for coalescing; aka generalized/improved `or_else`

It was noted in the await syntax thread that Option::or_else mixes poorly with await (and ?): https://github.com/rust-lang/rust/issues/57640#issuecomment-457518407 Since one can think of ? as and_then, would it make sense for there to be some kind of standard macro or operator for or_else? Like how C# has the null coalescing operator ?? (which spawned the or_else conversation) and SQL has the COALESCE function. As I’ve been spending a bunch of time with ops::Try lately, I realized we could do…