Something for coalescing; aka generalized/improved `or_else`

3

I think the only operator that would make sense is ||. It’s already lazy, as this one would be, and currently isn’t overloadable so it wouldn’t break anything. (And you can imagine a impl Try for bool where b? short-circuits on false which would be perfectly consistent with this definition, though I suspect we don’t actually want to allow ? on bool.)

OTOH, maybe just the macro would be enough. A bunch of the try{} discussions have talked about “what if I just want to return the first success?”, where it might be good not to have that use try/?, but instead use something like

coalesce!(
    fetch_from_primary(),
    fetch_from_secondary(),
    Err(NotFoundInEither),
)
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