[Accepted] SE-0206: Hashable Enhancements

Evolution Announcements
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Although you can cache the hash it isn’t obvious how you do this, is significant boiler plate, and has some overhead. Is it possible to add caching? EG:

struct Expensive {
    private var hash: Int?
    override func hash(into: inout Hasher) {
        guard h = hash else {
            into.combine( ... )
            ...
            hash = into.finalize()
        }
        into.previouslyCachedValue(h)
    }
}

IE I’m proposing adding previouslyCachedValue to Hasher. As a nice addition, this change gives a reason for finalize being a public function (at present the author of hash(into:) can call finalize but you shouldn’t!). An implication of this change is that a Hasher should be cheap to create so that the overhead of caching is low.

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