[Pitch] `Distinguishable` protocol for Quick Comparisons

[vanvoorden (Rick van Voorden)]

@lorentey

SE-0447 has already established the name isIdentical(to:). We added these to support simple tests for referential equality without having to conform Span/RawSpan to Equatable.

@lorentey I didn't follow along too closely with the Span review while it was taking place… but I just went back and searched through the pitches and reviews and I did not see a mention of the isIdentical function being discussed. Do you remember if there were any offline discussions about why this function should be named isIdentical? Were there any offline discussions about a potential mustBeEqual function instead?

@lorentey

I don't really see why we would need to define a standard protocol for this, though. (I'd particularly not like to have to think about types like Bool getting forced to conform to it.) If the protocol has widespread appeal, then it ought to be possible to demonstrate this by shipping it (and building on it) in a package first -- we don't need to immediately jump to defining it in the Standard Library.

Hmm… so the thinking here is we could go ahead and ship public isIdentical implementations on the appropriate standard library and Foundation types and then ship a potential Distinguishable protocol somewhere like swift-collections? And when a product engineer depends on swift-collections they get the Distinguishable protocol available for their own types as well as the adopting on types like Array and TreeDictionary?

I do believe there are legit generic context use-cases inside the standard library for us to optimize if Distinguishable shipped in there. It would be impactful to ship them sooner rather than later. I guess if we did ship in swift-collections and then incubate for one Swift release cycle that would put us about another six months in the future?