[swift-evolution] Pluggable hash algorithm for containers

[Ray Fix]

Last year, Howard Hinnant proposed a composable hash algorithm framework for C++ that I was impressed with.

His proposal is here: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3980.html <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3980.html>
A live presentation is here: https://www.youtube.com/watch?v=Njjp_MJsgt8 <https://www.youtube.com/watch?v=Njjp_MJsgt8>

A type with multiple fields you inevitably want to do a hash_combine but this pollutes the hashing algorithm.  It turns out that (nearly) all hashing algorithms can be abstracted into 3 phases: initialization, digestion and finalization.  It might be cool if Swift Hashable adopted this pluggable architecture.  

1. Easily adopt Hashable
2. Easily change and compare hash algorithms based on performance / security needs
3. Make good hash implementations trivial for users.

Wondering if anyone has thought about this already / if it would be worthwhile pursuing more.

Best wishes,
Ray

PS:  I have always thought that it is a bummer that compound types such as tuples can’t conform to protocols and therefore be Hashable.  

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