[Pitch] Make Collection Super-Convenient and Retire Sequence

[soroush (Soroush Khanlou)]

@nnnnnnnn

One way of thinking about this change is that it’s adding a new way to build a custom Collection type — before, you had to provide, at minimum, lower and upper index bounds and an (Index) -> Element subscript, now you’ll be able to just provide a () -> Element? method and be done with it.

There's one part of your table I don't follow — how do you get better indexing performance by implementing custom indexes? SequenceIndex keeps a reference to its element so you can return that in O(1) time.

In fact, you might be able to remove that entirely, because calling next() on Base.Iterator should give you the same value?

Regarding the proposal as a whole — I'm not sure. It seems a lot like using a sledgehammer to crack a nut, although it does solve a problems. My preference is still towards semantically changing Sequence to be multipass (a la [Pitch] Remove the single-pass requirement on Sequence), purely for the limited effects on existing source, but Dave's pitch also solves the associatedtype weirdness when jumping from Sequence to Collection.