[Proposal] Pointer family initialization improvements

[glessard]

Pitch to round out the API for memory initialization through UnsafeMutablePointer and its relatives, as well as adding more capabilities to slices of BufferPointer types.

Pitch thread: https://forums.swift.org/t/55689
Implementation PR: swiftlang/swift#41608

[rjmccall]

Review manager comments:

• (removed this point; I missed where you did discuss it in the proposal)
• I think you should discuss whether the existing element-by-element initialization APIs should be deprecated
• The document does respond to Jordan's feedback here about the single-element updates, but the argument seems weak. Is this basically an argument for deprecating modifications via pointee / subscript?

[glessard]

I think you should discuss whether the existing element-by-element initialization APIs should be deprecated

OK. FWIW I don't think they should be deprecated (it's a fine API for non-Collection Sequences,) but after implementing this, the stdlib can be modified to be stop relying on the fast paths implemented in the many versions of _copyContents; those are where the headaches lie.

the argument seems weak

I agree about the argument, it's not great. I slightly prefer to have updateElement(at:to:), but it may not be worth it. (I'll have to get over my impression that's something's missing when it's not included.)

[rjmccall]

Okay, if you think this is ready, do you have thoughts about scheduling? I can put it into review whenever — if you're okay with it, I'll put it in review from Wednesday through the 29th.

[glessard]

That's a good schedule. Thanks!

[glessard]

Thanks @rjmccall !