[pitch] Span-providing properties in the standard library

[glessard]

Continuing the work of adding Span, this is a proposal to add the ability to use Span as computed properties on standard library types, including Array.

[xwu]

Q: What is the behavior (and what are the performance characteristics) of this operation when a String has storage other than contiguous UTF-8? Would it be desirable to return an optional, or alternatively require the user to know or have called makeContiguousUTF8() on the String as a precondition?

[glessard]

Behind the scenes, we are implementing a new "lazy eager bridging" behaviour for bridged Array and bridged String. They will always succeed, but they will sometimes entail an allocation and copy (i.e. "usually O(1), sometimes O(n)".) An initial step is here. For native non-bridged instances, always O(1).

[xwu]

Cool! The implementation details would of course be out of scope for this text, but might be worth calling out here the performance implications for calling storage and bytes on these types, pros and cons for usability vs predictability, etc. (given that this is supposed to be a safe, performant API).

[glessard]

I appended a paragraph to the detailed design to spell this out.