[Pitch] 128 bit Integer Types

[wadetregaskis]

@lorentey

If wasted memory is a concern, engineers have the option to split their 128-bit integers into smaller chunks and store those, like Duration does.

If Int128 existed at the time Duration was defined, would it have been defined differently? e.g. to just use an Int128, or even to just be an Int128?

I'm not sure how important the answer is to this discussion, but maybe it hints at which way the balance is leaning currently.

I assume Duration won't change as a result of this proposal, for ABI compatibility reasons if nothing else?