A novel binary integer abstraction

[benrimmington]

@oscbyspro

Tangentially, StaticBigInt does not provide access to a buffer pointer, unlike its StaticString sibling.

The original pitch had a buffer pointer, but it was removed for various reasons.

StaticBigInt

Hmm. Guaranteeing that this is an array — and therefore also the count and order of the array elements — really ties this to the details of Swift's ABI. It would not be implementable to create one of these from generic code under a different ABI, and that seems like a line we shouldn't cross. Do you think it's especially important to expose this as an array (e.g. to allow the efficient creation of BigInts from a StaticBigInt), or can we reasonably abstract access in some way?

StaticBigInt

With that said, if StaticBigInt only exposes the underlying data as an UnsafeBufferPointer, that will probably interfere with constant evaluation. That might be another argument for providing a more "ABI-neutral" access to the data, like a method to return the kth least significant word.

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Future directions of StaticBigInt:

• Add an "unchecked" subscript, without the infinite sign extension?

• Wait for two-tiered iteration, using a Span<UInt> of one or more words?