[stdlib] Add StaticString.UnicodeScalarView

[lilyball]

Add a new type StaticString.UnicodeScalarView, accessible via a
property .unicodeScalars on StaticString. Also add initializers for
StaticString that take a UnicodeScalarView or a
Slice<UnicodeScalarView>.

The motivating reason for this is to make it possible to slice a
StaticString down to a substring that's still typed as StaticString,
without losing the guarantee of being well-formed UTF8.

Fixes rdar://problem/23382521

[radex]

Doesn't this require a swift-evolution proposal since it changes the stdlib API?

[gribozavr]

It does require a swift-evolution proposal. The proposal should include a condensed dump of the proposed API with documentation.

Please don't spend more time improving the code before the proposal is approved, but a general comment about tests is that every public API and every protocol conformance should be tested.

[gribozavr]

This tests should be added before runAllTests(), or they won't be run.

[lilyball]

Oops.

[lilyball]

Does every single stdlib API require a full proposal that has gone through the entire submission and review process? Seems like that could get very tedious. Or can relatively simple changes get by with just discussing it on the swift-evolution mailing list?

[gribozavr]

Does every single stdlib API require a full proposal that has gone through the entire submission and review process? Seems like that could get very tedious.

Yes, every API change that affects the user-visible API should go through swift-evolution. We have been using this process internally, and it worked well.

Or can relatively simple changes get by with just discussing it on the swift-evolution mailing list?

Relatively simple changes don't require long discussions and long proposals :) A simple email to swift-evolution and a small proposal that doesn't have much more than a simple statement of motivation and the API diff would be sufficient. Of course, assuming it is a simple change.

[lilyball]

PR updated to actually run the tests.

[lilyball]

The related proposal has now been merged as SE-0010 and is Awaiting Review.

[gottesmm]

Since the swift-evolution request was rejected with rational, can I close this pull request?

[lilyball]

I guess so. It's a shame, since there really should be some way of producing a slice of a StaticString, but it was rejected so I guess that's that.

[gottesmm]

Thanks!