String.Index has an encodedOffset-based initializer and computed
property that exists for serialization purposes. It was documented as
UTF-16 in the SE proposal introducing it, which was String's
underlying encoding at the time, but the dream of String even then was
to abstract away whatever encoding happend to be used.
Serialization needs an explicit encoding for serialized indices to
make sense: the offsets need to align with the view. With String
utilizing UTF-8 encoding for native contents in Swift 5, serialization
isn't necessarily the most efficient in UTF-16.
Furthermore, the majority of usage of encodedOffset in the wild is
buggy and operates under the assumption that a UTF-16 code unit was a
Swift Character, which isn't even valid if the String is known to be
all-ASCII (because CR-LF).
This change introduces a pair of semantics-preserving alternatives to
encodedOffset that explicitly call out the UTF-16 assumption. These
serve as a gentle off-ramp for current mis-uses of encodedOffset.
Add in a fast-path for Strings created from Substring which covers the
entire String. Put String-from-Substring behind a non-inlinable
resilience barrier for future flexibility.
After rebasing on master and incorporating more 32-bit support,
perform a bunch of cleanup, documentation updates, comments, move code
back to String declaration, etc.
Refactor and rename _StringGutsSlice, apply NFC-aware fast paths to a
new buffered iterator.
Also, fix bug in _typeName which used to assume ASCIIness and better
SIL optimizations on StringObject.
Add inlinability annotations to restore performance parity with 4.2 String.
Take advantage of known NFC as a fast-path for comparison, and
overhaul comparison dispatch.
RRC improvements and optmizations.
* Refactor out RRC implementation into dedicated file.
* Change our `_invariantCheck` pattern to generate efficient code in
asserts builds and make the optimizer job's easier.
* Drop a few Bidi shims we no longer need.
* Restore View decls to String, workaround no longer needed
* Cleaner unicode helper facilities
This is a giant squashing of a lot of individual changes prototyping a
switch of String in Swift 5 to be natively encoded as UTF-8. It
includes what's necessary for a functional prototype, dropping some
history, but still leaves plenty of history available for future
commits.
My apologies to anyone trying to do code archeology between this
commit and the one prior. This was the lesser of evils.
