This first pass only applies to the following two cases:
- Public functions returning a view type into an object they own
- Public ctors storing a view type
This catches a grand total of one (1) issue, which is fixed in
the previous commit.
Utf16FlyString more or less works exactly the same as FlyString. It will
store the raw encoded data of the string instance. If the string is a
short ASCII string, Utf16FlyString holds the ShortString bytes; else,
Utf16FlyString holds a pointer to the Utf16StringData.
The underlying storage used during string formatting is StringBuilder.
To support UTF-16 strings, this patch allows callers to specify a mode
during StringBuilder construction. The default mode is UTF-8, for which
StringBuilder remains unchanged.
In UTF-16 mode, we treat the StringBuilder's internal ByteBuffer as a
series of u16 code units. Appending a single character will append 2
bytes for that character (cast to a char16_t). Appending a StringView
will transcode the string to UTF-16.
Utf16String also gains the same memory optimization that we added for
String, where we hand-off the underlying buffer to Utf16String to avoid
having to re-allocate.
In the future, we may want to further optimize for ASCII strings. For
example, we could defer committing to the u16-esque storage until we
see a non-ASCII code point.
This is a strictly UTF-16 string with some optimizations for ASCII.
* If created from a short UTF-8 or UTF-16 string that is also ASCII,
then the string is stored in an inlined byte buffer.
* If created with a long UTF-8 or UTF-16 string that is also ASCII,
then the string is stored in an outlined char buffer.
* If created with a short or long UTF-8 or UTF-16 string that is not
ASCII, then the string is stored in an outlined char16 buffer.
We do not store short non-ASCII text in the inlined buffer to avoid
confusion with operations such as `length_in_code_units` and
`code_unit_at`. For example, "😀" would be stored as 4 UTF-8 bytes
in short string form. But we still want `length_in_code_units` to
be 2, and `code_unit_at(0)` to be 0xD83D.
