These functions are the equivalent of 23b424a46 for grapheme clusters.
In order to add the `is_grapheme_boundary` function we also need to
query whether a byte index lies on a character boundary, so this change
also adds `is_char_boundary`.
The new `RopeSliceExt::ceil_char_boundary` from the parent commits can
be used to implement `RopeSliceExt::byte_to_next_char` when used with
`RopeSlice::byte_to_char`. That function had only one caller and that
caller will eventually disappear when we switch to Ropey v2 and drop
character indexing, so we can drop `byte_to_next_char` now and replace
its caller with `byte_to_char` plus `ceil_char_boundary`.
This change keeps the unit tests for `byte_to_next_char` and checks them
against a polyfill of `byte_to_char` plus `ceil_char_boundary` to ensure
that `byte_to_next_char`'s intended behavior is not changed.
These functions mimic `str::floor_char_boundary` and
`str::floor_char_boundary` (currently unstable under
`round_char_boundary`). They're useful for correcting a byte index
which may not lie on a character boundary. For example you might limit
a search within a slice to some fixed number of bytes. The fixed number
might not lie on a boundary though so it needs to be corrected to
either the earlier (floor) or later (ceil) boundary.
Michael Davis
Rolo
rhogenson
Pascal Kuthe
Gabriel Dinner-David
Pascal Kuthe