If anyone wants a fairly-simple PR, if you submit one making them unstablyconst I'll merge it, assuming of course that the code change to do that is fairly straight-forward (not needing a bunch of reimplementing or weird internal features).
Then it would probably stabilize fairly quickly, but that would be a separate step.
EDIT: hmm, looks like it'll depend in is_char_boundary being const fn, so maybe start with that.
Even most of just regular slice manipulation is also nonconst currently. This is most likely due to that panicking with formatted message is nonconst, so making any panicking indexing operation const will require const_eval_selecting between a const panic or the current more informative panic message, and stop using the more convenient assert!s.
(Regular slices have the out of using slice patterns at least some of the time.)
But this pattern already exists and should likely be acceptable to use more widely, especially for things as widely useful (and possible to do via workaround) as slice manipulation.