...which translates to "/" because of the double slash -- the filesystem
root directory (*). But that's the same data which would be used by any
other proposed solution, too.
More or less, tho the "ideal" solution is to do that in the
completion-style code, which has a bit more knowledge about it.
Doing it in completion-style, though, would either require a relatively
awkward change in most/all styles (e.g. the "new dynamic variable" route),
or a more straightforward change in styles together with an incompatible
change in completion-all-completions.
Yup, hence the quotes around "ideal".
So on balance, would you say it's a good idea to a) use this approach in
minibuffer-completion-help, b) create a named function for it, for other
callers to take advantage of it as well?
Yes, while waiting for a new API it seems like a good stop gap.
Looks like completion--merge-suffix is the helper to use.
Yup.
In theory this approach can "do the wrong thing" with some completion
styles, but AFAIK they haven't been written yet. 🙂
So you figure that such theoretical style would return adjusted base-suffix
in -all-completions method, not just in -try-completion?
A completion style could make use of (and list) things after the
boundary. E.g. completing a file name like
foo/ba<|>ar/baz
could decide to list all the files that match
*f*o*o*/*b*a*a*r*/*b*a*z*
in which case the "end boundary" of the `completion-all-completions`
output should not be the `cdr` of the `completion-boundaries`.
I wouldn't worry about it, tho.