>> That being said, yes this may be nice.  But minibuffer-completion-help
>> already does sort the completions using display-sort-function, just like
>> completion-all-sorted-completions, so what's causing the difference in
>> behavior?
>
> It seems this is implementable in minibuffer-completion-help
> by copying this code from completion-all-sorted-completions:
>
>               (setq all (minibuffer--sort-by-position
>                          (minibuffer--sort-preprocess-history
>                           (substring string 0 base-size))
>                          all))

Oh, very interesting!  Maybe completions-sort should accept a new symbol
'history to behave this way?

I already think this would be very nice for project-switch-project, for
example, since I'm often switching between a few related projects.

Perhaps 'history should break ties by alphabetizing, otherwise
e.g. filenames you haven't visited before would be unsorted randomly.

Hmm, actually the case of filenames is a bit complex.  Because for
filenames, the strings we're completing over don't appear verbatim in
the history.  So perhaps read-file-name would need its own specialized
sorting function, which whenever you're completing in some directory,
sorts to the front any files in that directory whose full path appears
in file-name-history.

>> Honestly the main place I find myself wanting different sorting of
>> completions is for buffer completion - I'd prefer buffers to be sorted
>> by most-recently-used.  Maybe we can just add such an option?
>
> Such an option would be nice.  Maybe the right way to support it
> is to add a new sort function with the code above.  Then the caller
> could provide such a function in the completion metadata.

Oh, that's also interesting.  So there would be a function that the
completion metadata could specify as display-sort-function, which would
have the behavior of sorting based on history?

That also makes a lot of sense, and would allow commands like
project-switch-project and read-buffer to opt in on a command-by-command
basis, which might be more sensible.

So maybe adding 'history as a new option for completions-sort isn't a
good idea.  Instead we should just add user options for enabling history
sorting for files and buffers.  (And perhaps we could just enable
history sorting by default for project-switch-project.)

Also: it might be nice to switch between history-sorting and
alphabetized-sorting during the course of completion, both for files and
buffers.  Maybe we could do that by making a command which puts
completion into a mode where it just ignores the display-sort-function
specified by the completion metadata, and just always uses
completions-sort.