bug#48572: 28.0.50; Add `passthrough` completion style to minibuffer.el

[Daniel Mendler <mail@daniel-mendler.de>]

On 5/26/21 11:28 PM, Stefan Monnier via Bug reports for GNU Emacs, the
Swiss army knife of text editors wrote:
>>> The "passthrough" (elsewhere called "backend") completion-style is
>>> definitely something we need to add, yes.  But what it should do is pass
>>> all the args from `completion-all/try-completions` to the table and let
>>> the table do *all* the work, i.e. let the completion table implement the
>>> completion-style methods.  But we shouldn't call `all-completions` or
>>> `try-completions` for that.  Instead we should call the completion-table
>>> directly, as is done for the `completion-boundaries` method.
>> But `all-completions` and `try-completion` already calls the table
>> directly if the table is indeed a table.
> 
> Yes, they do, but if you call them, then *you* don't: they call the table in
> their own way for their own purpose (e.g. for `all-completions` they
> call it to get all the completions matching a given prefix and that's
> it, whereas the whole point of `passthrough` is to be able to fetch
> matches based on other criteria (defined by the completion table)).

Sorry, I don't get it. What is the difference in calling the backend
between the two cases:

(1) passthrough-all-completions -> all-completions -> backend

versus

(2) passthrough-all-completions -> backend?

In case (2) you call the backend directly but then you also have to
handle completion tables which are not functions and you end up
reimplementing parts of `all-completions` in elisp. `all-completions`
convers non-function completion tables to lists.

>> Basically `all-completions/try-completion` themselves are already
>> "passthrough" modulo some calling convention.
>> Am I missing something?
> 
> Passthrough is about passing the completion-style API directly to
> the backend.  `all-completions` is part of the backend API, not the
> completion-style API.

Yes, I am aware that `all-completions` is the backend API. Is anything
lost when going through `all-completions`? From what I've seen the
arguments are passed on directly to the backend table.

Daniel