Re: Allow xref to use other than current major-mode

[Stephen Leake <stephen_leake@stephe-leake.org>]

Dmitry Gutov <dgutov@yandex.ru> writes:

> On 08/25/2015 01:00 PM, Stephen Leake wrote:
>
>> To handle this, I propose changing both
>> xref-identifier-completion-table-function and xref-find-function to be
>> mode-local, instead of buffer-local.
>
> That seems unnecessarily limiting. First, mode-local facility is a
> part of CEDET, so it would need to be always loaded.

I realized after I posted that mode-local.el is not pre-loaded, so
elisp-mode.el can't require it. That said, mode-local.el is only one
file; nothing else in CEDET is required.

cl-generic.el is preloaded, so we could use a cl-defgeneric, and
dispatch on (mode (eql <major-mode>)).

> Second, how would a minor mode set an xref backend? It might not be
> tied to a particular major mode.

I don't see how a global minor mode can set an xref backend now.

To make such a minor mode work, xref--read-identifier could be a
cl-defgeneric that dispatches on (project <project-type). But if it also
dispatches on (mode (eql <major-mode>)), each backend would have to
define all combinations; that's a pain.

>> There are similar use cases; from a LaTeX document buffer, looking at the
>> description of an Ada function, find the Ada code that defines that
>> function. Or from a text mode notes file, look for a definition in any
>> one of several languages.
>
> All similar cases can be solved by a project-aware xref backend that
> knows about all languages used, and allows navigating to identifiers
> in all of them.

I've started writing such a thing, but it gets defeated by the fact that
xref--read-identifier uses the buffer-local value of
xref-identifier-completion-table-function, which is set by elisp-mode
(or c-mode, etc, or by default to etags). There's no way to change that.

We could require the user to manually enable an xref minor mode in each
buffer, but that's not friendly.

I guess a global xref backend could somehow reset that variable, in all
elisp buffers? Maybe a function on elisp-mode-hook.

The problem is that emacs-lisp-mode sets the buffer-local xref
variables, but I don't want it to.

So a better approach would be to define a way to disable emacs-lisp-mode
(and all other major-modes) setting the xref buffer-local variables, so
that the user choice of xref backend is respected. That would be much
simpler than having to add a function to all the major mode hooks (most
of which are unknown at load time).

We could define a new global variable xref-global-backend; if nil (the
default), major modes can set xref vars; if non-nil, it indicates which
particular global backend is in use. It could be a function that returns
a boolean indicating whether the global backend wants to control xref in
the current buffer. So a semi-global backend could provide C and Ada
xref, but not elisp xref, for example. It could be a list of such
functions; only if all return nil should the major mode set the xref
vars.

-- 
-- Stephe