bug#70408: 30.0.50; Eglot and Project integration

[Ergus via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

On Tue, Apr 16, 2024 at 02:51:10PM +0100, João Távora wrote:
>On Tue, Apr 16, 2024 at 1:56 PM Dmitry Gutov <dmitry@gutov.dev> wrote:
>
>> IIUC Ergus's request is primarily about a situation where an
>> "out-of-tree" build is used. Meaning, the directory for build artefacts
>> is not a subdirectory of the project root, but -- apparently -- some
>> sibling directory of it (e.g. "../build"). So it's somewhat atypical,
>> although I suppose the solution from the link above might work with it too.
>
>Ah, I know about that.  That's where compile-commands.json is generated
>by CMake.  But using that './build' as the project root passed via LSP
>to clangd
>(and likely any other server) will most likely fail: that's not  the
>project root
>and it doesn't have any versioned source files  (only auto-generated ones).
>
>Because of this, C++ projects usually have sth like:
>
>  ln -sf build/compile_commands.json compile_commands.json
>
>as a build step.
>
>Alternatively, you invoke clangd with `--compile-commands-dir=build`.
>I don't think there's anything project.el or Eglot can do or should do
>about this case.
>
What my POC workaround does is basically call a hook that updates
`eglot-workspace-configuration` as a root's directory-local variable
the first time project-current is called within a project.


```
(defun project-multi--set-eglot (plist)
   "Set the eglot variables in root's PLIST when possible."
   (when-let* ((compile-dir (project-extra-info (project-current) :compile-dir))
	      (file-exists-p (expand-file-name "compile_commands.json" compile-dir)))

     (let* ((symvars (intern (format "eglot-multi--%s" compile-dir)))
	   (eglot-complete (project-multi--merge-plist ;; merge with new values
			    (bound-and-true-p eglot-workspace-configuration)
			    `(:clangd (:initializationOptions
				       (:compilationDatabasePath ,compile-dir))))))

       ;; set the dir local variables, they will apply automatically to
       ;; all buffers open in the future within the project root
       (dir-locals-set-class-variables
        symvars
        `((nil . ((eglot-workspace-configuration . ,eglot-complete)))))

       (dir-locals-set-directory-class (project-root (project-current)) symvars)

       ;; set the variable manually in all the already opened buffers
       ;; TODO: JAM check if the variable is not already set in the other buffers??
       ;; Probably override only the value instead of replacing the whole variable?
       (mapc (lambda (buffer)
	      (with-current-buffer buffer
		(setq-local eglot-workspace-configuration eglot-complete)))
	    (project-buffers (project-current))))))
```

project-multi--merge-plist is just a hack function to merge the
eglot-workspace-configuration value without overriding the existing
sub-values if already set (in case the user sets some sub-values in the
dir locals then those takes precedence)

Then this restarts eglot

IIUC this is equivalent to call `--compile-commands-dir=build` and at
the moment is working for me.


>> This bug is split off from an emacs-devel discussion, where I posted a
>> draft solution of mine:
>> https://lists.gnu.org/archive/html/emacs-devel/2024-04/msg00279.html
>> I'm curious for any feedback - like would that be good enough for this
>> and related cases, or maybe if someone has an even simpler approach in mind.
>
>I'll pass, but wish you luck.  I've stated in the past that I think
>project.el should
>allow subprojects inside larger projects, and let users of
>project-current (direct
>or indirect) specify if they're interesting in the innermost,
>outermost, or intermediate
>projects when searching for projects to act on, via a combination of prefix
>arguments, arguments, customized special variables or let-bound special
>variables.
>
At the moment I assume the outer most only; which is the simpler one to
setup and implement woth the current project.el support. The main goal I
have is OOSC, not nested projects.

>João