Watcher compatibility

[xendk]

I implemented
flycheck-eldev--checker-substituted-arguments to handle the
'source-inplace to 'source conversion first. Then I ran the tests
and had lots of failures. Fixing up the --setup-first made all the
tests green again.

[doublep]

Please split into two PRs: one to fix things that got broken in the meanwhile (lexical-binding and so on), the other fully about new functionality.

[xendk]

I've rebased the branch and after updating the copyright year all but snapshot passes.

[doublep]

Looks fine, but still need some tests and a way to disable this.

About the failure on snapshot: this is caused by changes in Emacs: https://lists.gnu.org/archive/html/emacs-devel/2025-08/msg00332.html. I have managed to arrange some changes here, but they still haven't been committed (presumably Philip is on vacation). You may want to ping it after some time, would carry more weight if more people show up.

[xendk]

Looks fine, but still need some tests and a way to disable this.

Well, the existing tests cover this, and I'd like to advocate not making it disable-able.

For starters, we'd need to duplicate a lot of test between the two cases to ensure that they indeed do the same thing.

Secondly, the most invasive part of this change (the --setup-first change) builds upon existing code that eldev--package-dir-info doesn't find the original package file. If a (Emacs or other) change breaks that, there's a good chance that it would break the original too,

I would argue that this method is an improvement, in part because of a change I forgot somehow. With this PR, the following becomes unnecessary:

              ;; When checking project's main file, use the temporary as the main file
              ;; instead.
              "--setup"
              ,(flycheck-sexp-to-string
                `(when (and eldev-project-main-file (file-equal-p eldev-project-main-file ,real-filename))
                   (setf eldev-project-main-file ,filename)))

When we're not faking an empty file to eldev--package-dir-info for the original file in order to avoid clashes between it and the flycheck_ file, but instead feeding it the contents of the latter when it looks at the former, we don't need to fiddle with eldev-project-main-file, so this can be removed.

Lastly, in the case that flycheck-eldev--checker-substituted-arguments fails, or the user strong-arms it into using source-inplace anyway, the new version still works,

[doublep]

Well, the existing tests cover this

I added more tests for multifile "project" and rebased your branch. Seems to work fine.

I will think about what else to do tomorrow.

[doublep]

Sorry that it took so long.