Eglot: fix LSP "languageId" detection

This sweeping fix has been planned for a while, but a user recently
hit this bug as described in
https://github.com/joaotavora/eglot/discussions/1206.

More and more servers today are "multi-language", meaning can handle
more than one file type.  This relies on the ':languageId' string
being set to the correct value for every buffer managed by Eglot
(TextDocumentItem in LSP parlance).

Previously this string was calculated based on an imperfect heuristic
and was wrong quite often.  Many servers don't even care but some
others do, so we have to fix it.

* lisp/progmodes/eglot.el (eglot-lsp-server): Remove slots
'major-modes' and 'language-id'.  Add slot 'languages'.
(eglot--major-modes, eglot--language-ids): New helpers.
(eglot--lookup-mode): Simplify or maybe complicate.
(eglot--guess-contact): Use new eglot--looup-mode.  Change return
value.
(eglot): Rework docstring.
(eglot-reconnect): Use eglot--language-ids, not id.
(eglot--connect): Setup eglot--languages slot in server.
(eglot--TextDocumentItem): Finally, get correct language id.

* test/lisp/progmodes/eglot-tests.el (eglot--guessing-contact):
Enhance macro.
(eglot-test-server-programs-guess-lang): Update test.

1 files changed, 16 insertions, 13 deletions

diff --git a/test/lisp/progmodes/eglot-tests.el b/test/lisp/progmodes/eglot-tests.el
index efb0f4d8844..d96ba2ebf07 100644
--- a/test/lisp/progmodes/eglot-tests.el
+++ b/test/lisp/progmodes/eglot-tests.el
@@ -1041,7 +1041,8 @@ int main() {
 (cl-defmacro eglot--guessing-contact ((interactive-sym
                                        prompt-args-sym
                                        guessed-class-sym guessed-contact-sym
-                                       &optional guessed-lang-id-sym)
+                                       &optional guessed-major-modes-sym
+                                       guessed-lang-ids-sym)
                                       &body body)
   "Guess LSP contact with `eglot--guessing-contact', evaluate BODY.
 
@@ -1051,10 +1052,10 @@ BODY is evaluated twice, with INTERACTIVE bound to the boolean passed to
 If the user would have been prompted, PROMPT-ARGS-SYM is bound to
 the list of arguments that would have been passed to
 `read-shell-command', else nil.  GUESSED-CLASS-SYM,
-GUESSED-CONTACT-SYM and GUESSED-LANG-ID-SYM are bound to the
-useful return values of `eglot--guess-contact'.  Unless the
-server program evaluates to \"a-missing-executable.exe\", this
-macro will assume it exists."
+GUESSED-CONTACT-SYM, GUESSED-LANG-IDS-SYM and
+GUESSED-MAJOR-MODES-SYM are bound to the useful return values of
+`eglot--guess-contact'.  Unless the server program evaluates to
+\"a-missing-executable.exe\", this macro will assume it exists."
   (declare (indent 1) (debug t))
   (let ((i-sym (cl-gensym)))
     `(dolist (,i-sym '(nil t))
@@ -1070,8 +1071,9 @@ macro will assume it exists."
                          `(lambda (&rest args) (setq ,prompt-args-sym args) "")
                        `(lambda (&rest _dummy) ""))))
            (cl-destructuring-bind
-               (_ _ ,guessed-class-sym ,guessed-contact-sym
-                  ,(or guessed-lang-id-sym '_))
+               (,(or guessed-major-modes-sym '_)
+                _ ,guessed-class-sym ,guessed-contact-sym
+                  ,(or guessed-lang-ids-sym '_))
                (eglot--guess-contact ,i-sym)
              ,@body))))))
 
@@ -1166,16 +1168,17 @@ macro will assume it exists."
 (ert-deftest eglot-test-server-programs-guess-lang ()
   (let ((major-mode 'foo-mode))
     (let ((eglot-server-programs '((foo-mode . ("prog-executable")))))
-      (eglot--guessing-contact (_ nil _ _ guessed-lang)
-        (should (equal guessed-lang "foo"))))
+      (eglot--guessing-contact (_ nil _ _ _ guessed-langs)
+        (should (equal guessed-langs '("foo")))))
     (let ((eglot-server-programs '(((foo-mode :language-id "bar")
                                     . ("prog-executable")))))
-      (eglot--guessing-contact (_ nil _ _ guessed-lang)
-        (should (equal guessed-lang "bar"))))
+      (eglot--guessing-contact (_ nil _ _ _ guessed-langs)
+        (should (equal guessed-langs '("bar")))))
     (let ((eglot-server-programs '(((baz-mode (foo-mode :language-id "bar"))
                                     . ("prog-executable")))))
-      (eglot--guessing-contact (_ nil _ _ guessed-lang)
-        (should (equal guessed-lang "bar"))))))
+      (eglot--guessing-contact (_ nil _ _ modes guessed-langs)
+        (should (equal guessed-langs '("bar" "baz")))
+        (should (equal modes '(foo-mode baz-mode)))))))
 
 (defun eglot--glob-match (glob str)
   (funcall (eglot--glob-compile glob t t) str))