bug#60379: [PATCH] Eglot: don't use "nil" as minibuffer initial input

[João Távora]

Eli Zaretskii <eliz@gnu.org> writes:

>> Date: Wed, 28 Dec 2022 14:22:17 +0200
>> From:  Eshel Yaron via "Bug reports for GNU Emacs,
>>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
>> 
>> Currently, doing emacs -Q and followed by M-x eglot in the scratch
>> buffer results in a prompt for major mode to manage with LSP, but the
>> initial minibuffer input as well as the default value for the
>> completing-read call are both set to the string "nil", which is not very
>> helpful as that's not a valid input.
>> 
>> This patch tweaks the prompt a little and avoids this issue by leaving
>> the minibuffer's initial input empty and making sure we don't set the
>> default value to "nil" either.
>
> João, are you looking into this?  WDYT?

No, I wasn't until now.

The idea looks sane, but I think 'format-prompt' can't be used if Eglot
is to continue supporting Emacs 26 and 27.  Also I'd prefer if the patch
is slightly less disruptive to whitespace, like this:

diff --git a/lisp/progmodes/eglot.el b/lisp/progmodes/eglot.el
index 15cb1b6fad..eb3b51ed8b 100644
--- a/lisp/progmodes/eglot.el
+++ b/lisp/progmodes/eglot.el
@@ -991,6 +991,7 @@ eglot--guess-contact
 non-nil, maybe prompt user, else error as soon as something can't
 be guessed."
   (let* ((guessed-mode (if buffer-file-name major-mode))
+         (guessed-mode-name (and guessed-mode (symbol-name guessed-mode)))
          (main-mode
           (cond
            ((and interactive
@@ -1000,7 +1001,7 @@ eglot--guess-contact
              (completing-read
               "[eglot] Start a server to manage buffers of what major mode? "
               (mapcar #'symbol-name (eglot--all-major-modes)) nil t
-              (symbol-name guessed-mode) nil (symbol-name guessed-mode) nil)))
+              guessed-mode-name nil guessed-mode-name nil)))
            ((not guessed-mode)
             (eglot--error "Can't guess mode to manage for `%s'" 
(current-buffer)))
            (t guessed-mode)))

Other than that, it's fine to commit.

João