bug#48043: UTF-8 magic comment is unwelcome with recent Ruby versions

[Dmitry Gutov <dgutov@yandex.ru>]

Hi!

On 26.04.2021 21:28, Peter Oliver wrote:
> When saving a ruby-mode buffer, if the content is not plain ASCII, then 
> the default behaviour is to add a specially-formatted comment that 
> indicates the encoding to the Ruby interpreter.  E.g.,
> 
> # coding: utf-8
> 
> However, since Ruby 2.0 released in 2013, the default encoding for Ruby 
> has been UTF-8.  Consequently, users of other editors tend not to 
> include this comment when using UTF-8.  When you edit such a file with 
> Emacs, you end up with a messy diff.
> 
> Two patches are attached to address this:
> 
> - The first patch adds a new choice to 
> ruby-insert-encoding-magic-comment, unless-utf8, which causes the magic 
> comment not to be inserted if the encoding is UTF-8.
> 
> - The second patch, perhaps more controversially, makes this the default.

Both changes make sense to me.

However, I've looked at the existing code and found a prior change which 
intended for this to be more customizable already, yet had a minor bug.

Please try out the following patch:

diff --git a/lisp/progmodes/ruby-mode.el b/lisp/progmodes/ruby-mode.el
index 84ac8fdb28..35772827ce 100644
--- a/lisp/progmodes/ruby-mode.el
+++ b/lisp/progmodes/ruby-mode.el
@@ -291,6 +291,7 @@ ruby-deep-indent-paren-style

  (defcustom ruby-encoding-map
    '((us-ascii       . nil)       ;; Do not put coding: us-ascii
+    (utf-8          . nil)       ;; Default since Ruby 2.0
      (shift-jis      . cp932)     ;; Emacs charset name of Shift_JIS
      (shift_jis      . cp932)     ;; MIME charset name of Shift_JIS
      (japanese-cp932 . cp932))    ;; Emacs charset name of CP932
@@ -760,7 +761,7 @@ ruby--insert-coding-comment

  (defun ruby--detect-encoding ()
    (if (eq ruby-insert-encoding-magic-comment 'always-utf8)
-      "utf-8"
+      'utf-8
      (let ((coding-system
             (or save-buffer-coding-system
                 buffer-file-coding-system)))
@@ -769,12 +770,11 @@ ruby--detect-encoding
                  (or (coding-system-get coding-system 'mime-charset)
                      (coding-system-change-eol-conversion coding-system 
nil))))
        (if coding-system
-          (symbol-name
-           (if ruby-use-encoding-map
-               (let ((elt (assq coding-system ruby-encoding-map)))
-                 (if elt (cdr elt) coding-system))
-             coding-system))
-        "ascii-8bit"))))
+          (if ruby-use-encoding-map
+              (let ((elt (assq coding-system ruby-encoding-map)))
+                (if elt (cdr elt) coding-system))
+            coding-system)
+        'ascii-8bit))))

  (defun ruby--encoding-comment-required-p ()
    (or (eq ruby-insert-encoding-magic-comment 'always-utf8)
@@ -796,7 +796,7 @@ ruby-mode-set-encoding
                     (unless (string= (match-string 2) coding-system)
                       (goto-char (match-beginning 2))
                       (delete-region (point) (match-end 2))
-                     (insert coding-system)))
+                     (insert (symbol-name coding-system))))
                    ((looking-at "\\s *#.*coding\\s *[:=]"))
                    (t (when ruby-insert-encoding-magic-comment
                         (ruby--insert-coding-comment coding-system))))