emacs-29 cfd2b3504a: Fix encoding with 'utf-8-auto'

[Eli Zaretskii]

branch: emacs-29
commit cfd2b3504ab5de6eb5f3c7a0784cb447883e1326
Author: Eli Zaretskii <eliz@gnu.org>
Commit: Eli Zaretskii <eliz@gnu.org>

    Fix encoding with 'utf-8-auto'
    
    * src/coding.c (encode_coding_utf_8): Fix encoding with
    'utf-8-auto': it should produce BOM, per the documentation of
    the :bom attribute.  (Bug#60750)
    
    * lisp/international/mule.el (define-coding-system): Doc fix.
    
    * test/src/coding-tests.el (coding-tests): Use
    'with-coding-priority' instead of 'prefer-coding-system', as the
    latter has global persistent effect and affects further tests.
---
 lisp/international/mule.el |  3 ++-
 src/coding.c               |  2 +-
 test/src/coding-tests.el   | 30 +++++++++++++++---------------
 3 files changed, 18 insertions(+), 17 deletions(-)

diff --git a/lisp/international/mule.el b/lisp/international/mule.el
index 4f6addea38..eddd7b6407 100644
--- a/lisp/international/mule.el
+++ b/lisp/international/mule.el
@@ -863,7 +863,8 @@ This attribute is meaningful only when `:coding-type' is 
`utf-16' or
 VALUE must be `big' or `little' specifying big-endian and
 little-endian respectively.  The default value is `big'.
 
-This attribute is meaningful only when `:coding-type' is `utf-16'.
+Changing this attribute is only meaningful when `:coding-type'
+is `utf-16'.
 
 `:ccl-decoder' (required if :coding-type is `ccl')
 
diff --git a/src/coding.c b/src/coding.c
index 4e59f2b6a1..49dcd8634f 100644
--- a/src/coding.c
+++ b/src/coding.c
@@ -1431,7 +1431,7 @@ encode_coding_utf_8 (struct coding_system *coding)
   ptrdiff_t produced_chars = 0;
   int c;
 
-  if (CODING_UTF_8_BOM (coding) == utf_with_bom)
+  if (CODING_UTF_8_BOM (coding) != utf_without_bom)
     {
       ASSURE_DESTINATION (3);
       EMIT_THREE_BYTES (UTF_8_BOM_1, UTF_8_BOM_2, UTF_8_BOM_3);
diff --git a/test/src/coding-tests.el b/test/src/coding-tests.el
index 6bd8d1ae6c..b27907027b 100644
--- a/test/src/coding-tests.el
+++ b/test/src/coding-tests.el
@@ -148,21 +148,21 @@
 
 (defun coding-tests (content-type write-coding read-coding detected-coding
                                   &optional translator)
-  (prefer-coding-system 'utf-8-auto)
-  (let ((filename (coding-tests-filename content-type write-coding)))
-    (with-temp-buffer
-      (let ((coding-system-for-read read-coding)
-           (contents (coding-tests-file-contents content-type))
-           (disable-ascii-optimization nil))
-       (if translator
-           (setq contents (funcall translator contents)))
-       (insert-file-contents filename)
-       (if (and (coding-system-equal buffer-file-coding-system detected-coding)
-                (string= (buffer-string) contents))
-           nil
-         (list buffer-file-coding-system
-               (string-to-list (buffer-string))
-               (string-to-list contents)))))))
+  (with-coding-priority '(utf-8-auto)
+    (let ((filename (coding-tests-filename content-type write-coding)))
+      (with-temp-buffer
+        (let ((coding-system-for-read read-coding)
+             (contents (coding-tests-file-contents content-type))
+             (disable-ascii-optimization nil))
+         (if translator
+             (setq contents (funcall translator contents)))
+         (insert-file-contents filename)
+         (if (and (coding-system-equal buffer-file-coding-system 
detected-coding)
+                  (string= (buffer-string) contents))
+             nil
+           (list buffer-file-coding-system
+                 (string-to-list (buffer-string))
+                 (string-to-list contents))))))))
 
 (ert-deftest ert-test-coding-ascii ()
   (unwind-protect