(CODING_REQUIRE_ENCODING): Comment sync with HEAD.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/coding.h b/src/coding.h
index 41f4aab509c..93375d2c2eb 100644
--- a/src/coding.h
+++ b/src/coding.h
@@ -489,7 +489,10 @@ struct coding_system
 
 
 /* Return 1 if the coding context CODING requires code conversion on
-   encoding.  */
+   encoding.
+   The non-multibyte part of the condition is to support encoding of
+   unibyte strings/buffers generated by string-as-unibyte or
+   (set-buffer-multibyte nil) from multibyte strings/buffers.  */
 #define CODING_REQUIRE_ENCODING(coding)                         \
   ((coding)->src_multibyte                                      \
    || (coding)->common_flags & CODING_REQUIRE_ENCODING_MASK     \