Make describe-prefix-bindings say when there are no matches

* lisp/help.el (describe-prefix-bindings): Say when there are no
bindings under a prefix (bug#55875), for instance in `C-c C-h' in
a buffer with no `C-c' commands.

1 files changed, 17 insertions, 10 deletions

diff --git a/lisp/help.el b/lisp/help.el
index 4e0d807cb2c..abdce46edf3 100644
--- a/lisp/help.el
+++ b/lisp/help.el
@@ -392,16 +392,23 @@ If that doesn't give a function, return nil."
 The prefix described consists of all but the last event
 of the key sequence that ran this command."
   (interactive)
-  (let ((key (this-command-keys)))
-    (describe-bindings
-     (if (stringp key)
-	 (substring key 0 (1- (length key)))
-       (let ((prefix (make-vector (1- (length key)) nil))
-	     (i 0))
-	 (while (< i (length prefix))
-	   (aset prefix i (aref key i))
-	   (setq i (1+ i)))
-	 prefix)))))
+  (let* ((key (this-command-keys))
+         (prefix
+          (if (stringp key)
+	      (substring key 0 (1- (length key)))
+            (let ((prefix (make-vector (1- (length key)) nil))
+	          (i 0))
+	      (while (< i (length prefix))
+	        (aset prefix i (aref key i))
+	        (setq i (1+ i)))
+	      prefix))))
+    (describe-bindings prefix)
+    (with-current-buffer (help-buffer)
+      (when (< (buffer-size) 10)
+        (let ((inhibit-read-only t))
+          (insert (format "No commands with a binding that start with %s."
+                          (help--key-description-fontified prefix))))))))
+
 ;; Make C-h after a prefix, when not specifically bound,
 ;; run describe-prefix-bindings.
 (setq prefix-help-command 'describe-prefix-bindings)