accept-autosuggestion to return false if there was no autosuggestion to accept

Example usage:

    bind ctrl-space accept-autosuggestion and execute

Closes #10608

CHANGELOG.rst

@@ -180,6 +180,7 @@ New or improved bindings
 - New special input functions:
   - ``forward-char-passive`` and ``backward-char-passive`` are like their non-passive variants but do not accept autosuggestions or move focus in the completion pager (:issue:`10398`).
   - ``forward-token``, ``backward-token``, ``kill-token``, and ``backward-kill-token`` are similar to the ``*-bigword`` variants but for the whole argument token which includes escaped spaces (:issue:`2014`).
+- The ``accept-autosuggestion`` special input function now returns false when there was nothing to accept.
 - Vi mode has seen some improvements but continues to suffer from the lack of people working on it.
   - Insert-mode :kbd:`ctrl-n` accepts autosuggestions (:issue:`10339`).
   - Outside insert mode, the cursor will no longer be placed beyond the last character on the commandline.

doc_src/cmds/bind.rst

@@ -112,7 +112,7 @@ The following special input functions are available:
     only execute the next function if the previous succeeded (note: only some functions report success)
 
 ``accept-autosuggestion``
-    accept the current autosuggestion
+    accept the current autosuggestion. Returns false when there was nothing to accept.
 
 ``backward-char``
     move one character to the left.

src/reader.rs

@@ -2984,7 +2984,9 @@ fn handle_readline_command(&mut self, c: ReadlineCmd) {
                 self.input_data.function_set_status(success);
             }
             rl::AcceptAutosuggestion => {
+                let success = !self.autosuggestion.is_empty();
                 self.accept_autosuggestion(true, false, MoveWordStyle::Punctuation);
+                self.input_data.function_set_status(success);
             }
             rl::TransposeChars => {
                 let (elt, el) = self.active_edit_line();