Restore autosuggestion after corrected typo

Backspace signals that the user is not happy with the commandline,
and by extension the autosuggestion.

For this reason, backspace suppresses autosuggestions until the next
text insertion.

However if I
1. type something that has an autosuggestion
2. type *one* wrong letter (removing the autosuggestion)
3. type backspace

backspace does not visibly suppress any autosuggestion but rhater
signal that the user wants to go back to the previous state of the
commandline, which does have an autosuggestion.

Enable this scenario by caching the autosuggestion when it's
invalidated. On certain edits that make the cached autosuggestion
valid again, restore it from the cache.  Currently, only do this up
to a single backspace.  Could extend that in future.

This implementation is really bad.. but it's a start.
Weirdly, it does not restore the cache on undo; but that's
inconsequential because undo doesn't suppress autosuggestion as
of today.

Closes #3549

src/reader.rs

@@ -490,6 +490,8 @@ pub struct ReaderData {
     rendered_layout: LayoutData,
     /// The current autosuggestion.
     autosuggestion: Autosuggestion,
+    /// A previously valid autosuggestion.
+    saved_autosuggestion: Option<Autosuggestion>,
     /// Current pager.
     pager: Pager,
     /// The output of the pager.
@@ -1155,6 +1157,7 @@ fn new(history: Arc<History>, conf: ReaderConfig) -> Pin<Box<Self>> {
             command_line_has_transient_edit: false,
             rendered_layout: Default::default(),
             autosuggestion: Default::default(),
+            saved_autosuggestion: Default::default(),
             pager: Default::default(),
             current_page_rendering: Default::default(),
             suppress_autosuggestion: Default::default(),
@@ -1248,7 +1251,9 @@ fn command_line_changed(&mut self, elt: EditableLineTag, keep_autosuggestion: bo
                 // Update the gen count.
                 GENERATION.fetch_add(1, Ordering::Relaxed);
                 if !keep_autosuggestion {
-                    self.autosuggestion.clear();
+                    if !self.autosuggestion.is_empty() {
+                        self.saved_autosuggestion = Some(std::mem::take(&mut self.autosuggestion));
+                    }
                 }
             }
             EditableLineTag::SearchField => {
@@ -1771,7 +1776,27 @@ fn try_apply_edit_to_autosuggestion(&mut self, elt: EditableLineTag, edit: &Edit
     }
 
     fn push_edit_internal(&mut self, elt: EditableLineTag, edit: Edit, allow_coalesce: bool) {
-        let preserves_autosuggestion = self.try_apply_edit_to_autosuggestion(elt, &edit);
+        let mut preserves_autosuggestion = self.try_apply_edit_to_autosuggestion(elt, &edit);
+        if elt == EditableLineTag::Commandline {
+            let saved_autosuggestion = self.saved_autosuggestion.take();
+            if (self.autosuggestion.is_empty()
+                || !preserves_autosuggestion
+                || self.suppress_autosuggestion)
+                && saved_autosuggestion
+                    .as_ref()
+                    .is_some_and(|saved_autosuggestion| {
+                        self.conf.autosuggest_ok
+                            && self.history_search.is_at_end()
+                            && edit.replacement.is_empty()
+                            && edit.range.start == saved_autosuggestion.search_string_range.end
+                            && edit.range.len() == 1
+                    })
+            {
+                self.autosuggestion = saved_autosuggestion.unwrap();
+                self.suppress_autosuggestion = false;
+                preserves_autosuggestion = true;
+            }
+        }
         self.edit_line_mut(elt).push_edit(edit, allow_coalesce);
         self.command_line_changed(elt, preserves_autosuggestion);
     }
@@ -1860,9 +1885,9 @@ fn delete_char(&mut self, backward: bool /* = true */) {
                 break;
             }
         }
+        self.suppress_autosuggestion = true;
         self.erase_substring(elt, pos..pos_end);
         self.update_buff_pos(elt, None);
-        self.suppress_autosuggestion = true;
     }
 }
 
@@ -2800,10 +2825,10 @@ fn handle_readline_command(&mut self, c: ReadlineCmd) {
                     };
                     let begin = el.position() + bias - self.rls().yank_len;
                     let end = el.position() + bias;
+                    self.suppress_autosuggestion = true;
                     self.replace_substring(elt, begin..end, yank_str);
                     self.update_buff_pos(elt, None);
                     self.rls_mut().yank_len = new_yank_len;
-                    self.suppress_autosuggestion = true;
                 }
             }
             rl::BackwardDeleteChar => {

tests/checks/tmux-autosuggestion.fish

@@ -23,3 +23,10 @@ isolated-tmux send-keys C-u 'complete nofilecomp -f' Enter C-l 'nofilecomp ./CO'
 tmux-sleep
 isolated-tmux capture-pane -p
 # CHECK: prompt 2> : ./COMPL
+
+isolated-tmux send-keys C-u C-k C-l ': ./CO'
+tmux-sleep
+isolated-tmux send-keys A C-h
+tmux-sleep
+isolated-tmux capture-pane -p
+# CHECK: prompt 2> : ./COMPL