Assume access to nil root branch always exists

src/history.rs

@@ -340,14 +340,15 @@ impl<E: Edit, S: Slot> History<E, S> {
                 .for_each(|(_, child)| child.parent.index -= 1);
         }
 
-        // Handle new branch.
+        // Handle new branch by putting the tail into the empty root branch
+        // before we swap the root with the new branch.
         if !tail.is_empty() {
             let next = self.branches.insert(Branch::NIL);
             let new = At::new(next, head.index);
-            if let Some(branch) = self.branches.get_mut(head.root) {
-                branch.parent = new;
-                branch.entries = tail;
-            }
+            let root = self.branches.get_mut(head.root).unwrap();
+            debug_assert!(root.entries.is_empty());
+            root.parent = new;
+            root.entries = tail;
             self.set_root(new, rm_saved);
         }
 
@@ -396,10 +397,10 @@ impl<E: Edit, S: Slot> History<E, S> {
                 let (_, _, entries, rm_saved) = self.record.redo_and_push(target, entry);
                 if !entries.is_empty() {
                     let new = At::new(id, index);
-                    if let Some(branch) = self.branches.get_mut(self.root) {
-                        branch.parent = new;
-                        branch.entries = entries;
-                    }
+                    let root = self.branches.get_mut(self.root).unwrap();
+                    debug_assert!(root.entries.is_empty());
+                    root.parent = new;
+                    root.entries = entries;
                     self.set_root(new, rm_saved);
                 }
             }