rust cannot move out of `self.root` as enum variant `Some` which is behind a mutable reference

This error occurs when trying to move a value out of a borrowed mutable reference. In Rust, borrowing a value as mutable means that it cannot be moved out of the reference until the reference goes out of scope.

In this specific case, the error message is indicating that the code is trying to move the value of `self.root` out of a mutable reference to an `Option` variant `Some`. This is not allowed because `Some` is a non-trivial value that cannot be copied, and moving it out of the reference would leave the mutable reference in an invalid state.

To fix this error, the code needs to be modified to ensure that the value of `self.root` is not moved out of the mutable reference. One possible solution is to use the `take` method on the `Option` type, which replaces the value with `None` and returns the original value, allowing it to be moved out of the reference safely:

let root = self.root.take().unwrap();

This code first calls `take` on `self.root` to replace the value with `None` and return the original value. Then, it uses `unwrap` to extract the value from the `Option` variant `Some` and move it out of the reference. Since `self.root` is now `None`, the mutable reference is still valid and can be used for further operations.