how to fix error in trie implementation in rust?

I’m trying to implement a trie in rust to learn the language, especially around recursive structures. In the process, I"m seeing that the add_word method gives me an error. The implementation is

use std::array;
#[derive(Default)]
pub struct Trie {
    is_end: bool,
    pub children: [Option<Box<Trie>>; 26],
}

impl Trie {
    pub fn new() -> Box<Self> {
        Box::new(Trie {
            is_end: false,
            children: array::from_fn(|_| None),
        })
    }

    pub fn starts_with(&self, s: &str) -> bool {
        if s.is_empty() {
            return true;
        }
        let child = &self.children[(s.as_bytes()[0] - b'a') as usize];
        if child.is_none() {
            return false;
        }
        child
            .as_ref()
            .unwrap()
            .starts_with(s.get(1..).unwrap_or_default())
    }

    pub fn add_word(&mut self, s: &str) {
        if s.is_empty() {
            self.is_end = true;
        }
        let index = (s.as_bytes()[0] - b'a') as usize;
        let child = &self.children[index];
        if child.is_none() {
            return;
        }
        child
            .as_ref()
            .unwrap()
            .add_word(s.get(1..).unwrap_or_default());
    }
}

and the error is cannot borrow data in a & reference as mutable cannot borrow as mutable. I’m open to other fixes not directly related to my question that may yield more idiomatic rust code. Like, I believe I could have used unwrap_or_default in places but my attempts led to a bunch of other scary errors that didn’t get easily resolved by following clippy’s advice.

>Solution :

Try This:

use std::array;

#[derive(Default)]
pub struct Trie {
    is_end: bool,
    pub children: Vec<Option<Box<Trie>>>,
}

impl Trie {
    pub fn new() -> Box<Self> {
        Box::new(Trie {
            is_end: false,
            children: vec![None; 26],
        })
    }

    pub fn starts_with(&self, s: &str) -> bool {
        if s.is_empty() {
            return true;
        }
        let child = &self.children[(s.as_bytes()[0] - b'a') as usize];
        if child.is_none() {
            return false;
        }
        child
            .as_ref()
            .unwrap()
            .starts_with(s.get(1..).unwrap_or_default())
    }

    pub fn add_word(&mut self, s: &str) {
        if s.is_empty() {
            self.is_end = true;
        } else {
            let index = (s.as_bytes()[0] - b'a') as usize;
            let child = self.children.get_mut(index).unwrap();
            if child.is_none() {
                *child = Some(Trie::new());
            }
            child.as_mut().unwrap().add_word(s.get(1..).unwrap_or_default());
        }
    }
}