Stuck with borrowing rules error

[chino]

Hi,

I'm trying to build a todo app to learn Rust, but I'm a bit stuck.

Currently I have a todo_list struct that holds all todos, and I also operate on that one to create or delete items.

However, since I want to display daily, weekly, monthly and yearly todos later in different sections on the screens, I have getters for each one respectively.

Now having the code as below, I run into errors with the borrowing rules. Since I have a mutable borrow on the todo_list variable so I can add and delete items, I can't add another borrow for my specific getters such as daily_todos.

I understand the rules that I can have as many immutable borrows, but as soon as I have a mutable one that's is, this is the one and only. But now I can't think of how to make this work. I'm starting to think that my approach is flawed from the beginning regarding the data structure, but I wanted to ask for some help here first because I really am stuck.

#[derive(Debug, PartialEq)]
pub enum TodoType {
    Daily,
    Weekly,
}

#[derive(Debug, PartialEq)]
pub struct TodoItem {
    id: String,
    name: String,
    completed: bool,
    comment: Option<String>,
    todo_type: TodoType,
}

#[derive(Debug, Default)]
pub struct TodoList {
    todos: Vec<TodoItem>,
}

impl TodoItem {
    fn new(item: String, comment: Option<String>, todo_type: TodoType) -> Self {
        Self {
            id: "1".to_string(),
            name: item,
            completed: false,
            comment,
            todo_type,
        }
    }
}

impl TodoList {
    pub fn new() -> Self {
        Self { todos: vec![] }
    }

    // Getters
    pub fn get_daily(&self) -> Vec<&TodoItem> {
        self.get_todos(TodoType::Daily)
    }

    pub fn get_weekly(&self) -> Vec<&TodoItem> {
        self.get_todos(TodoType::Weekly)
    }

    fn get_todos(&self, todo_type: TodoType) -> Vec<&TodoItem> {
        self.todos
            .iter()
            .filter(|todo| todo.todo_type == todo_type)
            .collect()
    }

    // Adders
    pub fn add_daily_todo(&mut self, item: String, comment: Option<String>) {
        self.add_todo(item, comment, TodoType::Daily)
    }

    pub fn add_weekly_todo(&mut self, item: String, comment: Option<String>) {
        self.add_todo(item, comment, TodoType::Weekly)
    }

    fn add_todo(&mut self, item: String, comment: Option<String>, todo_type: TodoType) {
        self.todos.push(TodoItem::new(item, comment, todo_type))
    }

    // Removers
    pub fn remove_todo_by_id(&mut self, id: &String) {
        self.todos.retain_mut(|item| item.id != *id)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn can_delete_a_daily_todo() {
        let mut todo_list = TodoList::new();

        todo_list.add_daily_todo("D1".to_string(), None);
        todo_list.add_daily_todo("D2".to_string(), None);

        let daily_todos = todo_list.get_daily();

        // check initial state
        assert_eq!(daily_todos.len(), 2);
        assert_eq!(daily_todos[0].name, "D1".to_owned());
        assert_eq!(daily_todos[1].name, "D2".to_owned());

        // delete first todo
        todo_list.remove_todo_by_id(&daily_todos[0].id);

        // check updated todo list
        assert_eq!(daily_todos.len(), 1);
        assert_eq!(daily_todos[0].name, "D2".to_owned());
    }
}

This gives me the following error on the test:

But I can't change the get_daily to a mutable borrow, since then it complains about
cannot borrow todo_list as mutable more than once at a time

Would appreciate some help here

[zirconium-n]

Without mass refactoring your code, you should just call get_daily again.

With current code, ignoring daily_todos does not borrow todo_list, it's still a disjoint Vec that will not magically shrink when something happens to todo_list. i.e. Even if you translate your code to python, it still won't work as expected.

[chino]

Thank you for your reply.

Hmm yeah I didn't think of that. It looks like a big refactor is in order.

What would be the smartest way to set this up? I'm thinking having separate structs for daily and weekly todos?

[zirconium-n]

It depends. There's no definitive answer. Since you are just learning Rust, I think calling get_daily again is not that bad. You can return Vec<TodoItem> by cloning the items. I suggest avoid "complicated" stuff at first.

In the end, what's the best pattern depends on your api usage pattern.

[chino]

Thanks! I will try that.

I will most likely try a few different solutions side by side to see which one works the best.