Code shared from the Rust Playground

playground.rs

use std::collections::BTreeMap;
use std::io::{self, Write};

pub struct Todo {
    name: String,
    checked: bool,
    priority: i32,
}

impl Todo {
    pub fn write<W: Write>(&self, w: &mut W) {
        write!(w, "{}:\n", self.name);
        write!(w, "Checked: {}\n", self.checked);
        write!(w, "Priority: {}\n", self.priority);
        write!(w, "\n");
    }
    
    pub fn print(&self) {
        self.write(&mut io::stdout());
    }
}

#[derive(Default)]
pub struct TodoList {
    list: BTreeMap<i32, Vec<Todo>>,
}

impl TodoList {
    pub fn new() -> TodoList {
        TodoList {
            list: BTreeMap::new(),
        }
    }

    pub fn add(&mut self, todo: Todo) {
        let list = self.list.entry(todo.priority).or_insert_with(Vec::new);
        list.push(todo);
    }

    pub fn todos(&self) -> impl Iterator<Item = &Todo> {
        self.list.iter().flat_map(|(_, list)| list.iter())
    }

    pub fn unchecked(&self) -> impl Iterator<Item = &Todo> {
        self.todos().filter(|t| !t.checked)
    }

    pub fn checked(&self) -> impl Iterator<Item = &Todo> {
        self.todos().filter(|t| t.checked)
    }
}

fn main() {
    let mut list = TodoList::new();
    list.add(Todo {
        name: "A".to_string(),
        checked: false,
        priority: 10,
    });
    list.add(Todo {
        name: "B".to_string(),
        checked: false,
        priority: 5,
    });
    list.add(Todo {
        name: "C".to_string(),
        checked: false,
        priority: 6,
    });
    list.add(Todo {
        name: "D".to_string(),
        checked: true,
        priority: 1,
    });
    
    println!("Unchecked todos:");
    for todo in list.unchecked() {
        todo.print();
    }

    println!("Checked todos:");
    for todo in list.checked() {
        todo.print();
    }
}