willdo 0.0.1

Task manager with DAG
Documentation 
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use std::sync::Arc;

#[derive(Debug)]
pub struct Stack<T: 'static>(Link<'static, T>);

impl<T> Default for Stack<T> {
    fn default() -> Self {
        Self(Default::default())
    }
}

impl<T> FromIterator<T> for Stack<T> {
    fn from_iter<I: IntoIterator<Item = T>>(iter: I) -> Self {
        let mut list = Stack::new();
        for item in iter {
            list += item;
        }
        list
    }
}

impl<T> core::ops::Add<T> for Stack<T> {
    type Output = Self;
    fn add(self, item: T) -> Self::Output {
        Self(Link::Own(Arc::new(Entry {
            item,
            next: self.0.clone(),
        })))
    }
}
impl<T> core::ops::Add<T> for &Stack<T> {
    type Output = Stack<T>;
    fn add(self, item: T) -> Self::Output {
        Stack(Link::Own(Arc::new(Entry {
            item,
            next: self.0.clone(),
        })))
    }
}

impl<T> core::ops::AddAssign<T> for Stack<T> {
    fn add_assign(&mut self, rhs: T) {
        *self = core::mem::take(self) + rhs
    }
}

impl<'a, T> Stack<T> {
    pub fn new() -> Self {
        Self(Link::Empty)
    }
    pub fn iter(&'a self) -> impl 'a + Iterator<Item = &'a T> {
        &self.0
    }
    pub(crate) fn head(&self) -> Option<&T> {
        self.0.as_ref().map(|e| &e.item)
    }
}
#[derive(Debug)]
enum Link<'a, T> {
    Empty,
    Ref(&'a Entry<'a, T>),
    Own(Arc<Entry<'a, T>>),
}
impl<'a, T> Link<'a, T> {
    fn as_ref(&'a self) -> Option<&'a Entry<'a, T>> {
        Some(match self {
            Link::Empty => return None,
            Link::Ref(entry) => entry,
            Link::Own(entry) => entry.as_ref(),
        })
    }
}
impl<T> Default for Link<'_, T> {
    fn default() -> Self {
        Self::Empty
    }
}
impl<T> Clone for Link<'_, T> {
    fn clone(&self) -> Self {
        match self {
            Self::Empty => Self::Empty,
            Self::Ref(entry) => Self::Ref(entry),
            Self::Own(entry) => Self::Own(entry.clone()),
        }
    }
}

#[derive(Debug, Default)]
struct Entry<'a, T> {
    item: T,
    next: Link<'a, T>,
}

impl<'a, T> Iterator for &'a Link<'a, T> {
    type Item = &'a T;
    fn next(&mut self) -> Option<Self::Item> {
        let entry = match self {
            Link::Empty => return None,
            Link::Ref(entry) => entry,
            Link::Own(entry) => entry.as_ref(),
        };
        *self = &entry.next;
        Some(&entry.item)
    }
}

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

    #[test]
    fn add() {
        let stack = Stack::new() + true + false;
        assert_eq!(stack.iter().collect::<Vec<_>>(), vec![&false, &true]);
    }

    #[test]
    fn add_assign() {
        let mut stack = Stack::new();
        stack += "a";
        stack += "b";
        assert_eq!(stack.iter().collect::<Vec<_>>(), vec![&"b", &"a"]);
    }

    #[test]
    fn collect() {
        let stack = [1, 2, 3].into_iter().collect::<Stack<usize>>();
        assert_eq!(stack.iter().collect::<Vec<_>>(), vec![&3, &2, &1]);
    }
}