memory_storage 0.9.30

A memory storage comparable to a Vec where removing items doesn't shift all the items after the removed item to the left and doesn't invalidate their IDs. It allows you to remove items with high speed and access items via an ID that was returned after adding them.
Documentation 
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use core::fmt::{Debug, Formatter};

/// A slot representing a spot in the storage.
pub enum Slot<T> {
    Taken(T),
    NextFreeSlot(Option<usize>),
}

impl<T> Slot<T> {
    pub fn is_taken(&self) -> bool {
        matches!(*self, Slot::Taken(_))
    }

    pub fn is_free(&self) -> bool {
        !self.is_taken()
    }

    pub fn taken(&self) -> Option<&T> {
        if let Slot::Taken(item) = self {
            Some(item)
        } else {
            None
        }
    }

    pub fn taken_mut(&mut self) -> Option<&mut T> {
        if let Slot::Taken(item) = self {
            Some(item)
        } else {
            None
        }
    }

    pub fn unwrap_taken(self) -> T {
        if let Slot::Taken(kill_switch) = self {
            kill_switch
        } else {
            panic!("Slot wasn't taken!")
        }
    }

    pub fn unwrap_next_free(self) -> Option<usize> {
        if let Slot::NextFreeSlot(next_free_slot) = self {
            next_free_slot
        } else {
            panic!("Slot wasn't free!")
        }
    }

    pub fn next_free(&self) -> Option<usize> {
        if let Slot::NextFreeSlot(next_free_slot) = self {
            *next_free_slot
        } else {
            panic!("Slot wasn't free!")
        }
    }
}

impl<T: Debug> Debug for Slot<T> {
    fn fmt(&self, f: &mut Formatter<'_>) -> core::fmt::Result {
        match self {
            Slot::Taken(item) =>
                write!(f, "Slot::Taken({:#?})", item),
            Slot::NextFreeSlot(next_free_slot) =>
                write!(f, "Slot::NextFreeSlot({:#?})", next_free_slot),
        }
    }
}