treaty -


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use core::{
    marker::PhantomData,
    mem::{ManuallyDrop, MaybeUninit},
};

/// Tiny version of dungeon-cell's DungeonCore.
///
/// Can be used to erase types. This container **will not**
/// run the stored value's Drop impl. Also this container does not
/// track what type it is storing. Also, also this container
/// does not align it's value so no making borrows of the inner value!.
///
/// This type **is** safe to use in const environments.
pub struct Erased<const N: usize> {
    /// A value of some unknown type that is N bytes long.
    ///
    /// We don't need a UnsafeCell here because we never give out a borrow
    /// of the value.
    bytes: MaybeUninit<[u8; N]>,

    _marker: PhantomData<*const ()>,
}

impl<const N: usize> Erased<N> {
    /// Erase the type of a value.
    pub const fn new<T>(value: T) -> Self {
        const {
            assert!(core::mem::size_of::<T>() <= N);
        }

        let value = ManuallyDrop::new(value);

        #[repr(C)]
        union Transmute<T, const N: usize> {
            value: ManuallyDrop<T>,
            bytes: MaybeUninit<[u8; N]>,
        }

        Self {
            #[allow(unsafe_code)]
            bytes: unsafe { Transmute { value }.bytes },
            _marker: PhantomData,
        }
    }

    /// Extract the original value.
    ///
    /// # Safety
    /// `T` must be the same type used when [`Self::new`] was called.
    #[allow(unsafe_code)]
    pub const unsafe fn into_inner<T>(self) -> T {
        const {
            // This should already be covered by new, but here for completeness.
            assert!(core::mem::size_of::<T>() <= N);
        }

        #[repr(C)]
        union Transmute<T, const N: usize> {
            bytes: MaybeUninit<[u8; N]>,
            value: ManuallyDrop<T>,
        }

        ManuallyDrop::into_inner(unsafe { Transmute { bytes: self.bytes }.value })
    }
}