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use core::fmt;

use crate::error::IntoRepr;
use crate::pointer::Packable;
use crate::traits::ZeroCopy;

mod sealed {
    use crate::mem::MaybeUninit;
    use crate::pointer::Pointee;
    use crate::traits::ZeroCopy;

    pub trait Sealed {}

    impl<T> Sealed for MaybeUninit<T> where T: Pointee {}
    impl<T> Sealed for T where T: ZeroCopy {}
    impl<T> Sealed for [T] where T: ZeroCopy {}
    impl Sealed for str {}
}

/// The trait for a value that can be pointed to by a [`Ref<T>`].
///
/// This ultimately determines the layout of [`Ref<T>`] as for unsized types it
/// needs to accommodate the size of the pointed-to type as well.
///
/// ```
/// use std::mem::size_of;
///
/// use musli_zerocopy::Ref;
///
/// assert_eq!(size_of::<Ref::<u32>>(), 4);
/// assert_eq!(size_of::<Ref::<[u32]>>(), 8);
/// ```
///
/// [`Ref<T>`]: crate::Ref
pub trait Pointee: self::sealed::Sealed {
    /// Metadata associated with a pointee.
    type Metadata: Copy + fmt::Debug + Packable + IntoRepr;
}

impl<T> Pointee for T
where
    T: ZeroCopy,
{
    type Metadata = ();
}

impl<T> Pointee for [T]
where
    T: ZeroCopy,
{
    type Metadata = usize;
}

impl Pointee for str {
    type Metadata = usize;
}