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use core::marker::Unsize;
use core::ptr::{self, DynMetadata, NonNull, Pointee};

/// A type whose pointer metadata can be stored in a [`DynDynTable`](crate::DynDynTable).
#[const_trait]
pub trait DynDynCastTarget: private::Sealed {
    /// The root type that the trait object metadata is for.
    type Root: ?Sized;

    /// Splits a fat pointer into its data pointer and metadata.
    fn ptr_into_parts(ptr: NonNull<Self>) -> (NonNull<()>, DynMetadata<Self::Root>);

    /// Combines a data pointer with the provided metadata to produce a fat pointer.
    fn ptr_from_parts(data: NonNull<()>, meta: DynMetadata<Self::Root>) -> NonNull<Self>;

    /// Gets the metadata that would be used for an object of concrete type `U` when cast to this type.
    fn meta_for_ty<U: Unsize<Self>>() -> DynMetadata<Self::Root>;
}

impl<M: ?Sized, T: Pointee<Metadata = DynMetadata<M>> + ?Sized> const DynDynCastTarget for T {
    type Root = M;

    fn ptr_into_parts(ptr: NonNull<Self>) -> (NonNull<()>, DynMetadata<M>) {
        (ptr.cast(), ptr::metadata(ptr.as_ptr()))
    }

    fn ptr_from_parts(data: NonNull<()>, meta: DynMetadata<M>) -> NonNull<Self> {
        // SAFETY: If data is not null, then the result of attaching metadata to it is not null either
        unsafe { NonNull::new_unchecked(ptr::from_raw_parts_mut(data.as_ptr(), meta)) }
    }

    fn meta_for_ty<U: Unsize<Self>>() -> DynMetadata<M> {
        ptr::metadata(ptr::null::<U>() as *const Self)
    }
}

mod private {
    use core::ptr::{DynMetadata, Pointee};

    pub trait Sealed {}

    impl<M: ?Sized, T: Pointee<Metadata = DynMetadata<M>> + ?Sized> Sealed for T {}
}