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use ::core::{mem::ManuallyDrop, ptr::read};

use crate::Destructure;

pub trait Destructuring {}

pub trait DestructuringFor<T>: Destructuring {
    type Destructurer: Destructurer<Inner = T>;
}

pub trait Destructurer {
    type Inner: Destructure;

    fn new(inner: Self::Inner) -> Self;

    fn inner(&self) -> &Self::Inner;

    fn inner_mut(&mut self) -> &mut Self::Inner;
}

pub trait Test<'a> {
    type Test;

    /// # Safety
    ///
    /// `test` may not be called.
    unsafe fn test(&'a mut self) -> Self::Test;
}

pub struct Ref<T>(T);

impl<T: Destructure> Destructurer for Ref<T> {
    type Inner = T;

    fn new(inner: T) -> Self {
        Self(inner)
    }

    fn inner(&self) -> &Self::Inner {
        &self.0
    }

    fn inner_mut(&mut self) -> &mut Self::Inner {
        &mut self.0
    }
}

impl<'a, T: 'a + Destructure> Test<'a> for Ref<T> {
    type Test = &'a T::Underlying;

    unsafe fn test(&'a mut self) -> Self::Test {
        // SAFETY: `test` may not be called.
        unsafe { &*T::underlying(&mut self.0) }
    }
}

pub struct Value<T>(ManuallyDrop<T>);

impl<T: Destructure> Destructurer for Value<T> {
    type Inner = T;

    fn new(inner: T) -> Self {
        Self(ManuallyDrop::new(inner))
    }

    fn inner(&self) -> &Self::Inner {
        &self.0
    }

    fn inner_mut(&mut self) -> &mut Self::Inner {
        &mut self.0
    }
}

impl<'a, T: 'a + Destructure> Test<'a> for Value<T>
where
    T::Underlying: Sized,
{
    type Test = T::Underlying;

    unsafe fn test(&'a mut self) -> Self::Test {
        // SAFETY: `test` may not be called.
        unsafe { read(T::underlying(&mut self.0)) }
    }
}