yoke 0.8.2

Abstraction allowing borrowed data to be carried along with the backing data it borrows from
Documentation 
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// This file is part of ICU4X. For terms of use, please see the file
// called LICENSE at the top level of the ICU4X source tree
// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).

// In this case consistency between impls is more important
// than using pointer casts
#![allow(clippy::transmute_ptr_to_ptr)]

use crate::Yokeable;
use core::{mem::ManuallyDrop, ptr};

macro_rules! copy_yoke_impl {
    () => {
        #[inline]
        fn transform(&self) -> &Self::Output {
            self
        }
        #[inline]
        fn transform_owned(self) -> Self::Output {
            self
        }
        #[inline]
        unsafe fn make(this: Self::Output) -> Self {
            this
        }
        #[inline]
        fn transform_mut<F>(&'a mut self, f: F)
        where
            F: 'static + for<'b> FnOnce(&'b mut Self::Output),
        {
            f(self)
        }
    };
}
macro_rules! impl_copy_type {
    ($ty:ty) => {
        // Safety: all the types that this macro is used to generate impls of Yokeable for do not
        // borrow any memory.
        unsafe impl<'a> Yokeable<'a> for $ty {
            type Output = Self;
            copy_yoke_impl!();
        }
    };
}

impl_copy_type!(());
impl_copy_type!(u8);
impl_copy_type!(u16);
impl_copy_type!(u32);
impl_copy_type!(u64);
impl_copy_type!(u128);
impl_copy_type!(usize);
impl_copy_type!(i8);
impl_copy_type!(i16);
impl_copy_type!(i32);
impl_copy_type!(i64);
impl_copy_type!(i128);
impl_copy_type!(isize);
impl_copy_type!(char);
impl_copy_type!(bool);

// This is for when we're implementing Yoke on a complex type such that it's not
// obvious to the compiler that the lifetime is covariant
//
// Safety: the caller of this macro must ensure that `Self` is indeed covariant in 'a.
macro_rules! unsafe_complex_yoke_impl {
    () => {
        fn transform(&'a self) -> &'a Self::Output {
            // Safety: equivalent to casting the lifetime. Macro caller ensures covariance.
            unsafe { &*(self as *const Self as *const Self::Output) }
        }

        fn transform_owned(self) -> Self::Output {
            // Safety: equivalent to casting the lifetime. Macro caller ensures covariance.
            unsafe {
                let ptr: *const Self::Output = (&self as *const Self).cast();
                let _ = ManuallyDrop::new(self);
                ptr::read(ptr)
            }
        }

        unsafe fn make(from: Self::Output) -> Self {
            debug_assert!(size_of::<Self::Output>() == size_of::<Self>());
            let ptr = &from as *const Self::Output as *const Self;
            let _ = ManuallyDrop::new(from);
            // Safety: `ptr` is certainly valid, aligned and points to a properly initialized value, as
            // it comes from a value that was moved into a ManuallyDrop.
            unsafe { ptr::read(ptr) }
        }

        fn transform_mut<F>(&'a mut self, f: F)
        where
            F: 'static + for<'b> FnOnce(&'b mut Self::Output),
        {
            crate::utils::transform_mut_yokeable(self, f)
        }
    };
}

// Safety: since T implements Yokeable<'a>, Option<T<'b>> must be covariant on 'b or the Yokeable
// implementation on T would be unsound.
unsafe impl<'a, T: 'static + for<'b> Yokeable<'b>> Yokeable<'a> for Option<T> {
    type Output = Option<<T as Yokeable<'a>>::Output>;
    unsafe_complex_yoke_impl!();
}

// Safety: since T1, T2 implement Yokeable<'a>, (T1<'b>, T2<'b>) must be covariant on 'b or the Yokeable
// implementation on T would be unsound.
unsafe impl<'a, T1: 'static + for<'b> Yokeable<'b>, T2: 'static + for<'b> Yokeable<'b>> Yokeable<'a>
    for (T1, T2)
{
    type Output = (<T1 as Yokeable<'a>>::Output, <T2 as Yokeable<'a>>::Output);
    unsafe_complex_yoke_impl!();
}

// Safety: since T implements Yokeable<'a>, [T<'b>; N] must be covariant on 'b or the Yokeable
// implementation on T would be unsound.
unsafe impl<'a, T: 'static + for<'b> Yokeable<'b>, const N: usize> Yokeable<'a> for [T; N] {
    type Output = [<T as Yokeable<'a>>::Output; N];
    unsafe_complex_yoke_impl!();
}