Function try_array_init_impl

src/lib.rs:193–315

(
    mut initializer: F,
)

#[inline]
fn try_array_init_impl<Err, F, T, const N: usize, const D: i8>(
    mut initializer: F,
) -> Result<[T; N], Err>
where
    F: FnMut(usize) -> Result<T, Err>,
{
    // The implementation differentiates two cases:
    //   A) `T` does not need to be dropped. Even if the initializer panics
    //      or returns `Err` we will not leak memory.
    //   B) `T` needs to be dropped. We must keep track of which elements have
    //      been initialized so far, and drop them if we encounter a panic or `Err` midway.
    if !mem::needs_drop::<T>() {
        let mut array: MaybeUninit<[T; N]> = MaybeUninit::uninit();
        // pointer to array = *mut [T; N] <-> *mut T = pointer to first element
        let mut ptr_i = array.as_mut_ptr() as *mut T;

        // # Safety
        //
        //   - for D > 0, we are within the array since we start from the
        //     beginning of the array, and we have `0 <= i < N`.
        //   - for D < 0, we start at the end of the array and go back one
        //     place before writing, going back N times in total, finishing
        //     at the start of the array.
        unsafe {
            if D < 0 {
                ptr_i = ptr_i.add(N);
            }
            for i in 0..N {
                let value_i = initializer(i)?;
                // We overwrite *ptr_i previously undefined value without reading or dropping it.
                if D < 0 {
                    ptr_i = ptr_i.sub(1);
                }
                ptr_i.write(value_i);
                if D > 0 {
                    ptr_i = ptr_i.add(1);
                }
            }
            Ok(array.assume_init())
        }
    } else {
        // else: `mem::needs_drop::<T>()`

        /// # Safety
        ///
        ///   - `base_ptr[.. initialized_count]` must be a slice of init elements...
        ///
        ///   - ... that must be sound to `ptr::drop_in_place` if/when
        ///     `UnsafeDropSliceGuard` is dropped: "symbolic ownership"
        struct UnsafeDropSliceGuard<Item> {
            base_ptr: *mut Item,
            initialized_count: usize,
        }

        impl<Item> Drop for UnsafeDropSliceGuard<Item> {
            fn drop(self: &'_ mut Self) {
                unsafe {
                    // # Safety