[PATCH 6/7] rust: init: Add functions to create array initializers

[Alice Ryhl <aliceryhl@google.com>]

Benno Lossin <benno.lossin@proton.me> writes:
> Add two functions `pin_init_array_from_fn` and `init_array_from_fn` that
> take a function that generates initializers for `T` from usize, the added
> functions then return an initializer for `[T; N]` where every element is
> initialized by an element returned from the generator function.
> 
> Suggested-by: Asahi Lina <lina@asahilina.net>
> Signed-off-by: Benno Lossin <benno.lossin@proton.me>
> ---
> +/// Initializes an array by initializing each element via the provided initializer.
> +///
> +/// # Examples
> +///
> +/// ```rust
> +/// let array: Box<[usize; 1000_000_000]>= Box::init(init_array_from_fn(|i| i)).unwrap();
> +/// println!("{array:?}");
> +/// ```
> +pub fn init_array_from_fn<I, const N: usize, T, E>(
> +    mut make_init: impl FnMut(usize) -> I,
> +) -> impl Init<[T; N], E>
> +where
> +    I: Init<T, E>,
> +{
> +    let init = move |slot: *mut [T; N]| {
> +        let slot = slot.cast::<T>();
> +        for i in 0..N {
> +            let init = make_init(i);
> +            // SAFETY: since 0 <= `i` < N, it is still in bounds of `[T; N]`.
> +            let ptr = unsafe { slot.add(i) };
> +            // SAFETY: The pointer is derived from `slot` and thus satisfies the `__init`
> +            // requirements.
> +            match unsafe { init.__init(ptr) } {
> +                Ok(()) => {}
> +                Err(e) => {
> +                    // We now free every element that has been initialized before:
> +                    for j in 0..i {
> +                        let ptr = unsafe { slot.add(j) };
> +                        // SAFETY: The value was initialized in a previous iteration of the loop
> +                        // and since we return `Err` below, the caller will consider the memory at
> +                        // `slot` as uninitialized.
> +                        unsafe { ptr::drop_in_place(ptr) };
> +                    }

The loop can be simplified like this:

ptr::drop_in_place(ptr::slice_from_raw_parts(slot, i));

Yes, this actually works and will run the destructor of each value.

Alice