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

[Björn Roy Baron]

On Saturday, June 24th, 2023 at 11:25, Benno Lossin <benno.lossin@xxxxxxxxx> wrote:

> 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@xxxxxxxxxxxxx>
> Signed-off-by: Benno Lossin <benno.lossin@xxxxxxxxx>

Reviewed-by: Björn Roy Baron <bjorn3_gh@xxxxxxxxxxxxxx>

> ---
>  rust/kernel/init.rs | 90 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 90 insertions(+)
> 
> diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
> index 44bc3e77419a..c9ea4bf71987 100644
> --- a/rust/kernel/init.rs
> +++ b/rust/kernel/init.rs
> @@ -867,6 +867,96 @@ pub fn uninit<T, E>() -> impl Init<MaybeUninit<T>, E> {
>      unsafe { init_from_closure(|_| Ok(())) }
>  }
> 
> +/// 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) };
> +                    }
> +                    return Err(e);
> +                }
> +            }
> +        }
> +        Ok(())
> +    };
> +    // SAFETY: The initializer above initializes every element of the array. On failure it drops
> +    // any initialized elements and returns `Err`.
> +    unsafe { init_from_closure(init) }
> +}
> +
> +/// Initializes an array by initializing each element via the provided initializer.
> +///
> +/// # Examples
> +///
> +/// ```rust
> +/// let array: Arc<[Mutex<usize>; 1000_000_000]>=
> +///     Arc::pin_init(init_array_from_fn(|i| new_mutex!(i))).unwrap();
> +/// println!("{array:?}");
> +/// ```
> +pub fn pin_init_array_from_fn<I, const N: usize, T, E>(
> +    mut make_init: impl FnMut(usize) -> I,
> +) -> impl PinInit<[T; N], E>
> +where
> +    I: PinInit<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 `__pinned_init`
> +            // requirements.
> +            match unsafe { init.__pinned_init(ptr) } {
> +                Ok(()) => {}
> +                Err(e) => {
> +                    // We now have to free every element that has been initialized before, since we
> +                    // have to abide by the drop guarantee.
> +                    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) };
> +                    }
> +                    return Err(e);
> +                }
> +            }
> +        }
> +        Ok(())
> +    };
> +    // SAFETY: The initializer above initializes every element of the array. On failure it drops
> +    // any initialized elements and returns `Err`.
> +    unsafe { pin_init_from_closure(init) }
> +}
> +
>  // SAFETY: Every type can be initialized by-value.
>  unsafe impl<T, E> Init<T, E> for T {
>      unsafe fn __init(self, slot: *mut T) -> Result<(), E> {
> --
> 2.41.0