[PATCH v2 07/12] rust: init: Add functions to create array initializers

[Benno Lossin <benno.lossin@proton.me>]

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>
Reviewed-by: Björn Roy Baron <bjorn3_gh@protonmail.com>
Signed-off-by: Benno Lossin <benno.lossin@proton.me>
---
v1 -> v2:
* fix warnings and errors in doctests
* replace dropping loop with `drop_in_place` and `slice_from_raw_parts_mut`
  inside of `{pin_}init_array_from_fn` functions

 rust/kernel/init.rs | 86 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 86 insertions(+)

diff --git a/rust/kernel/init.rs b/rust/kernel/init.rs
index 460f808ebf84..fa1ebdbf5f4b 100644
--- a/rust/kernel/init.rs
+++ b/rust/kernel/init.rs
@@ -875,6 +875,92 @@ 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
+/// use kernel::{error::Error, init::init_array_from_fn};
+/// let array: Box<[usize; 1_000_000_000]>= Box::init::<Error>(init_array_from_fn(|i| i)).unwrap();
+/// pr_info!("{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:
+                    // SAFETY: The loop initialized exactly the values from 0..i and since we
+                    // return `Err` below, the caller will consider the memory at `slot` as
+                    // uninitialized.
+                    unsafe { ptr::drop_in_place(ptr::slice_from_raw_parts_mut(slot, i)) };
+                    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
+/// use kernel::{sync::{Arc, Mutex}, init::pin_init_array_from_fn, new_mutex};
+/// let array: Arc<[Mutex<usize>; 1_000_000_000]>=
+///     Arc::pin_init(pin_init_array_from_fn(|i| new_mutex!(i))).unwrap();
+/// pr_info!("{}", array.len());
+/// ```
+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.
+                    // SAFETY: The loop initialized exactly the values from 0..i and since we
+                    // return `Err` below, the caller will consider the memory at `slot` as
+                    // uninitialized.
+                    unsafe { ptr::drop_in_place(ptr::slice_from_raw_parts_mut(slot, i)) };
+                    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