[PATCH] rust: alloc: implement Box::pin_slice()

[Vitaly Wool]

From: Alice Ryhl <aliceryhl@xxxxxxxxxx>

Add a new constructor to Box to facilitate Box creation from a pinned
slice of elements. This allows to efficiently allocate memory for e.g.
slices of structrures containing spinlocks or mutexes. Such slices may
be used in kmemcache like or zpool API implementations.

Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Signed-off-by: Vitaly Wool <vitaly.wool@xxxxxxxxxxx>
---

This patch supersedes "rust: extend kbox with a new constructor" posted
a day earlier.

 rust/kernel/alloc/kbox.rs | 61 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index 1fef9beb57c8..f0be307f7242 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -290,6 +290,67 @@ pub fn pin(x: T, flags: Flags) -> Result<Pin<Box<T, A>>, AllocError>
         Ok(Self::new(x, flags)?.into())
     }
 
+    /// Construct a pinned slice of elements `Pin<Box<[T], A>>`.
+    ///
+    /// This is a convenient means for creation of e.g. slices of structrures containing spinlocks
+    /// or mutexes.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #[pin_data]
+    /// struct Example {
+    ///     c: u32,
+    ///     #[pin]
+    ///     d: SpinLock<Inner>,
+    /// }
+    ///
+    /// impl Example {
+    ///     fn new() -> impl PinInit<Self> {
+    ///         pin_init!(Self {
+    ///             c: 10,
+    ///             d <- new_spinlock!(Inner { a: 20, b: 30 }),
+    ///         })
+    ///     }
+    /// }
+    /// // Allocate a boxed slice of 10 `Example`s.
+    /// let s = KBox::pin_slice(
+    ///     | _i | Example::new(),
+    ///     10,
+    ///     GFP_KERNEL
+    /// )?;
+    /// assert_eq!(s[5].c, 10);
+    /// assert_eq!(s[3].d.lock().a, 20),
+    /// ```
+    pub fn pin_slice<Func, Item, E>(
+        mut init: Func,
+        len: usize,
+        flags: Flags,
+    ) -> Result<Pin<Box<[T], A>>, E>
+    where
+        Func: FnMut(usize) -> Item,
+        Item: PinInit<T, E>,
+        E: From<AllocError>,
+    {
+        let mut buffer = super::Vec::<T, A>::with_capacity(len, flags)?;
+        for i in 0..len {
+            let ptr = buffer.spare_capacity_mut().as_mut_ptr().cast();
+            // SAFETY:
+            // - `ptr` is a valid pointer to uninitialized memory.
+            // - `ptr` is not used if an error is returned.
+            // - `ptr` won't be moved until it is dropped, i.e. it is pinned.
+            unsafe { init(i).__pinned_init(ptr)? };
+            // SAFETY:
+            // - `i + 1 <= len` => we don't exceed the capacity
+            // - this new value is initialized
+            unsafe { buffer.inc_len(1) };
+        }
+        let (ptr, _, _) = buffer.into_raw_parts();
+        let slice = core::ptr::slice_from_raw_parts_mut(ptr, len);
+        // SAFETY: `slice` is not a NULL pointer because it is a valid pointer to [T]
+        Ok(Pin::from(unsafe { Box::from_raw(slice) }))
+    }
+
     /// Convert a [`Box<T,A>`] to a [`Pin<Box<T,A>>`]. If `T` does not implement
     /// [`Unpin`], then `x` will be pinned in memory and can't be moved.
     pub fn into_pin(this: Self) -> Pin<Self> {
-- 
2.39.2