rust: alloc: allow different error types in `KBox::pin_slice`

Previously, `KBox::pin_slice` required the initializer error type to match
the return error type via `E: From<AllocError>`. This prevented using
infallible initializers like `new_mutex!` inside `pin_slice`, because
`Infallible` does not implement `From<AllocError>`.

Introduce a separate type parameter `E2` for the initializer error type
and require `E: From<AllocError>` and `E: From<E2>` instead. This allows
the initializer to return a different error type that can be converted
into the final error type, enabling use of infallible pin initializers
in fallible allocation contexts.

Link: https://rust-for-linux.zulipchat.com/#narrow/channel/288089-General/topic/.E2.9C.94.20Constructing.20Mutex.20from.20PinInit.3CT.2C.20Error.3E
Co-developed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://patch.msgid.link/20260605-pin-slice-init-v2-1-b197e84040ba@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

1 files changed, 4 insertions, 3 deletions

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index c63d6acdbb6f..2c929eb1c3d6 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -345,7 +345,7 @@ where
     /// }
     ///
     /// // Allocate a boxed slice of 10 `Example`s.
-    /// let s = KBox::pin_slice(
+    /// let s = KBox::pin_slice::<_, _, Error, _>(
     ///     | _i | Example::new(),
     ///     10,
     ///     GFP_KERNEL
@@ -355,15 +355,16 @@ where
     /// assert_eq!(s[3].d.lock().a, 20);
     /// # Ok::<(), Error>(())
     /// ```
-    pub fn pin_slice<Func, Item, E>(
+    pub fn pin_slice<Func, Item, E, E2>(
         mut init: Func,
         len: usize,
         flags: Flags,
     ) -> Result<Pin<Box<[T], A>>, E>
     where
         Func: FnMut(usize) -> Item,
-        Item: PinInit<T, E>,
+        Item: PinInit<T, E2>,
         E: From<AllocError>,
+        E: From<E2>,
     {
         let mut buffer = super::Vec::<T, A>::with_capacity(len, flags)?;
         for i in 0..len {