[PATCH v2] rust: alloc: allow different error types in `KBox::pin_slice`

[Andreas Hindborg <a.hindborg@kernel.org>]

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.

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>
---
Changes in v2:
- Use `E: From<E2>` (and keep `E: From<AllocError>`) instead of `Into<E>`
  bounds, which allows the body to use `?` instead of explicit `match`
  blocks (Danilo).
- Add Benno's Co-developed-by and Signed-off-by tags (Benno).
- Link to v1: https://msgid.link/20260214-pin-slice-init-v1-1-0b174fbb1844@kernel.org

To: Danilo Krummrich <dakr@kernel.org>
To: Lorenzo Stoakes <ljs@kernel.org>
To: Vlastimil Babka <vbabka@kernel.org>
To: "Liam R. Howlett" <liam@infradead.org>
To: Uladzislau Rezki <urezki@gmail.com>
To: Miguel Ojeda <ojeda@kernel.org>
To: Boqun Feng <boqun@kernel.org>
To: Gary Guo <gary@garyguo.net>
To: Björn Roy Baron <bjorn3_gh@protonmail.com>
To: Benno Lossin <lossin@kernel.org>
To: Andreas Hindborg <a.hindborg@kernel.org>
To: Alice Ryhl <aliceryhl@google.com>
To: Trevor Gross <tmgross@umich.edu>
Cc: rust-for-linux@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 rust/kernel/alloc/kbox.rs | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/rust/kernel/alloc/kbox.rs b/rust/kernel/alloc/kbox.rs
index bd6da02c7ab8..c2e89e523eae 100644
--- a/rust/kernel/alloc/kbox.rs
+++ b/rust/kernel/alloc/kbox.rs
@@ -298,7 +298,7 @@ pub fn pin(x: T, flags: Flags) -> Result<Pin<Box<T, A>>, AllocError>
     /// }
     ///
     /// // 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
@@ -308,15 +308,16 @@ pub fn pin(x: T, flags: Flags) -> Result<Pin<Box<T, A>>, AllocError>
     /// 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 {

---
base-commit: 7fd2df204f342fc17d1a0bfcd474b24232fb0f32
change-id: 20260214-pin-slice-init-e8ef96fc07b9

Best regards,
--  
Andreas Hindborg <a.hindborg@kernel.org>