[PATCH v3] rust: alloc: use `spare_capacity_mut` to reduce unsafe

[Tamir Duberstein]

Use `spare_capacity_mut` in the implementation of `push` to reduce the
use of `unsafe`. Both methods were added in commit 2aac4cd7dae3 ("rust:
alloc: implement kernel `Vec` type").

Reviewed-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
Reviewed-by: Benno Lossin <benno.lossin@xxxxxxxxx>
Signed-off-by: Tamir Duberstein <tamird@xxxxxxxxx>
---
Changes in v3:
- Clarify unsafe operation by factoring out safe call. (Benno Lossin)
- Link to v2: https://lore.kernel.org/r/20250317-vec-push-use-spare-v2-1-5dc6583f732c@xxxxxxxxx

Changes in v2:
- Use `slice::get_unchecked_mut` to ensure we avoid a bounds check.
  (Benno Lossin)
- Link to v1: https://lore.kernel.org/r/20250317-vec-push-use-spare-v1-1-7e025ef4ae14@xxxxxxxxx
---
 rust/kernel/alloc/kvec.rs | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index ae9d072741ce..f5b1f201a80a 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -285,15 +285,10 @@ pub fn spare_capacity_mut(&mut self) -> &mut [MaybeUninit<T>] {
     pub fn push(&mut self, v: T, flags: Flags) -> Result<(), AllocError> {
         self.reserve(1, flags)?;
 
-        // SAFETY:
-        // - `self.len` is smaller than `self.capacity` and hence, the resulting pointer is
-        //   guaranteed to be part of the same allocated object.
-        // - `self.len` can not overflow `isize`.
-        let ptr = unsafe { self.as_mut_ptr().add(self.len) };
+        let spare = self.spare_capacity_mut();
 
-        // SAFETY:
-        // - `ptr` is properly aligned and valid for writes.
-        unsafe { core::ptr::write(ptr, v) };
+        // SAFETY: The call to `reserve` was successful so the spare capacity is at least 1.
+        unsafe { spare.get_unchecked_mut(0) }.write(v);
 
         // SAFETY: We just initialised the first spare entry, so it is safe to increase the length
         // by 1. We also know that the new length is <= capacity because of the previous call to

---
base-commit: cf25bc61f8aecad9b0c45fe32697e35ea4b13378
change-id: 20250317-vec-push-use-spare-27484fd016a9

Best regards,
-- 
Tamir Duberstein <tamird@xxxxxxxxx>