[PATCH v4 2/4] rust: alloc: add `Vec::dec_len`

Tamir Duberstein
2025-04-16T17:15:53Z

Add `Vec::dec_len` that reduces the length of the receiver. This method
is intended to be used from methods that remove elements from `Vec` such
as `truncate`, `pop`, `remove`, and others. This method is intentionally
not `pub`.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
rust/kernel/alloc/kvec.rs | 19 +++++++++++++++++++
1 file changed, 19 insertions(+)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index ca30fad90de5..a84a907acae4 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -201,6 +201,25 @@ pub unsafe fn set_len(&mut self, new_len: usize) {
self.len = new_len;
}

+ /// Decreases `self.len` by `count`.
+ ///
+ /// Returns a mutable slice to the elements forgotten by the vector. It is the caller's
+ /// responsibility to drop these elements if necessary.
+ ///
+ /// # Safety
+ ///
+ /// - `count` must be less than or equal to `self.len`.
+ unsafe fn dec_len(&mut self, count: usize) -> &mut [T] {
+ debug_assert!(count <= self.len());
+ // INVARIANT: We relinquish ownership of the elements within the range `[self.len - count,
+ // self.len)`, hence the updated value of `set.len` represents the exact number of elements
+ // stored within `self`.
+ self.len -= count;
+ // SAFETY: The memory after `self.len()` is guaranteed to contain `count` initialized
+ // elements of type `T`.
+ unsafe { slice::from_raw_parts_mut(self.as_mut_ptr().add(self.len), count) }
+ }
+
/// Returns a slice of the entire vector.
#[inline]
pub fn as_slice(&self) -> &[T] {

--
2.49.0