[PATCH v4 3/4] rust: alloc: refactor `Vec::truncate` using `dec_len`

Tamir Duberstein
2025-04-16T17:15:54Z

Use `checked_sub` to satisfy the safety requirements of `dec_len` and
replace nearly the whole body of `truncate` with a call to `dec_len`.

Reviewed-by: Andrew Ballance <andrewjballance@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
---
rust/kernel/alloc/kvec.rs | 24 ++++++++----------------
1 file changed, 8 insertions(+), 16 deletions(-)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index a84a907acae4..87dc37ecb94d 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -488,23 +488,15 @@ pub fn reserve(&mut self, additional: usize, flags: Flags) -> Result<(), AllocEr
/// # Ok::<(), Error>(())
/// ```
pub fn truncate(&mut self, len: usize) {
- if len >= self.len() {
- return;
+ if let Some(count) = self.len().checked_sub(len) {
+ // SAFETY: `count` is `self.len() - len` so it is guaranteed to be less than or
+ // equal to `self.len()`.
+ let ptr: *mut [T] = unsafe { self.dec_len(count) };
+
+ // SAFETY: the contract of `dec_len` guarantees that the elements in `ptr` are
+ // valid elements whose ownership has been transferred to the caller.
+ unsafe { ptr::drop_in_place(ptr) };
}
-
- let drop_range = len..self.len();
-
- // SAFETY: `drop_range` is a subrange of `[0, len)` by the bounds check above.
- let ptr: *mut [T] = unsafe { self.get_unchecked_mut(drop_range) };
-
- // SAFETY: By the above bounds check, it is guaranteed that `len < self.capacity()`.
- unsafe { self.set_len(len) };
-
- // SAFETY:
- // - the dropped values are valid `T`s by the type invariant
- // - we are allowed to invalidate [`new_len`, `old_len`) because we just changed the
- // len, therefore we have exclusive access to [`new_len`, `old_len`)
- unsafe { ptr::drop_in_place(ptr) };
}
}

--
2.49.0