[PATCH 1/3] rust: alloc: add Vec::truncate method

Andrew Ballance
2025-03-15T02:43:21Z

implements the equivalent to the std's Vec::truncate
on the kernel's Vec type.

Signed-off-by: Andrew Ballance <andrewjballance@gmail.com>
---
rust/kernel/alloc/kvec.rs | 36 ++++++++++++++++++++++++++++++++++++
1 file changed, 36 insertions(+)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index ae9d072741ce..75e9feebb81f 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -452,6 +452,42 @@ pub fn reserve(&mut self, additional: usize, flags: Flags) -> Result<(), AllocEr

Ok(())
}
+
+ /// Shortens the vector, setting the length to `len` and drops the removed values.
+ /// If `len` is greater than or equal to the current length, this does nothing.
+ ///
+ /// This has no effect on the capacity and will not allocate.
+ /// # Examples
+ /// ```
+ /// let mut v = kernel::kvec![1, 2, 3]?;
+ /// v.truncate(1);
+ /// assert_eq!(v.len(), 1);
+ /// assert_eq!(&v, &[1]);
+ ///
+ /// # Ok::<(), Error>(())
+ /// ```
+ pub fn truncate(&mut self, len: usize) {
+ if len >= self.len() {
+ return;
+ }
+
+ // [new_len, len) is guaranteed to be valid because [0, len) is guaranteed to be valid
+ let drop_range = len..self.len();
+
+ // SAFETY:
+ // we can safely ignore the bounds check because we already did our own check
+ let ptr: *mut [T] = unsafe { self.get_unchecked_mut(drop_range) };
+
+ // SAFETY:
+ // it is safe to shrink the length because the new length is
+ // guaranteed to be less than the old length
+ unsafe { self.set_len(len) };
+
+ // SAFETY:
+ // - the dropped values are valid `T`s
+ // - we are allowed to invalidate [new_len, old_len) because we just changed the len
+ unsafe { ptr::drop_in_place(ptr) };
+ }
}

impl<T: Clone, A: Allocator> Vec<T, A> {
--
2.48.1