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

Benno Lossin
2025-03-15T10:09:34Z

On Sat Mar 15, 2025 at 3:42 AM CET, Andrew Ballance wrote:
> 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) };

What's this `get_unchecked_mut` method, I don't see it in `rust-next` or
`alloc-next`.

> +
> + // SAFETY:
> + // it is safe to shrink the length because the new length is
> + // guaranteed to be less than the old length

Please take a look at the documentation of `set_len`, in the safety
section you'll find what you need to justify here.

> + unsafe { self.set_len(len) };
> +
> + // SAFETY:

A couple points missing:
- why is the pointer valid?

> + // - the dropped values are valid `T`s
> + // - we are allowed to invalidate [new_len, old_len) because we just changed the len

This should justify why the value will never be accessed again.

---
Cheers,
Benno

> + unsafe { ptr::drop_in_place(ptr) };
> + }
> }
>
> impl<T: Clone, A: Allocator> Vec<T, A> {