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

[Benno Lossin <benno.lossin@proton.me>]

On Sun Mar 16, 2025 at 11:32 PM CET, Tamir Duberstein wrote:
> 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`.

I think it should be `pub`. Otherwise we're loosing functionality
compared to now. If one decides to give the raw pointer to some C API
that takes ownership of the pointer, then I want them to be able to call
`dec_len` manually. 

>
> Signed-off-by: Tamir Duberstein <tamird@gmail.com>
> ---
>  rust/kernel/alloc/kvec.rs | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
>
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> index d43a1d609434..5d604e04b9a5 100644
> --- a/rust/kernel/alloc/kvec.rs
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -195,6 +195,21 @@ pub unsafe fn inc_len(&mut self, additional: usize) {
>          self.len += additional;
>      }
>  
> +    /// Decreases `self.len` by `count`.
> +    ///
> +    /// Returns a mutable reference to the removed elements.

s/reference/slice/

I would also mention here that the elements won't be dropped when the
user doesn't do that manually using the slice. So explain that this is a
low-level operation and `clear` or `truncate` should be used instead
where possible.

> +    ///
> +    /// # Safety
> +    ///
> +    /// - `count` must be less than or equal to `self.len`.

I also think that we should use saturating_sub instead and then not have
to worry about this. (It should still be documented in the function
though). That way this can also be a safe function.

---
Cheers,
Benno

> +    unsafe fn dec_len(&mut self, count: usize) -> &mut [T] {
> +        debug_assert!(count <= self.len());
> +        self.len -= count;
> +        // SAFETY: The memory between `self.len` and `self.len + count` is guaranteed to contain
> +        // `self.len` 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] {