Re: [PATCH 4/5] rust: alloc: add Vec::drain_all

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

On Thu Mar 20, 2025 at 2:52 PM CET, Alice Ryhl wrote:
> This is like the stdlib method drain, except that it's hard-coded to use
> the entire vector's range. Rust Binder uses it in the range allocator to
> take ownership of everything in a vector in a case where reusing the
> vector is desirable.

Is the reason for not implementing `drain` complexity?

> Implementing `DrainAll` in terms of `slice::IterMut` lets us reuse some
> nice optimizations in core for the case where T is a ZST.
>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>

The code is good, but I'd like to know the answer to the above question
before giving my RB.

> ---
>  rust/kernel/alloc/kvec.rs | 57 +++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 57 insertions(+)
>
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> index df930ff0d0b85b8b03c9b7932a2b31dfb62612ed..303198509885f5e24b74da5a92382b518de3e1c0 100644
> --- a/rust/kernel/alloc/kvec.rs
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -564,6 +564,30 @@ pub fn truncate(&mut self, len: usize) {
>          //   len, therefore we have exclusive access to [`new_len`, `old_len`)
>          unsafe { ptr::drop_in_place(ptr) };
>      }
> +
> +    /// Takes ownership of all items in this vector without consuming the allocation.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// let mut v = kernel::kvec![0, 1, 2, 3]?;
> +    ///
> +    /// for (i, j) in v.drain_all().enumerate() {
> +    ///     assert_eq!(i, j);
> +    /// }
> +    ///
> +    /// assert!(v.capacity() >= 4);
> +    /// ```
> +    pub fn drain_all(&mut self) -> DrainAll<'_, T> {
> +        let len = self.len();
> +        // INVARIANT: The first 0 elements are valid.
> +        self.len = 0;

Why not `set_len`?

> +        // INVARIANT: The first `len` elements of the spare capacity are valid values, and as we
> +        // just set the length to zero, we may transfer ownership to the `DrainAll` object.
> +        DrainAll {
> +            elements: self.spare_capacity_mut()[..len].iter_mut(),
> +        }
> +    }
>  }
>  
>  impl<T: Clone, A: Allocator> Vec<T, A> {
> @@ -1049,3 +1073,36 @@ fn into_iter(self) -> Self::IntoIter {
>          }
>      }
>  }
> +
> +/// An iterator that owns all items in a vector, but does not own its allocation.
> +///
> +/// # Invariants
> +///
> +/// Every `&mut MaybeUninit<T>` returned by the iterator contains a valid `T` owned by this
> +/// `DrainAll`.
> +pub struct DrainAll<'vec, T> {
> +    elements: slice::IterMut<'vec, MaybeUninit<T>>,
> +}
> +
> +impl<'vec, T> Iterator for DrainAll<'vec, T> {
> +    type Item = T;
> +
> +    fn next(&mut self) -> Option<T> {
> +        let elem = self.elements.next()?;
> +        // SAFETY: By the type invariants, we may take ownership of the value in this
> +        // `MaybeUninit<T>`.
> +        Some(unsafe { elem.assume_init_read() })
> +    }
> +
> +    fn size_hint(&self) -> (usize, Option<usize>) {
> +        self.elements.size_hint()
> +    }
> +}
> +
> +impl<'vec, T> Drop for DrainAll<'vec, T> {
> +    fn drop(&mut self) {
> +        if core::mem::needs_drop::<T>() {

This is neat!

---
Cheers,
Benno

> +            while self.next().is_some() {}
> +        }
> +    }
> +}