Re: [PATCH v6 14/26] rust: alloc: implement `IntoIterator` for `Vec`

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

On 16.08.24 02:10, Danilo Krummrich wrote:
> Implement `IntoIterator` for `Vec`, `Vec`'s `IntoIter` type, as well as
> `Iterator` for `IntoIter`.
> 
> `Vec::into_iter` disassembles the `Vec` into its raw parts; additionally,
> `IntoIter` keeps track of a separate pointer, which is incremented
> correspondingsly as the iterator advances, while the length, or the count
> of elements, is decremented.
> 
> This also means that `IntoIter` takes the ownership of the backing
> buffer and is responsible to drop the remaining elements and free the
> backing buffer, if it's dropped.
> 
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  rust/kernel/alloc.rs      |   1 +
>  rust/kernel/alloc/kvec.rs | 184 ++++++++++++++++++++++++++++++++++++++
>  2 files changed, 185 insertions(+)
> 
> diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
> index e88c7e10ee9b..4ff4df4597a3 100644
> --- a/rust/kernel/alloc.rs
> +++ b/rust/kernel/alloc.rs
> @@ -19,6 +19,7 @@
>  pub use self::kbox::KVBox;
>  pub use self::kbox::VBox;
> 
> +pub use self::kvec::IntoIter;
>  pub use self::kvec::KVVec;
>  pub use self::kvec::KVec;
>  pub use self::kvec::VVec;
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> index 89afc0f25bd4..3b79f977b65e 100644
> --- a/rust/kernel/alloc/kvec.rs
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -11,6 +11,7 @@
>      ops::DerefMut,
>      ops::Index,
>      ops::IndexMut,
> +    ptr,
>      ptr::NonNull,
>      slice,
>      slice::SliceIndex,
> @@ -627,3 +628,186 @@ fn eq(&self, other: &$rhs) -> bool { self[..] == other[..] }
>  __impl_slice_eq! { [A: Allocator] [T], Vec<U, A> }
>  __impl_slice_eq! { [A: Allocator, const N: usize] Vec<T, A>, [U; N] }
>  __impl_slice_eq! { [A: Allocator, const N: usize] Vec<T, A>, &[U; N] }
> +
> +impl<'a, T, A> IntoIterator for &'a Vec<T, A>
> +where
> +    A: Allocator,
> +{
> +    type Item = &'a T;
> +    type IntoIter = slice::Iter<'a, T>;
> +
> +    fn into_iter(self) -> Self::IntoIter {
> +        self.iter()
> +    }
> +}
> +
> +impl<'a, T, A: Allocator> IntoIterator for &'a mut Vec<T, A>
> +where
> +    A: Allocator,
> +{
> +    type Item = &'a mut T;
> +    type IntoIter = slice::IterMut<'a, T>;
> +
> +    fn into_iter(self) -> Self::IntoIter {
> +        self.iter_mut()
> +    }
> +}
> +
> +/// An `Iterator` implementation for `Vec<T,A>` that moves elements out of a vector.

Please make both links.

> +///
> +/// This structure is created by the `Vec::into_iter` method on [`Vec`] (provided by the

Ditto.

> +/// [`IntoIterator`] trait).
> +///
> +/// # Examples
> +///
> +/// ```
> +/// let v = kernel::kvec![0, 1, 2]?;
> +/// let iter = v.into_iter();
> +///
> +/// # Ok::<(), Error>(())
> +/// ```
> +pub struct IntoIter<T, A: Allocator> {
> +    ptr: *mut T,
> +    buf: NonNull<T>,

No invariants for these two fields?

> +    len: usize,
> +    cap: usize,
> +    _p: PhantomData<A>,
> +}
> +
> +impl<T, A> IntoIter<T, A>
> +where
> +    A: Allocator,
> +{
> +    fn as_raw_mut_slice(&mut self) -> *mut [T] {
> +        ptr::slice_from_raw_parts_mut(self.ptr, self.len)
> +    }
> +}
> +
> +impl<T, A> Iterator for IntoIter<T, A>
> +where
> +    A: Allocator,
> +{
> +    type Item = T;
> +
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// let v = kernel::kvec![1, 2, 3]?;
> +    /// let mut it = v.into_iter();
> +    ///
> +    /// assert_eq!(it.next(), Some(1));
> +    /// assert_eq!(it.next(), Some(2));
> +    /// assert_eq!(it.next(), Some(3));
> +    /// assert_eq!(it.next(), None);
> +    ///
> +    /// # Ok::<(), Error>(())
> +    /// ```

AFAIK documentation on functions in trait implementations won't show up
in rustdoc (I just checked this). So I would remove it.

> +    fn next(&mut self) -> Option<T> {
> +        if self.len == 0 {
> +            return None;
> +        }
> +
> +        let ptr = self.ptr;
> +        if !Vec::<T, A>::is_zst() {
> +            // SAFETY: We can't overflow; `end` is guaranteed to mark the end of the buffer.
> +            unsafe { self.ptr = self.ptr.add(1) };
> +        } else {
> +            // For ZST `ptr` has to stay where it is to remain aligned, so we just reduce `self.len`
> +            // by 1.

Note that `<*mut T>::add` advances the pointer by `size_of::<T>()`
bytes. So in the case that `T` is a ZST, it won't be advanced.
So you could remove this `if`.

> +        }
> +        self.len -= 1;
> +
> +        // SAFETY: `ptr` is guaranteed to point at a valid element within the buffer.
> +        Some(unsafe { ptr.read() })
> +    }
> +
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// let v: KVec<u32> = kernel::kvec![1, 2, 3]?;
> +    /// let mut iter = v.into_iter();
> +    /// let size = iter.size_hint().0;
> +    ///
> +    /// iter.next();
> +    /// assert_eq!(iter.size_hint().0, size - 1);
> +    ///
> +    /// iter.next();
> +    /// assert_eq!(iter.size_hint().0, size - 2);
> +    ///
> +    /// iter.next();
> +    /// assert_eq!(iter.size_hint().0, size - 3);
> +    ///
> +    /// # Ok::<(), Error>(())
> +    /// ```
> +    fn size_hint(&self) -> (usize, Option<usize>) {
> +        (self.len, Some(self.len))
> +    }
> +}
> +
> +impl<T, A> Drop for IntoIter<T, A>
> +where
> +    A: Allocator,
> +{
> +    fn drop(&mut self) {
> +        // SAFETY: Drop the remaining vector's elements in place, before we free the backing
> +        // memory.

This comment explains why you are doing it, not why it's ok to do it.

> +        unsafe { ptr::drop_in_place(self.as_raw_mut_slice()) };
> +
> +        // If `cap == 0` we never allocated any memory in the first place.
> +        if self.cap != 0 {
> +            // SAFETY: `self.buf` was previously allocated with `A`.
> +            unsafe { A::free(self.buf.cast()) };
> +        }
> +    }
> +}
> +
> +impl<T, A> IntoIterator for Vec<T, A>
> +where
> +    A: Allocator,
> +{
> +    type Item = T;
> +    type IntoIter = IntoIter<T, A>;
> +
> +    /// Consumes the `Vec<T, A>` and creates an `Iterator`, which moves each value out of the
> +    /// vector (from start to end).
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// let v = kernel::kvec![1, 2]?;
> +    /// let mut v_iter = v.into_iter();
> +    ///
> +    /// let first_element: Option<u32> = v_iter.next();
> +    ///
> +    /// assert_eq!(first_element, Some(1));
> +    /// assert_eq!(v_iter.next(), Some(2));
> +    /// assert_eq!(v_iter.next(), None);
> +    ///
> +    /// # Ok::<(), Error>(())
> +    /// ```
> +    ///
> +    /// ```
> +    /// let v = kernel::kvec![];
> +    /// let mut v_iter = v.into_iter();
> +    ///
> +    /// let first_element: Option<u32> = v_iter.next();
> +    ///
> +    /// assert_eq!(first_element, None);
> +    ///
> +    /// # Ok::<(), Error>(())
> +    /// ```

I feel a bit bad that you wrote all of this nice documentation for
functions that receive their documentation from the trait...

---
Cheers,
Benno

> +    #[inline]
> +    fn into_iter(self) -> Self::IntoIter {
> +        let (ptr, len, cap) = self.into_raw_parts();
> +
> +        IntoIter {
> +            ptr,
> +            // SAFETY: `ptr` is either a dangling pointer or a pointer to a valid memory
> +            // allocation, allocated with `A`.
> +            buf: unsafe { NonNull::new_unchecked(ptr) },
> +            len,
> +            cap,
> +            _p: PhantomData::<A>,
> +        }
> +    }
> +}
> --
> 2.46.0
>