Re: [PATCH v3 16/25] rust: alloc: implement `IntoIterator` for `Vec`

[Danilo Krummrich <dakr@kernel.org>]

On Thu, Aug 01, 2024 at 05:07:48PM +0200, Alice Ryhl wrote:
> On Thu, Aug 1, 2024 at 2:08 AM Danilo Krummrich <dakr@kernel.org> 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 | 186 ++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 187 insertions(+)
> >
> > diff --git a/rust/kernel/alloc.rs b/rust/kernel/alloc.rs
> > index bd93140f3094..f2998ad57456 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 04cc85f7d92c..50e7705e5686 100644
> > --- a/rust/kernel/alloc/kvec.rs
> > +++ b/rust/kernel/alloc/kvec.rs
> > @@ -12,6 +12,8 @@
> >      ops::DerefMut,
> >      ops::Index,
> >      ops::IndexMut,
> > +    ptr,
> > +    ptr::NonNull,
> >      slice,
> >      slice::SliceIndex,
> >  };
> > @@ -581,3 +583,187 @@ 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 that moves out of a vector.
> > +///
> > +/// This `struct` is created by the `into_iter` method on [`Vec`] (provided by the [`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>,
> > +    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>(())
> > +    /// ```
> > +    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.
> > +        }
> > +        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.
> > +        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()) };
> > +        }
> 
> Is this ok for ZST?

Yes, for ZST `self.cap` is always zero.

> 
> > +    }
> > +}
> > +
> > +impl<T, A> IntoIterator for Vec<T, A>
> > +where
> > +    A: Allocator,
> > +{
> > +    type Item = T;
> > +    type IntoIter = IntoIter<T, A>;
> > +
> > +    /// Creates a consuming iterator, that is, one that moves each value out of
> > +    /// the vector (from start to end). The vector cannot be used after calling
> > +    /// this.
> > +    ///
> > +    /// # 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>(())
> > +    /// ```
> > +    #[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>,
> > +        }
> > +    }
> > +}
> > --
> > 2.45.2
> >
>