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

[Danilo Krummrich <dakr@kernel.org>]

On Tue, Sep 10, 2024 at 08:04:27PM +0000, Benno Lossin wrote:
> 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?

Suggestions?

> 
> > +    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.

They don't, but the KUnit tests are still executed. :)

> 
> > +    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...

No worries, I really only added them for the KUnit tests.

> 
> ---
> 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
> > 
>