rust: alloc: implement IntoIterator for Vec

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
correspondingly 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.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20241004154149.93856-18-dakr@kernel.org
[ Fixed typos. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: Danilo Krummrich

rust/kernel/alloc.rs

@@ -20,6 +20,7 @@ pub use self::kbox::KBox;
 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;

rust/kernel/alloc/kvec.rs

@@ -647,3 +647,173 @@ impl_slice_eq! {
     [A: Allocator, const N: usize] Vec<T, A>, [U; N],
     [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`] that moves elements out of a vector.
+///
+/// This structure is created by the [`Vec::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,
+    layout: ArrayLayout<T>,
+    _p: PhantomData<A>,
+}
+
+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 current = self.ptr;
+
+        // SAFETY: We can't overflow; decreasing `self.len` by one every time we advance `self.ptr`
+        // by one guarantees that.
+        unsafe { self.ptr = self.ptr.add(1) };
+
+        self.len -= 1;
+
+        // SAFETY: `current` is guaranteed to point at a valid element within the buffer.
+        Some(unsafe { current.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: `self.ptr` is guaranteed to be valid by the type invariant.
+        unsafe { ptr::drop_in_place(ptr::slice_from_raw_parts_mut(self.ptr, self.len)) };
+
+        // SAFETY:
+        // - `self.buf` was previously allocated with `A`.
+        // - `self.layout` matches the `ArrayLayout` of the preceding allocation.
+        unsafe { A::free(self.buf.cast(), self.layout.into()) };
+    }
+}
+
+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>(())
+    /// ```
+    #[inline]
+    fn into_iter(self) -> Self::IntoIter {
+        let buf = self.ptr;
+        let layout = self.layout;
+        let (ptr, len, _) = self.into_raw_parts();
+
+        IntoIter {
+            ptr,
+            buf,
+            len,
+            layout,
+            _p: PhantomData::<A>,
+        }
+    }
+}