Re: [PATCH v7 15/26] rust: alloc: implement `collect` for `IntoIter`

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

On 12.09.24 00:52, Danilo Krummrich wrote:
> Currently, we can't implement `FromIterator`. There are a couple of
> issues with this trait in the kernel, namely:
> 
>   - Rust's specialization feature is unstable. This prevents us to
>     optimze for the special case where `I::IntoIter` equals `Vec`'s
>     `IntoIter` type.
>   - We also can't use `I::IntoIter`'s type ID either to work around this,
>     since `FromIterator` doesn't require this type to be `'static`.
>   - `FromIterator::from_iter` does return `Self` instead of
>     `Result<Self, AllocError>`, hence we can't properly handle allocation
>     failures.
>   - Neither `Iterator::collect` nor `FromIterator::from_iter` can handle
>     additional allocation flags.
> 
> Instead, provide `IntoIter::collect`, such that we can at least convert
> `IntoIter` into a `Vec` again.
> 
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  rust/kernel/alloc/kvec.rs | 86 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 86 insertions(+)

One comment below, but feel free to keep it as-is.

Reviewed-by: Benno Lossin <benno.lossin@proton.me>

> +    pub fn collect(self, flags: Flags) -> Vec<T, A> {
> +        let (mut ptr, buf, len, mut cap) = self.into_raw_parts();
> +        let has_advanced = ptr != buf.as_ptr();
> +
> +        if has_advanced {
> +            // Copy the contents we have advanced to at the beginning of the buffer.
> +            //
> +            // SAFETY:
> +            // - `ptr` is valid for reads of `len * size_of::<T>()` bytes,
> +            // - `buf.as_ptr()` is valid for writes of `len * size_of::<T>()` bytes,
> +            // - `ptr` and `buf.as_ptr()` are not be subject to aliasing restrictions relative to
> +            //   each other,
> +            // - both `ptr` and `buf.ptr()` are properly aligned.
> +            unsafe { ptr::copy(ptr, buf.as_ptr(), len) };
> +            ptr = buf.as_ptr();
> +        }
> +
> +        // This can never fail, `len` is guaranteed to be smaller than `cap`.
> +        let layout = core::alloc::Layout::array::<T>(len).unwrap();
> +
> +        // SAFETY: `buf` points to the start of the backing buffer and `len` is guaranteed to be
> +        // smaller than `cap`. Depending on `alloc` this operation may shrink the buffer or leaves
> +        // it as it is.
> +        ptr = match unsafe { A::realloc(Some(buf.cast()), layout, flags) } {
> +            // If we fail to shrink, which likely can't even happen, continue with the existing
> +            // buffer.
> +            Err(_) => ptr,
> +            Ok(ptr) => {
> +                cap = len;
> +                ptr.as_ptr().cast()
> +            }
> +        };

Would it make sense to only do the resize if the iterator has advanced?
If it hasn't, doing `into_iter().collect()` would be a no-op, which
would make sense IMO.

---
Cheers,
Benno

> +
> +        // SAFETY: If the iterator has been advanced, the advanced elements have been copied to
> +        // the beginning of the buffer and `len` has been adjusted accordingly.
> +        //
> +        // - `ptr` is guaranteed to point to the start of the backing buffer.
> +        // - `cap` is either the original capacity or, after shrinking the buffer, equal to `len`.
> +        // - `alloc` is guaranteed to be unchanged since `into_iter` has been called on the original
> +        //   `Vec`.
> +        unsafe { Vec::from_raw_parts(ptr, len, cap) }
> +    }
>  }
> 
>  impl<T, A> Iterator for IntoIter<T, A>
> --
> 2.46.0
>