linux-kernel - Re: [PATCH v2 1/4] rust: alloc: implement `Borrow` and `BorrowMut` for `Vec`

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Message-Id: <DAMHZIN35QQ9.3N5WSB58J4WG9@kernel.org>
Date: Sat, 14 Jun 2025 21:19:55 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Alexandre Courbot" <acourbot@...dia.com>, "Danilo Krummrich"
 <dakr@...nel.org>, "Miguel Ojeda" <ojeda@...nel.org>, "Alex Gaynor"
 <alex.gaynor@...il.com>, "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo"
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Andreas Hindborg" <a.hindborg@...nel.org>,
 "Alice Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>
Cc: <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/4] rust: alloc: implement `Borrow` and `BorrowMut`
 for `Vec`

On Fri Jun 13, 2025 at 3:46 PM CEST, Alexandre Courbot wrote:
> Implement `Borrow<[T]>` and `BorrowMut<[T]>` for `Vec<T>`. This allows
> `Vec<T>` to be used in generic APIs asking for types implementing those
> traits. `[T; N]` and `&mut [T]` also implement those traits allowing
> users to use either owned, borrowed and heap-owned values.
>
> The implementation leverages `as_slice` and `as_mut_slice`.
>
> Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
> Signed-off-by: Alexandre Courbot <acourbot@...dia.com>

One comment below, with that fixed:

Reviewed-by: Benno Lossin <lossin@...nel.org>

> ---
>  rust/kernel/alloc/kvec.rs | 57 +++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 57 insertions(+)
>
> diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
> index 1a0dd852a468ccda6ea1b521bc1e7dbc8d7fc79c..3f368d4a67683ac5a0ff87d7df33a3bb640ced59 100644
> --- a/rust/kernel/alloc/kvec.rs
> +++ b/rust/kernel/alloc/kvec.rs
> @@ -8,6 +8,7 @@
>      AllocError, Allocator, Box, Flags,
>  };
>  use core::{
> +    borrow::{Borrow, BorrowMut},
>      fmt,
>      marker::PhantomData,
>      mem::{ManuallyDrop, MaybeUninit},
> @@ -890,6 +891,62 @@ fn deref_mut(&mut self) -> &mut [T] {
>      }
>  }
>  
> +/// Allows `Vec<T>` to be used as a `Borrow<[T]>`.

I personally would vote against this first line description here. I
don't think that it will show up in a summary view of rust doc (since
trait impls don't appear in searches or module overviews). Additionally,
this first sentence seems like this kind of comment:

    // call `foo`:
    foo();

So let's just remove it and directly start with the examples :)

Also for the other cases.

---
Cheers,
Benno

> +///
> +/// # Examples
> +///
> +/// ```
> +/// # use core::borrow::Borrow;
> +/// struct Foo<B: Borrow<[u32]>>(B);
> +///
> +/// // Owned array.
> +/// let foo_array = Foo([1, 2, 3]);
> +///
> +/// // Owned vector.
> +/// let foo_vec = Foo(KVec::from_elem(0, 3, GFP_KERNEL)?);
> +///
> +/// let arr = [1, 2, 3];
> +/// // Borrowed slice from `arr`.
> +/// let foo_borrowed = Foo(&arr[..]);
> +/// # Ok::<(), Error>(())
> +/// ```
> +impl<T, A> Borrow<[T]> for Vec<T, A>
> +where
> +    A: Allocator,
> +{
> +    fn borrow(&self) -> &[T] {
> +        self.as_slice()
> +    }
> +}
> +
> +/// Allows `Vec<T>` to be used as a `BorrowMut<[T]>`.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// # use core::borrow::BorrowMut;
> +/// struct Foo<B: BorrowMut<[u32]>>(B);
> +///
> +/// // Owned array.
> +/// let foo_array = Foo([1, 2, 3]);
> +///
> +/// // Owned vector.
> +/// let foo_vec = Foo(KVec::from_elem(0, 3, GFP_KERNEL)?);
> +///
> +/// let mut arr = [1, 2, 3];
> +/// // Borrowed slice from `arr`.
> +/// let foo_borrowed = Foo(&mut arr[..]);
> +/// # Ok::<(), Error>(())
> +/// ```
> +impl<T, A> BorrowMut<[T]> for Vec<T, A>
> +where
> +    A: Allocator,
> +{
> +    fn borrow_mut(&mut self) -> &mut [T] {
> +        self.as_mut_slice()
> +    }
> +}
> +
>  impl<T: Eq, A> Eq for Vec<T, A> where A: Allocator {}
>  
>  impl<T, I: SliceIndex<[T]>, A> Index<I> for Vec<T, A>