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

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Message-Id: <DADKPKS4EAWU.D1UJEEBXKS8R@kernel.org>
Date: Wed, 04 Jun 2025 09:34:00 +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 1/4] rust: alloc: implement `Borrow` and `BorrowMut` for
 `Vec`

On Mon Jun 2, 2025 at 3:13 AM CEST, Alexandre Courbot wrote:
> Hi Benno,
>
> On Mon Jun 2, 2025 at 1:11 AM JST, Benno Lossin wrote:
>> On Sun Jun 1, 2025 at 5:00 AM CEST, Alexandre Courbot wrote:
>>> Implement these two common traits, which allow generic types to store
>>> either an owned value or a reference to it.
>>
>> I don't understand the second part of the sentence.
>
> I want to say that Borrow allows you to do something like:
>
>     struct Foo<B: Borrow<u32>>(B);
>
>     // `foo1` owns its value...
>     let foo1 = Foo(0x12);
>
>     let i = 0x24;
>     // ... but `foo2` just borrows it, subject to the lifetime of `i`.
>     let foo2 = Foo(&i);
>
> And the implementations in this series also let you do:
>
>     // `foo3`'s value is owned, but heap-allocated
>     let foo3 = Arc::new(KBox::new(0x56, GFP_KERNEL)?);
>
>     let j = Arc::new(0x78, GFP_KERNEL)?;
>     // `foo4`'s value is shared and its lifetime runtime-managed.
>     let foo4 = Foo(j.clone());

How about something like:

    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.

Also note this paragraph from the docs:

    In particular `Eq`, `Ord` and `Hash` must be equivalent for borrowed
    and owned values: `x.borrow() == y.borrow()` should give the same
    result as `x == y`.

(This holds for the types that you implement it for, but I wanted to
mention it)

---
Cheers,
Benno