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

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Message-Id: <DAL6OLAG7R5W.3U3TNG58QUO04@nvidia.com>
Date: Fri, 13 Jun 2025 15:15:47 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Boqun Feng" <boqun.feng@...il.com>
Cc: "Benno Lossin" <lossin@...nel.org>, "Danilo Krummrich"
 <dakr@...nel.org>, "Miguel Ojeda" <ojeda@...nel.org>, "Alex Gaynor"
 <alex.gaynor@...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>, <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 Tue Jun 3, 2025 at 12:06 AM JST, Boqun Feng wrote:
> On Mon, Jun 02, 2025 at 10:13:22AM +0900, 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());
>
> Maybe you could put these in the "# Examples" section before impl
> blocks. E.g
>
> 	/// # Examples
> 	/// ```
> 	/// <you case above>
> 	/// ```
> 	impl<T, A> Borrow<[T]> for Vec<T, A> ...

I've started doing this, but it felt like I was writing down the obvious
as this kind of use is precisely what Borrow is intended for. I think
Benno's suggestion for the commit log addresses your request for
clarification, but let me add these on v2 anyway - we can always remove
them if we conclude they are unnecessary.