linux-kernel - Re: [PATCH v1] rust: add improved version of `ForeignOwnable::borrow

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Message-ID: <87r0nu40cs.fsf@metaspace.dk>
Date:   Wed, 23 Aug 2023 10:40:54 +0200
From:   "Andreas Hindborg (Samsung)" <nmi@...aspace.dk>
To:     Alice Ryhl <aliceryhl@...gle.com>
Cc:     alex.gaynor@...il.com, benno.lossin@...ton.me,
        bjorn3_gh@...tonmail.com, boqun.feng@...il.com, gary@...yguo.net,
        linux-kernel@...r.kernel.org, ojeda@...nel.org,
        patches@...ts.linux.dev, rust-for-linux@...r.kernel.org,
        wedsonaf@...il.com
Subject: Re: [PATCH v1] rust: add improved version of
 `ForeignOwnable::borrow_mut`


Alice Ryhl <aliceryhl@...gle.com> writes:

> Andreas Hindborg <nmi@...aspace.dk> writes:
>>> +    /// Borrows a foreign-owned object mutably.
>>> +    ///
>>> +    /// This method provides a way to access a foreign-owned value from Rust mutably. It provides
>>> +    /// you with exactly the same abilities as an `&mut Self` when the value is Rust-owned, except
>>> +    /// that this method does not let you swap the foreign-owned object for another. (That is, it
>>> +    /// does not let you change the address of the void pointer that the foreign code is storing.)
>> 
>> How about this:
>> 
>> "For a smart pointer P<T> this method provides mutable access to T if
>> &mut P<T> would allow mutable access to T. Otherwise it provides
>> immutable access to T."
>> 
>> The point is that the method provides access to the pointee, not the
>> smart pointer itself. In fact it is perfectly fine to do a mem::swawp()
>> for the pointee in the case of Box and depending on interpretation the
>> sentence "does not let you swap the foreign-owned object for another" is
>> confusing.
>
> Yeah, I agree that the phrasing is somewhat confusing.
>
> How about this:
>
> /// This method provides a way to access a foreign-owned value from Rust mutably. It provides
> /// you with exactly the same abilities as an `&mut Self` when the value is Rust-owned, except
> /// that the address of the object must not be changed.
> ///
> /// Note that for types like [`Arc`], an `&mut Arc<T>` only gives you immutable access to the
> /// inner value, so this method also only provides immutable access in that case.
> ///
> /// In the case of `Box<T>`, this method gives you the ability to modify the inner `T`, but it
> /// does not let you change the box itself. That is, you cannot change which allocation the box
> /// points at.

Sounds good 👍

BR Andreas