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

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Message-ID: <87a5uh4u5h.fsf@metaspace.dk>
Date:   Wed, 23 Aug 2023 18:09:15 +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:
>>>> I am not sure this makes sense. How about splitting the trait in two,
>>>> immutable and mutable and only implementing the immutable one or Arc?
>>>
>>> I used this design based on what would make sense for a linked list. The
>>> idea is that we can have two different types of cursors for a linked
>>> list: immutable and mutable. The immutable cursor lets you:
>>>
>>>  * move around the linked list
>>>  * access the values using `borrow`
>>>
>>> The mutable cursor lets you:
>>>
>>>  * move around the linked list
>>>  * delete or add items to the list
>>>  * access the values using `borrow_mut`
>>>
>>> The mutable cursor gives you extra abilities beyond the `borrow` vs
>>> `borrow_mut` distinction, so we want to provide both types of cursors
>>> even if the pointer type is Arc. To do that, we need a trait that
>>> defines what it means to have mutable access to an Arc.
>> 
>> I don't see how that prevents this trait from being split in two?
>
> Well, you could split the trait, but if you make the mutable iterator
> require the `borrow_mut` trait, then you have to implement the mutable
> trait for `Arc` too if you want the mutable iterator to work with `Arc`.
>
> And if you're always going to implement both traits, then maybe it
> makes more sense to not split the traits?

Maybe. Do you have a prototype for the list cursor available?

BR Andreas