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

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Date:   Wed, 23 Aug 2023 10:34:58 +0000
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     nmi@...aspace.dk
Cc:     alex.gaynor@...il.com, aliceryhl@...gle.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`

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?

Alice