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Message-ID: <3c917bbd-7d53-23e2-d067-7ce8fce95307@gmail.com>
Date:   Thu, 13 Jul 2023 00:28:17 -0300
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Subject: Re: [PATCH v1] rust: add improved version of
 `ForeignOwnable::borrow_mut`
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To:     Alice Ryhl <aliceryhl@google.com>, rust-for-linux@vger.kernel.org
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>, Gary Guo <gary@garyguo.net>,
        =?UTF-8?Q?Bj=c3=b6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Benno Lossin <benno.lossin@proton.me>,
        linux-kernel@vger.kernel.org, patches@lists.linux.dev
References: <20230710074642.683831-1-aliceryhl@google.com>
From:   Martin Rodriguez Reboredo <yakoyoku@gmail.com>
In-Reply-To: <20230710074642.683831-1-aliceryhl@google.com>
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On 7/10/23 04:46, Alice Ryhl wrote:
> Previously, the `ForeignOwnable` trait had a method called `borrow_mut`
> that was intended to provide mutable access to the inner value. However,
> the method accidentally made it possible to change the address of the
> object being modified, which usually isn't what we want. (And when we
> want that, it can be done by calling `from_foreign` and `into_foreign`,
> like how the old `borrow_mut` was implemented.)
> 
> In this patch, we introduce an alternate definition of `borrow_mut` that
> solves the previous problem. Conceptually, given a pointer type `P` that
> implements `ForeignOwnable`, the `borrow_mut` method gives you the same
> kind of access as an `&mut P` would, except that it does not let you
> change the pointer `P` itself.
> 
> This is analogous to how the existing `borrow` method provides the same
> kind of access to the inner value as an `&P`.
> 
> Note that for types like `Arc`, having an `&mut Arc<T>` only gives you
> immutable access to the inner `T`. This is because mutable references
> assume exclusive access, but there might be other handles to the same
> reference counted value, so the access isn't exclusive. The `Arc` type
> implements this by making `borrow_mut` return the same type as `borrow`.
> 
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
> [...]

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>