Re: [PATCH 5/5] rust: add improved version of `ForeignOwnable::borrow_mut`

[Andreas Hindborg <a.hindborg@kernel.org>]

"Tamir Duberstein" <tamird@gmail.com> writes:

> From: Alice Ryhl <aliceryhl@google.com>
>
> 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: Boqun Feng <boqun.feng@gmail.com>
> Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>


Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>

Best regards,
Andreas Hindborg