Re: [PATCH 0/3] Groundwork for Lock<T> when T is pinned

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Jul 30, 2025 at 02:14:43PM -0300, Daniel Almeida wrote:
> It's currently impossible to have a pinned struct within the Lock<T> type.
> This is problematic, because drivers might want to do this for various
> reasons, specially as they grow in complexity.
> 
> A trivial example is:
> 
> struct Foo {
>   #[pin]
>   bar: Mutex<Bar>,
>   #[pin]
>   p: PhantomPinned,
> }
> 
> struct Bar {
>   #[pin]
>   baz: Mutex<Baz>,
>   #[pin]
>   p: PhantomPinned,
> }
> 
> Note that Bar is pinned, so having it in a Mutex makes it impossible to
> instantiate a Foo that pins the Bar in bar. This is specially undesirable,
> since Foo is already pinned, and thus, it could trivially enforce that its
> bar field is pinned as well.
> 
> This can be trivially solved by using Pin<KBox<Bar>> instead of
> structurally pinning, at the cost of an extra (completely unneeded)
> allocation and ugly syntax.
> 
> This series lays out the groundwork to make the above possible without any
> extra allocations.
> 
> - Patch 1 structurally pins the 'data' field in Lock<T>
> - Patch 2 constrains the DerefMut implementation for safety reasons
> - Patch 3 adds an accessor to retrieve a Pin<&mut T>
> 
> Note that this is just the beginning of the work needed to make a Pin<&mut
> T> actually useful due to pin projections being currently unsupported.
> 
> In other words, it is currently impossible (even with the current patch) to
> do this:
> 
> let mut data: MutexGuard<'_, Data> = mutex.lock();
> let mut data: Pin<&mut Data> = data.as_mut();
> let foo = &mut data.foo; // <- won't compile
> 
> The above is something that Benno is working on.
> 
> Thanks Boqun, Benno and the rest of the team for brainstorming the issue
> and for and laying out a series of steps to implement a solution.
> 
> ---
> Daniel Almeida (3):
>       rust: lock: pin the inner data
>       rust: lock: guard: add T: Unpin bound to DerefMut
>       rust: lock: add a Pin<&mut T> accessor

With the things that Benno said fixed:

Reviewed-by: Alice Ryhl <aliceryhl@google.com>