Re: [PATCH v4 1/3] rust: add UnsafePinned type

["Benno Lossin" <lossin@kernel.org>]

On Thu Jun 5, 2025 at 7:03 PM CEST, Christian Schrefl wrote:
> On 11.05.25 8:21 PM, Christian Schrefl wrote:
>> +/// This type provides a way to opt-out of typical aliasing rules;
>> +/// specifically, `&mut UnsafePinned<T>` is not guaranteed to be a unique pointer.
>> +///
>> +/// However, even if you define your type like `pub struct Wrapper(UnsafePinned<...>)`, it is still
>> +/// very risky to have an `&mut Wrapper` that aliases anything else. Many functions that work
>> +/// generically on `&mut T` assume that the memory that stores `T` is uniquely owned (such as
>> +/// `mem::swap`). In other words, while having aliasing with `&mut Wrapper` is not immediate
>> +/// Undefined Behavior, it is still unsound to expose such a mutable reference to code you do not
>> +/// control! Techniques such as pinning via [`Pin`](core::pin::Pin) are needed to ensure soundness.
>> +///
>> +/// Similar to [`UnsafeCell`], [`UnsafePinned`] will not usually show up in
>> +/// the public API of a library. It is an internal implementation detail of libraries that need to
>> +/// support aliasing mutable references.
>> +///
>> +/// Further note that this does *not* lift the requirement that shared references must be read-only!
>> +/// Use [`UnsafeCell`] for that.
>
> The upstream rust PR [0] that changes this was just merged. So now `UnsafePinned` includes
> `UnsafeCell` semantics. It's probably best to also change this in the kernel docs.
> Though it's still the case that removing the guarantee is simpler than adding it back later,
> so let me know what you all think.

Depends on how "stable" this decision is. I haven't followed the
discussion, but given that this once changed to the "non-backwards"
compatible case it feels permanent.

How close is it to stabilization?

If it's close-ish, then I'd suggest we change this to reflect the new
semantics. If not, then we should leave it as-is.

---
Cheers,
Benno

> [0]: https://github.com/rust-lang/rust/pull/140638