Propose goal: Field Projections

src/2025h2/field-projections.md

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+# Design a language feature to solve Field Projections
+
+| Metadata         |                                                                                  |
+|:-----------------|----------------------------------------------------------------------------------|
+| Point of contact | @BennoLossin                                                                     |
+| Teams            | <!-- TEAMS WITH ASKS -->                                                         |
+| Task owners      | <!-- TASK OWNERS -->                                                             |
+| Status           | Proposed                                                                         |
+| Tracking issue   |                                                                                  |
+| Zulip channel    | N/A                                                                              |
+
+## Summary
+
+Figure out the best design for field projections. Update the existing [Field Projections RFC] or
+author a new one and implement it for use in nightly via a lang experiment.
+
+[Field Projections RFC]: https://github.com/rust-lang/rfcs/pull/3735
+
+## Motivation
+
+Rust makes extensive use of smart pointers (`Box<T>`, `Rc<T>`, `Arc<T>`), modified references (`&mut
+MaybeUninit<T>`, `Pin<&mut T>`) and custom pointer types (`NonNull<T>`).
+
+Some of these types implement the `Deref[Mut]` trait(s) allowing one to access fields of the type
+`T`. But not all of them can implement it due to various reasons. However, they often *can* support
+operations that "index" into the fields of the type `T`. For example `&mut MaybeUninit<Struct>`
+conceptually has fields of type `&mut MaybeUninit<Field>`.
+
+### The status quo
+
+Rust has a lot of container types that make it difficult to directly interact with fields of structs
+that they wrap. For example:
+- `MaybeUninit<T>`,
+- `UnsafeCell<T>`,
+- `Cell<T>`
+
+It also has several pointer-like types that could support a natural pointer-to-field operation. For
+example:
+- `NonNull<T>`,
+- `*const T` / `*mut T`,
+- `cell::Ref<'_, T>` / `cell::RefMut<'_, T>`
+
+Additionally, there is `Pin<&mut T>`, which already has a well-established name for this operation:
+pin-projections. The ecosystem provides several crates to add this operation to the struct itself.
+
+#### Custom types
+
+A plethora of types making use of field projections are found in the context of Rust for Linux.
+Therefore they might -- with high probability -- come up in other embedded projects too.
+
+- `VolatilePtr<'a, T>` like `*mut T`, but with a lifetime & all accesses to the pointer are
+  volatile.
+- `Ptr<'a, T>` like a `&'a T` but without certain rust guarantees (most likely `&'a UnsafePinned<T>`
+  under the hood).
+- `RcuMutex<T>` a safe abstraction for RCU (a special synchronization primitive in the kernel)
+  working together with a `Mutex<T>` to synchronize accesses to data (this requires complex
+  projections, only allowing certain fields to be projected).
+- `SeqLockRef<'_, T>`
+- `AtomicPtr<T>` where `T` is a small enough type composed of integers.
+- `UserPtr<T>` a pointer into userspace
+
+Additionally, Rust for Linux could take advantage of field information present in the current
+proposal. Essentially answering the question "does this type have a field of type X at offset Y?"
+via traits.
+
+Note that the projections listed above are also very important to Rust for Linux. Virtually all
+types are pinned in the kernel, so `Pin<&mut T>` comes up a lot in drivers. We're also handling raw
+pointers very often where we could use `NonNull<T>` instead if they had better field access.
+
+### The next 6 months
+
+Have design meetings with the relevant parties & update the existing or write a new RFC.
+
+### The "shiny future" we are working towards
+
+Have field projections available in stable Rust.
+
+## Design axioms
+
+- **Effortless Syntax.** Using field projections in a non-generic context should look very similar
+  to normal field accesses.
+- **Broad Solution.** Field projections should be very general and solve complex projection problems
+  such as pin-projections and `RcuMutex<T>`.
+
+## Ownership and team asks
+
+| Task                 | Owner(s) or team(s)                 | Notes                                                               |
+|----------------------|-------------------------------------|---------------------------------------------------------------------|
+| Design meeting       | ![Team][] [lang]                    | Possibly more than one required as well as discussions on zulip.    |
+| Lang-team experiment | @dingxiangfei2009, @BennoLossin, ![Team][] [lang] |                                                       |
+| Author RFC           | @BennoLossin                        |                                                                     |
+| Lang-team champion   | ![Team][] [lang]                    | *Champion Needed*                                                   |
+| Secondary RFC review | ![Team][] [types]                   | might be a good idea?                                               |
+| RFC decision         | ![Team][] [lang]                    |                                                                     |
+
+
+## Frequently asked questions
+