Design a language feature to solve Field Projections

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.

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.

Current proposals

In addition to Field Projections RFC v2 already mentioned above, there is a newer proposal that improves upon it.

For historical context, there also is the Field Projections RFC v1.

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	lang	Possibly more than one required as well as discussions on zulip.
Lang-team experiment	lang	Ding Xiang Fei, Benno Lossin
Author RFC	Benno Lossin

Frequently asked questions

Why not push for RFC decision?

The design is still too early to expect a final decision on the RFC.