Design a language feature to solve Field Projections

[nikomatsakis]

Metadata
Point of contact	@BennoLossin
Team(s)	lang
Goal document	2025h2/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.

Tasks and status

• Design meeting (lang )
• Lang-team experiment (lang )
• Author RFC (@BennoLossin)

[nikomatsakis]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[BennoLossin]

Key Developments

• coordinating with #![feature(pin_ergonomics)] (Tracking issue for pin ergonomics rust#130494) to ensure compatibility between the two features (allow custom pin projections to be the same as the ones for &pin mut T)
• identified connection to auto reborrowing
  • Reborrow traits #399
  • Tracking Issue for Reborrow trait lang experiment rust#145612
• held a design meeting
  • very positive feedback from the language team
  • approved lang experiment
  • got a vibe check on design axioms
• created a new Zulip channel #t-lang/custom-refs for all new features needed to make custom references more similar to &T/&mut T such as field projections, auto reborrowing and more
• created the tracking issue for #![feature(field_projections)]
• opened Add field representing types rust#146307 to implement field representing types (FRTs) in the compiler

Next Steps

• Get Add field representing types rust#146307 reviewed & merged

Help Wanted

• When the PR for FRTs lands, try out the feature & provide feedback on FRTs
• if possible using the field-projection crate and provide feedback on projections

Internal Design Updates

Shared & Exclusive Projections

We want users to be able to have two different types of projections analogous to &T and &mut T. Each field can be projected independently and a single field can only be projected multiple times in a shared way. The current design uses two different traits to model this. The two traits are almost identical, except for their safety documentation.

We were thinking if it is possible to unify them into a single trait and have coercions similar to autoreborrowing that would allow the borrow checker to change the behavior depending on which type is projected.

Syntax

There are lots of different possibilities for which syntax we can choose, here are a couple options: @x->f/@mut x->f, @x.f/@mut x.f, x.@f/x.mut@f, x.ref.@f/x.@f. Also many alternatives for the sigils used: x@f, x~f, x.@.f.

We have yet to decide on a direction we want to go in. If we are able to merge the two project traits, we can also settle on a single syntax which would be great.

Splitting Projections into Containers & Pointers

There are two categories of projections: Containers and Pointers:

• Containers are types like MaybeUninit<T>, Cell<T>, UnsafeCell<T>, ManuallyDrop<T>. They are repr(transparent) and apply themselves to each field, so MaybeUninit<MyStruct> has a field of type MaybeUninit<MyField> (if MyStruct has a field of type MyField).
• Pointers are types like &T, &mut T, cell::Ref[Mut]<'_, T>, *const T/*mut T, NonNull<T>. They support projecting Pointer<'_, Struct> to Pointer<'_, Field>.

In the current design, these two classes of projections are unified by just implementing Pointer<'_, Container<Struct>> -> Pointer<'_, Container<Field>> manually for the common use-cases (for example &mut MaybeUninit<Struct> -> &mut MaybeUninit<Field>). However this means that things like &Cell<MaybeUninit<Struct>> doesn't have native projections unless we explicitly implement them.

We could try to go for a design that has two different ways to implement projections -- one for containers and one for pointers. But this has the following issues:

• there are two ways to implement projections, which means that some people will get confused which one they should use.
• making projections through multiple container types work out of the box is great, however this means that when defining a new container type and making it available for projections, one needs to consider all other container types and swear coherence with them. If we instead have an explicit way to opt in to projections through multiple container types, the implementer of that trait only has to reason about the types involved in that operation.
  • so to rephrase, the current design allows more container types that users actually use to be projected whereas the split design allows arbitrary nestings of container types to be projected while disallowing certain types to be considered container types.
• The same problem exists for allowing all container types to be projected by pointer types, if I define a new pointer type I again need to reason about all container types and if it's sound to project them.

We might be able to come up with a sensible definition of "container type" which then resolves these issues, but further investigation is required.

Projections for &Custom<U>

We want to be able to have both a blanket impl<T, F: Field<Base = T>> Project<F> for &T as well as allow people to have custom projections on &Custom<U>. The motivating example for custom projections is the Rust-for-Linux Mutex that wants these projections for safe RCU abstractions.

During the design meeting, it was suggested we could add a generic to Project that only the compiler is allowed to insert, this would allow disambiguation between the two impls. We have now found an alternative approach that requires less specific compiler magic:

• Add a new marker trait ProjectableBase that's implemented for all types by default.
• People can opt out of implementing it by writing impl !ProjectableBase for MyStruct; (needs negative impls for marker traits).
• We add where T: ProjectableBase to the impl Project for &T.
• The compiler needs to consider the negative impls in the overlap check for users to be able to write their own impl<U, F> Project<F> for &Custom<U> where ... (needs negative impl overlap reasoning)

We probably want negative impls for marker traits as well as improved overlap reasoning for different reasons too, so it is probably fine to depend on them here.

enum support

enum and union shouldn't be available for projections by default, take for example &Cell<Enum>, if we project to a variant, someone else could overwrite the value with a different variant, invalidating our &Cell<Field>. This also needs a new trait, probably AlwaysActiveField (needs more name bikeshedding, but too early for that) that marks fields in structs and tuples.

To properly project an enum, we need:

• a new CanProjectEnum (TBB) trait that provides a way to read the discriminant that's currently inhabiting the value.
  • it also needs to guarantee that the discriminant doesn't change while fields are being projected (this rules out implementing it for &Cell)
• a new match operator that will project all mentioned fields (for &Enum this already is the behavior for match)

Field Representing Types (FRTs)

While implementing rust-lang/rust#146307 we identified the following problems/design decisions:

• a FRT is considered local to the orphan check when each container base type involved in the field path is local or a tuple (see the top comment on the PR for more infos)
• FRTs cannot implement Drop
• the Field trait is not user-implementable
• types with fields that are dynamically sized don't have a statically known offset, which complicates the UnalignedField trait,

I decided to simplify the first implementation of FRTs and restrict them to sized structs and tuples. It also doesn't support packed structs. Future PRs will add support for enums, unions and packed structs as well as dynamically sized types.