In this blog post, I present an idea on how we can specify the borrow checker behavior in the context of the field projection design effort. This work builds upon the blog post The Algebra of Loans in Rust by Nadrieril. I will give a more field projection focused proposal, whereas that blog post looked at the general way the borrow checker works.

Jump to The Table.

The main proposal for field projection is focused on virtual places, which were introduced by Nadrieril. Similarly now, I will give a way to specify the borrow checker behavior of the built-in reference types as well as custom types with places at the focal point.

Additionally, I will present an idea on how to better include the borrow checker's lifetime into our proposal.

There are several issues/open questions with the current design By current design I mean the design introduced by Truly First-Class Custom Smart Pointers and developed further through discussions on Zulip. :

• What does the lifetime on PlaceBorrow mean when we have BORROW_KIND = Untracked?
• How does lifetime shortening fit into the picture here, we only have one lifetime?
• What are the actual semantics of BorrowKind?

I hope to answer all of these with this blog post. And additionally provide a simpler explanation of borrowing in Rust.

# Informal Writeup

The Algebra of Loans in Rust is a prerequisite of this post, it gives a good overview of the basic concepts, such as what a place is, taking a borrow and loans. The post also gives three tables, which specify what kind of operations each reference has available. In this post I will replicate the latter two tables The first table is encoded by implementing PlaceBorrow for the right types. ; not via specifying them directly, but rather by specifying an underlying mechanism.

This underlying mechanism consists of two concepts:

1. What kind of access are we granting to the place. For example, &mut requires exclusive access, & only needs shared access and *const doesn't need any kind of access Raw pointers can obviously be used to read or write their pointee, but that operation is not governed by the borrow checker, which is what we're interested in here. So from the borrow checker's perspective, *const does not ask for any kind of access. .
2. What state should the place be in when the borrow starts and should it be changed when the borrow ends? For example, & requires the place to be initialized. &mut requires that the place is not pinned. *const doesn't require anything. As a last important example there is &own, which requires the state to be initialized, not pinned and transform that state on expiry to uninitialized.

These concepts also explain how all place operations interact with the borrow checker:

• PlaceDrop requires exclusive access & changes the state from initialized to uninitialized.
• PlaceRead requires shared access to the place and expects it to be initialized.
• PlaceMove additionally requires the place to be not pinned and changes the state to uninitialized.
• PlaceWrite requires exclusive access; if the state is initialized, it performs PlaceDrop first, so it requires an uninitialized state, otherwise it expects the place to be uninitialized.
• PlaceDeref requires the same kind of access that the following operation on the derefed pointer needs.
• PlaceBorrow specifies its access & expected state + state change.

# Formal Explanation

This part essentially is like the "reference-level explanation" section of an RFC. We model the access kind and place state using enums:

// also called `BorrowKind` in previous proposals
pub enum PlaceAccess {
    Shared,
    Exclusive,
    Untracked,
}

pub enum PlaceState {
    Initialized(PinnedState),
    Uninitialized,
}

pub enum PinnedState {
    NotPinned,
    Pinned,
}

We can now add two constants on PlaceBorrow or even on HasPlace:

pub trait HasPlace {
    const ACCESS: PlaceAccess;

    const STATE: PlaceState;

    type Target: ?Sized;
}

(Note that to properly support places that allow several states, we'd probably need a set or another enum instead of PlaceState.)

We then also need a constant AFTER: PlaceState in PlaceBorrow, which specifies the state the place should be in after the borrow ends.

To obtain the second table "If a loan was taken and is live, what can I still do to the place", we only need to consider the ACCESS constants of the two custom pointers (types that implement HasPlace). If one of them is Untracked or both are Shared, they may coexist; otherwise, a borrow check error is thrown.

Examples:

• &mut T and &own T cannot coexist, as they both want Exclusive.
• &T and ArcMap<T, U> can coexist, as both only need Shared access.
• UniqueArcMap<T, U> and *const T can coexist, as raw pointers have Untracked access.

For the third table, one only needs to consider the state of the place after the borrow expires. In this model, it is useful to make Untracked borrows expire immediately.

Examples:

• &own T changes the state to Uninitialized, so a subsequent &mut T borrow is not allowed, as that expects an initialized state. A borrow using &uninit T is allowed, since that expects uninitialized memory.
• &mut T doesn't change the state, so a subsequent &T borrow is allowed.

# The Table

A couple of notes:

• &uninit overlaps heavily with in-place init. The borrow checker needs to understand control flow here, as the error path leaves the memory uninitialized, but the happy path initializes it. Not really sure how we would track this using PlaceState. Since in-place init has not settled on a design, we do not need to support it right away.
• PlaceWrite expects the place to be uninitialized. This matches the current behavior of Rust, where a drop_in_place is inserted before a write is performed. This behavior would of course be kept for custom pointers as well.
• PlaceDeref is pretty special, we need more time to iron out its design anyways. From this posts perspective, we probably want to copy the borrow checker behavior from the operation that is performed afterwards on the returned pointer.
• The last column could also be encoded via a PlaceAction enum:

  pub enum PlaceAction {
      Nothing,
      Initialize(PinnedState),
      Uninitialize,
  }

  This would allow us to better specify the "unchanged" semantics.

# Conclusion

I believe that this idea is very much on the right track, since it has many great properties at the same time:

• It overall is a simple explanation, we only have two small enums to keep track of and combine.
• Interactions between references are defined immediately through specifying just three properties of both references and there is no room for ambiguity.
• It covers the existing Place* operations.
• It matches my intuitive understanding of how the borrow checker works very well.

If we had a Move trait instead of making pin a place state, we'd have an even simpler picture. Another piece of evidence in favor of making Move a reality.

# Open Questions

• What is the best way to accommodate multiple PlaceState in the same borrow?
• How do we support different changes to the PlaceState depending on various things? Do we even want to support that?
• Should we specify a PlaceAction (which encodes a state change) instead of giving the state after the borrow ends?
• How does PlaceDeref work?
• Does it make sense for Untracked access to "immediately end" the borrow?

# Bonus: Lifetime in PlaceAccess

Taking inspiration from Nadrieril He had the idea to use the existing reference types to specify the behavior for new ones. , we can try to make the lifetime available depending on whether the access kind is untracked. My idea is to just use the type system instead of an enum:

#[sealed]
pub trait PlaceAccess {}

pub struct Owned;
impl PlaceAccess for Owned {}

pub struct Shared<'a>(PhantomData<&'a ()>);
impl PlaceAccess for Shared<'_> {}

pub struct Exclusive<'a>(PhantomData<&'a mut ()>);
impl PlaceAccess for Exclusive<'_> {}

When we now implement HasPlace, we must give a lifetime in shared and exclusive cases:

impl<'a, T> HasPlace for &'a mut T {
    type Access = Exclusive<'a>;
    const STATE: PlaceState = PlaceState::Initialized { pinned: false };
    type Target = T;
}

But in the untracked case, we now do not have a lifetime at all:

impl<T> HasPlace for *const T {
    type Access = Untracked;
    const STATE: PlaceState = PlaceState::Any;
    type Target = T;
}

impl<P: Projection> PlaceBorrow<P, *const P::Target> for *const P::Source {
    // ...
}

This conditional binding of the lifetime to PlaceAccess also allows us to allow the borrow checker to choose that lifetime. It essentially acts as a marker of the borrow-checker-controlled lifetime. This way, we get lifetime shortening, since we can now write the following impl:

impl<'a, 'b, P> PlaceBorrow<P, &'a mut P::Target> for &'b mut P::Source
where
    'b: 'a,
    P: Projection,
{
    // ...
}

And since &'a mut P::Target has Access = Exclusive<'a>, the borrow checker can shorten that lifetime as is appropriate.