Crate selfref

An experimental approach to self-referential structs in Rust.

This crate provides an alternative approach to self-referential structs, where instead of providing you with a macro or framework where you define a self-referential struct and it handles all of the details for you, we try to expose the abstractions and building blocks for making self-referential structs work well in safe Rust.

For example, a Holder is a safe wrapper around a self-referential struct, providing safe APIs for constructing and manipulating a self-referential struct. However, and unlike other self-referential crates, it does not dictate the backing storage of the struct. The Opaque trait is used to identify a self-referential struct for use with a Holder - since Rust does not support higher kinded types (HKTs), this crate uses generic associated types (GATs) as a workaround.

To use the crate, first define a self-referential struct in plain Rust:

use std::cell::Cell;

// Your self-referential struct.
struct MySelfRefStruct<'this> {
    // Rust uses RAII-like struct construction, as a result this must be
    // somehow initialized after the struct. We can use an Option in a Cell
    // for this.
    this: Cell<Option<&'this MySelfRefStruct<'this>>>,
}

Then, define a type to implement the Opaque. This can be done automatically with the opaque macro:

use selfref::opaque;

// A "marker type" that implements `Opaque`.
// This follows the "type family" GAT pattern.
struct MySelfRefStructKey;

opaque! {
    impl Opaque for MySelfRefStructKey {
        type Kind<'this> = MySelfRefStruct<'this>;
    }
}

// Alternatively, it is possible to implement `Opaque` on, for example,
// `MySelfRefStruct<'static>`, but the added lifetime adds verbosity which
// may be considered unnecessary/undesired.

Now you can construct a Holder and pick a storage for it. For example, in a Box:

use selfref::Holder;

fn main() {
    // first, construct the struct
    let holder = Box::pin(Holder::<'_, MySelfRefStructKey>::new_with(
        |foo| foo.build({
            MySelfRefStruct {
                this: Cell::new(None)
            }
        })
    ));

    // then, build the self-reference
    holder.as_ref().operate_in(
        |this| {
            this.this.set(Some(this.get_ref()));
        }
    );
}

Examples

This is a more complex example borrowing from an external lifetime:

use core::cell::Cell;
use core::marker::PhantomData;
use core::pin::Pin;
 
use selfref::Holder;
use selfref::opaque;

struct Foo<'a, 'b: 'a> {
    foo: Cell<Option<&'a Foo<'a, 'b>>>,
    t: &'b str,
}

struct FooKey<'b>(PhantomData<&'b str>);
opaque! {
    impl['b] Opaque for FooKey<'b> {
        type Kind<'a> = Foo<'a, 'b>;
    }
}

fn main() {
    // a non-'static &str
    let stack_array: [u8; 5] = *b"hello";
    let stack_str = core::str::from_utf8(&stack_array).unwrap();

    // construct the struct
    let holder = Box::pin(Holder::<'_, FooKey<'_>>::new_with(|foo| {
        foo.build(Foo {
            foo: Default::default(),
            t: stack_str,
        })
    }));

    holder.as_ref().operate_in(|foo| {
        foo.foo.set(Some(foo.get_ref()));
    });
}

Features

Due to PhantomData is unsound we currently require the following features for T: ?Sized support in selfref::opaque!:

• alloc - selfref::opaque! for T: ?Sized is provided by Box.
• nightly - selfref::opaque! for T: ?Sized is provided by a wrapper around PhantomData, which works around the above issue. we call this “PhantomDrop”.

When enabling both features, nightly takes over and we use the wrapper always. This doesn’t make a significant difference since the generated UB check is dead code anyway, but PhantomDrop doesn’t depend on alloc and can be used in no_std environments.

If not using either feature, T: ?Sized support requires unsafely implementing Opaque.

Note that we do not enable any features by default! We assume most folks aren’t coming to this crate for its T: ?Sized support, so these are the best defaults for crates to depend on. If they do need the ?Sized support they can just enable one of these (probably alloc).

Modules

srce

SRCE is a way to open and close an LCellOwner. This provides the full expressive power of selfref, with the ability to use zeroish-cost LCells in the struct.

Macros

opaque

Creates an opaqueified self-referential struct “key”.

Structs

Builder

Helper for creating a Holder.

Holder

Holds an “opaqueified” T::Kind.

OperateIn

Wrapper around a Pin<&'k T::Kind<'k>> for implied bounds.

Traits

Opaque

An opaqueified self-referential struct “key”.