rust-gc

Simple tracing (mark and sweep) garbage collector for Rust

The design and motivation is illustrated in this blog post, with a sketch of the code in this gist.

There is another post about the initial design of cgc, its experimental concurrent branch.

How to use

To include in your project, add the following to your Cargo.toml:

[dependencies]
gc = { version = "0.5", features = ["derive"] }

This can be used pretty much like Rc, with the exception of interior mutability.

While this can be used pervasively, this is intended to be used only when needed, following Rust's "pay only for what you need" model. Avoid using Gc where Rc or Box would be equally usable.

Types placed inside a Gc must implement Trace and Finalize. The easiest way to do this is to use the gc_derive crate:

use gc::{Finalize, Gc, Trace};

#[derive(Trace, Finalize)]
struct Foo {
    x: Option<Gc<Foo>>,
    y: u8,
    // ...
}

// now, `Gc<Foo>` may be used

Finalize may also be implemented directly on the struct, in order to add custom finalizer behavior:

use gc::{Finalize, Trace};

#[derive(Trace)]
struct Foo {...}

impl Finalize for Foo {
    fn finalize(&self) {
        // Clean up resources for Foo, because we think it will be destroyed.
        // Foo may not be destroyed after a call to finalize, as another
        // finalizer may create a reference to it due to reference cycles.
        // Recursive calls of Finalize::finalize on fields are not required here
        // because this is handled by Trace::finalize_glue already.
    }
}

For types defined in the stdlib, please file an issue on this repository (use the unsafe_ignore_trace method shown below to make things work in the meantime).

Note that Trace is only needed for types which transitively contain a Gc, if you are sure that this isn't the case, you may use the unsafe_empty_trace! macro on your types. Alternatively, use the #[unsafe_ignore_trace] annotation on the struct field. Incorrect usage of unsafe_empty_trace and unsafe_ignore_trace may lead to unsafety.

use gc::{Finalize, Gc, Trace};
use bar::Baz;

#[derive(Trace, Finalize)]
struct Foo {
    x: Option<Gc<Foo>>,
    #[unsafe_ignore_trace]
    y: Baz, // we are assuming that `Baz` doesn't contain any `Gc` objects
    // ...
}

To use Gc, simply call Gc::new:

let x = Gc::new(1_u8);
let y = Gc::new(Box::new(Gc::new(1_u8)));

#[derive(Trace, Finalize)]
struct Foo {
    a: Gc<u8>,
    b: u8
}

let z = Gc::new(Foo {a: x.clone(), b: 1})

Calling clone() on a Gc will create another garbage collected reference to the same object. For the most part, try to use borrowed references to the inner value instead of cloning the Gc wherever possible -- Gc implements Deref and is compatible with borrowing.

Gc is an immutable container. Much like with Rc, to get mutability, we must use a cell type. The regular RefCell from the stdlib will not work with Gc (as it does not implement Trace), instead, use GcCell. GcCell behaves very similar to RefCell, except that it internally helps keep track of GC roots.

#[derive(Trace, Finalize)]
struct Foo {
    cyclic: GcCell<Option<Gc<Foo>>>,
    data: u8,
}

let foo1 = Gc::new(Foo {cyclic: GcCell::new(None), data: 1});
let foo2 = Gc::new(Foo {cyclic: GcCell::new(Some(foo1.clone())), data: 2});
let foo3 = Gc::new(Foo {cyclic: GcCell::new(Some(foo2.clone())), data: 3});
*foo1.cyclic.borrow_mut() = Some(foo3.clone());

Known issues

• Destructors should not access Gc/GcCell values. This is enforced by the Trace custom derive automatically implementing Drop with a safe empty drop method. Finalize should be used for cleanup instead.
• There needs to be a better story for cross-crate deriving.
• The current GC is not concurrent and the GCed objects are confined to a thread. There is an experimental concurrent collector in this pull request.

Related projects

• RuScript: Uses single-thread rust-gc to allocate memory for various objects