dumpster: A cycle-tracking garbage collector for Rust

dumpster is an cycle-detecting garbage collector for Rust. It detects unreachable allocations and automatically frees them.

Why should you use this crate?

In short, dumpster offers a great mix of usability, performance, and flexibility.

• dumpster's API is a drop-in replacement for std's reference-counted shared allocations (Rc and Arc).
• It's very performant and has builtin implementations of both thread-local and concurrent garbage collection.
• There are no restrictions on the reference structure within a garbage-collected allocation (references may point in any way you like).
• It's trivial to make a custom type Trace using the provided derive macros.
• You can even store ?Sized data in a garbage-collected pointer!

How it works

dumpster is unlike most tracing garbage collectors. Other GCs keep track of a set of roots, which can then be used to perform a sweep and find out which allocations are reachable and which are not. Instead, dumpster extends reference-counted garbage collection (such as std::rc::Rc) with a cycle-detection algorithm, enabling it to effectively clean up self-referential data structures.

For a deeper dive, check out this blog post.

What this library contains

dumpster actually contains two garbage collector implementations: one thread-local, non-Send garbage collector in the module unsync, and one thread-safe garbage collector in the module sync. These garbage collectors can be safely mixed and matched.

This library also comes with a derive macro for creating custom Trace types.

Examples

use dumpster::{Trace, unsync::Gc};

#[derive(Trace)]
struct Foo {
    ptr: RefCell<Option<Gc<Foo>>>,
}

// Create a new garbage-collected Foo.
let foo = Gc::new(Foo {
    ptr: RefCell::new(None),
});

// Insert a circular reference inside of the foo.
*foo.ptr.borrow_mut() = Some(foo.clone());

// Render the foo inaccessible.
// This may trigger a collection, but it's not guaranteed.
// If we had used `Rc` instead of `Gc`, this would have caused a memory leak.
drop(foo);

// Trigger a collection.
// This isn't necessary, but it guarantees that `foo` will be collected immediately (instead of
// later).
dumpster::unsync::collect();

Installation

To install, simply add dumpster as a dependency to your project.

[dependencies]
dumpster = "1.0.0"

Optional features

dumpster has two optional features: derive and coerce-unsized.

derive is enabled by default. It enables the derive macro for Trace, which makes it easy for users to implement their own Trace types.

use dumpster::{unsync::Gc, Trace};
use std::cell::RefCell;

#[derive(Trace)] // no manual implementation required
struct Foo(RefCell<Option<Gc<Foo>>>);

let my_foo = Gc::new(Foo(RefCell::new(None)));
*my_foo.0.borrow_mut() = Some(my_foo.clone());

drop(my_foo); // my_foo will be automatically cleaned up

coerce-unsized is disabled by default. This enables the implementation of CoerceUnsized for each garbage collector, making it possible to use Gc with !Sized types conveniently.

use dumpster::unsync::Gc;

// this only works with "coerce-unsized" enabled while compiling on nightly Rust
let gc1: Gc<[u8]> = Gc::new([1, 2, 3]);

To use coerce-unsized, edit your installation to Cargo.toml to include the feature.

[dependencies]
dumpster = { version = "1.0.0", features = ["coerce-unsized"]}

License

This code is licensed under the Mozilla Public License, version 2.0. For more information, refer to LICENSE.md.