#garbage-collection #gc #garbage #collection

nightly sys bronze_gc

The Bronze garbage collector for Rust. This version only includes the API for creating and using GC references; it does not actually collect anything. For experimental purposes only.

4 releases (2 breaking)

0.2.1 Jan 28, 2022
0.2.0 Jan 19, 2022
0.1.0 Apr 11, 2021
0.0.1 Mar 26, 2021

#518 in Memory management

MIT license

855 lines



To use Bronze, you must use the nightly Rust tools. To install them, run rustup default nightly on the command line.


Sometimes, the requirements of ownership in Rust can be challenging to address. For example, implementing a doubly-linked list in Rust is notoriously hard because each node must be referenced twice. Also, especially for people who are learning Rust, it may be easier to avoid the every variable has one owner restriction for the time being.

Bronze relaxes some of the restrictions that Rust has by introducing a new smart pointer type, GcRef, which describes a pointer to a garbage-collected heap location. When using Bronze, data located on the stack has all the usual Rust ownership requirements. However, Bronze allows moving data to the heap. If a value of type T is on the heap, Bronze allows an arbitrary number of references of type GcRef<T> to that value.

For example, without Bronze, we have to carefully manage references and lifetimes:

pub struct IntContainer {
    n: i32,

pub fn set(c: &mut IntContainer, n: i32) {
    c.n = n;

pub fn test() {
    let c1 = IntContainer{n: 42};
    let mut c2 = c1;
    // Can't use c1 anymore because it's been moved to c2
    set(&mut c2, 42);

With Bronze, types that are not Copy can be referenced through smart pointers, which are Copy. Temporary immutable and immutable borrows can be obtained via borrow() and borrow_mut; these implement Deref, allowing convenient access.

#[derive(Trace, Finalize)]
pub struct IntContainer {
    n: i32,

pub fn set(mut c: GcRef<IntContainer>, n: i32) {
    c.borrow_mut().n = n;

pub fn test() {
    let c1 = GcRef::new(IntContainer{n: 42});
    let c2 = c1; 
    // Now c1 and c2 both reference the same object.
    set(c2, 42);
    set(c1, 43);
    // Now they both reference an object with value 43.

To create a new GcRef<T>, call GcRef::new(e), where e is an expression whose value you want to be on the GC heap.

Advanced Usage

If you need to remove data from the GC heap, you can use a GcNullableRef rather than a GcRef. You can create one with Gc::new_nullable. GcNullableRef is like GcRef but adds a method remove. The first call to remove returns an Option populated with the data that was previously in the GC heap. Future calls to remove return None.


An earlier version of Bronze allowed users to obtain multiple mutable references to a GC object. This should no longer be possible; instead, Bronze dynamically tracks borrows (using a very similar mechanism to that used in RefCell). To obtain an immutable or mutable borrowed reference to a GC object, call borrow() or borrow_mut() on the GcRef.

Experimental Implementation

This implementation is experimental. In particular, the collector will not run; be aware that you will eventually run out of memory. However, the present version is suitable for experimentation and prototyping.


~40K SLoC