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https://github.com/kyren/gc-arena

Incremental garbage collection from safe Rust
https://github.com/kyren/gc-arena

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Incremental garbage collection from safe Rust

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## gc-arena

This repo is home to the `gc-arena` crate, which provides Rust with garbage
collected arenas and a means of safely interacting with them.

The `gc-arena` crate, along with its helper crate `gc-arena-derive`, provides
allocation with safe, incremental, exact, cycle-detecting garbage collection
within a closed "arena". There are two techniques at play that make this system
sound:

* Garbage collected objects are traced using the `Collect` trait, which must
be implemented correctly to ensure that all reachable objects are found. This
trait is therefore `unsafe`, but it *can* safely be implemented by procedural
macro, and the `gc-arena-derive` provides such a safe procedural macro.

* In order for garbage collection to take place, the garbage collector must
first have a list of "root" objects which are known to be reachable. In our
case, the user of `gc-arena` chooses a single root object for the arena, but
this is not sufficient for safe garbage collection. If garbage collection
were to take place when there are garbage collected pointers anywhere on the
Rust stack, such pointers would also need to be considered as "root" objects
to prevent memory unsafety. `gc-arena` solves this by strictly limiting where
garbage collected pointers can be stored, and when they can be alive. The
arena can only be accessed through a single `mutate` method which takes a
callback, and all garbage collected pointers inside this callback are branded
with an invariant lifetime which is unique to that single callback call. Thus,
when outside of this `mutate` method, the rust borrow checker ensures that
it is not possible for garbage collected pointers to be alive anywhere on
the stack, nor is it possible for them to have been smuggled outside of the
arena's root object. Since all pointers can be proven to be reachable from the
single root object, safe garbage collection can take place.

In other words, the `gc-arena` crate does *not* retrofit Rust with a globally
accessible garbage collector, rather it *only* allows for limited garbage
collection in isolated garbage collected arenas. All garbage collected pointers
must forever live inside only this arena, and pointers from different arenas are
prevented from being stored in the wrong arena.

See [this blog post](https://kyju.org/blog/rust-safe-garbage-collection/) for a
more in-depth tour of the crate's design. It is quite dense, but it explains
everything necessary to fully understand the machinery used in the included
[linked list example](examples/linked_list.rs).

## Use cases

This crate was developed primarily as a means of writing VMs for garbage
collected languages in safe Rust, but there are probably many more uses than
just this.

## Current status and TODOs

Basically usable and safe! It is used by the Adobe Flash Player emulator
[Ruffle](https://github.com/ruffle-rs/ruffle) for its ActionScript VM as well
as some other projects (like my own stackless Lua runtime
[piccolo](https://github.com/kyren/piccolo), for which the crate was originally
designed)

The collection algorithm is an incremental mark-and-sweep algorithm very similar
to the one in PUC-Rio Lua, and is optimized primarily for low pause time. During
mutation, allocation "debt" is accumulated, and this "debt" determines the
amount of work that the next call to `Arena::collect` will do.

The pointers held in arenas (spelled `Gc<'gc, T>`) are zero-cost raw pointers.
They implement `Copy` and are pointer sized, and no bookkeeping at all is done
during mutation.

Some notable current limitations:

* Allocating DSTs is currently somewhat painful due to limitations in Rust. It
is possible to have `Gc` pointers to DSTs, and there is a replacement for
unstable `Unsize` coercion, but there is no support for directly allocating
arbitrarily sized DSTs.

* There is no support at all for multi-threaded allocation and collection.
The basic lifetime and safety techniques here would still work in an arena
supporting multi-threading, but this crate does not support this. It is
optimized for single threaded use and multiple, independent arenas.

* The `Collect` trait does not provide a mechanism to move objects once they are
allocated, so this limits the types of collectors that could be written. This
is achievable but no work has been done towards this.

## Prior Art

The ideas here are mostly not mine. Much of the user-facing design is borrowed
heavily from [rust-gc](
https://manishearth.github.io/blog/2015/09/01/designing-a-gc-in-rust/),
and the idea of using "generativity" comes from [You can't spell trust without
Rust](https://raw.githubusercontent.com/Gankro/thesis/master/thesis.pdf). The
design of the actual garbage collection system itself borrows heavily from the
incremental mark-and-sweep collector in Lua 5.4.

## License

Everything in this repository is licensed under either of:

* MIT license [LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT
* Creative Commons CC0 1.0 Universal Public Domain Dedication
[LICENSE-CC0](LICENSE-CC0) or
https://creativecommons.org/publicdomain/zero/1.0/

at your option.