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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.