Garbage collection is contrarian

4 months ago

The Nova JavaScript engine models garbage collection using Rust's borrow checker, ensuring unrooted handles are not kept past garbage collection safepoints.
The current model is criticized for turning code into a 'soup' of bind and unbind calls, with some calling it worse than C++.
The author initially assumed the model was correct, but limitations led to exploring contravariant lifetimes for garbage collected handles.
Garbage collected handles on the heap should have a 'static lifetime, while those on the stack should have a 'local lifetime, shorter than 'static.
The current covariant lifetime model in Nova prevents safe rooting of local handles into the heap, requiring unsafe Rust workarounds.
Contravariant lifetimes allow shortening the lifetime of a handle from the heap to a local lifetime, aligning with garbage collection semantics.
Contravariant references act as 'sinks' where values can be written but not safely read without additional proof, complicating safe API design.
The proposed contravariant handle model would simplify Nova's code by removing many unbind/bind calls, improving ergonomics.
Contravariant references may have broader applications in self-referential data structures, though their safe use in Rust requires further exploration.
Feedback highlighted that a fully safe representation of unrooted handles is possible using invariance, as demonstrated by gc-arena.

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