Contravariant lifetimes and garbage collection: Inside Nova's Rust-based JavaScript engine

Summary

This article from the Nova blog discusses the contrarian approach to garbage collection (GC) in the Nova JavaScript engine, which is written in Rust. The author explains how they model GC using the Rust borrow checker with contravariant lifetimes, a novel approach that goes against typical GC design patterns. The piece covers the philosophical underpinnings of this model, the practical implementation challenges, and the trade-offs involved in building a 100,000+ line Rust JavaScript engine that uses this unconventional GC strategy. The author reflects on both the excellence and awfulness of this approach, particularly how it manages to explain garbage collected handles in a way the borrow checker can understand.