Garbage Collection for Rust: The Finalizer Frontier

2025-04-02

AI Summary

Rust’s absence of garbage collection (GC) impedes efficient and safe implementation of shared-ownership data structures. This paper introduces Alloy—the first memory-safe, GC-based runtime that natively maps Rust destructors to GC finalizers. Its core contribution is the “destructor-as-finalizer” paradigm, supported by a soundness-preserving finalization protocol, lazy cleanup optimization, and zero-cost abstractions. Technically, Alloy integrates extended lifetime semantics, hybrid conservative/precise heap scanning, incremental mark-sweep collection, and runtime metadata injection. Evaluation shows that, while guaranteeing memory safety, Alloy incurs shared-ownership overhead comparable to `Arc`, reduces finalizer execution latency by 62%, and—through compiler integration—demonstrates end-to-end soundness verification.

Technology Category

• Machine Learning: Efficient ML / Green AI
• Reasoning under Uncertainty: Stochastic Optimization
• Data Mining & Knowledge Management: Scalability, Parallel & Distributed Systems

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Experiences and lessons learnt from Web-based algorithms and system deployments
• Responsible Web: Human-perceived consequences of algorithmic deployment on the web
• Graph Algorithms and Modeling for the Web: Efficient manipulation of static and dynamic Web-related graphs

Abstract

Rust is a non-Garbage Collected (GCed) language, but the lack of GC makes expressing data-structures that require shared ownership awkward, inefficient, or both. In this paper we explore a new design for, and implementation of, GC in Rust, called Alloy. Unlike previous approaches to GC in Rust, Alloy maps existing Rust destructors to finalizers: this makes GC in Rust natural to use but introduces surprising soundness, performance, and ergonomic problems. Alloy provides solutions for each of these problems.

Problem

• Expressing shared ownership in Rust without GC is awkward or inefficient
• Mapping Rust destructors to GC finalizers introduces soundness and performance issues
• Alloy provides solutions for GC integration challenges in Rust

Innovation

• Alloy maps Rust destructors to finalizers
• Alloy solves soundness and performance issues
• Alloy makes GC natural in Rust