Garbage Collection for Rust: The Finalizer Frontier

Summary

Garbage Collection for Rust: The Finalizer Frontier October, 2025 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 allows existing Rust destructors to be automatically used as GC finalizers: this makes Alloy integrate better with existing Rust code than previous solutions but introduces surprising soundness and performance problems. Alloy provides novel solutions for the core problems: finalizer safety analysis rejects unsound destructors from automatically being reused as finalizers; finalizer elision optimises away unnecessary finalizers; and premature finalizer prevention ensures that finalizers are only run when it is provably safe to do so. 1 Introduction Amongst the ways one can classify programming languages are whether they are Garbage Collected (GCed) or not: GCed languages enable implicit memory management; non-GCed languages require explicit memory management (e.g C's malloc / free functions). Rust's use of affine types [25, p. 5] and ownership does not fit within this classification: it is not GCed but it has implicit scope-based memory management. Most portions of Rust programs are as succinct as a GCed equivalent, but ownership is too inflexible to express shared ownership for data-structures that require multiple owners (e.g. doubly linked lists). Workarounds such as reference counting impose an extra burden on the programmer, make mistakes more likely, and often come with a performance penalty. In an attempt to avoid such problems, there are now a number of GCs for Rust (e.g. [2, 11, 14, 32, 33]). Most introduce a user-visible type Gc<T> which takes a value v of type T and moves v to the 'GC heap'. The Gc<T> value itself is a wrapper around a pointer to v on the GC heap. Gc<T> can be cloned (i...