- Sources: Bytecode Alliance post, Wasmtime v47.0.0 release, HN 48981665
- Summary: Nick Fitzgerald described Wasmtime's implementation of the WebAssembly garbage collection and exception handling proposals in a post dated 2026-07-20, the day Wasmtime 47.0.0 was released. The collector is a Cheney-style semi-space copying design with bump-pointer allocation and no read or write barriers. The GC heap lives inside a WebAssembly linear memory and object references are 32-bit indices rather than native pointers, so the post states that a collector bug corrupting the heap still cannot let a malicious module escape the sandbox. Effort has gone to correctness rather than performance, and the post says throughput and latency will not match the collectors in V8 or SpiderMonkey. Component model integration is named as the next milestone, and the
wasm-smith fuzzer cannot yet generate non-nullable references. - Why it matters: Managed languages targeting Wasm no longer have to ship a collector inside the module to run on Wasmtime, and indices rather than pointers keep the sandbox boundary intact when that collector is wrong.
- Follow-up: Watch for component model integration and for the first performance numbers against a mature collector.
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