A few week ago, I finished implementing support for the WebAssembly reference types proposal in Wasmtime. Wasmtime is a standalone, outside-the-Web WebAssembly runtime, and the reference types proposal is WebAssembly’s first foray beyond simple integers and floating point numbers, into the exciting world of garbage-collected references. This article will explain what the reference types proposal enables, what it leaves for future proposals, and how it is implemented in Wasmtime.

What are Reference Types?

Without the reference types proposal, WebAssembly can only manipulate simple integer and floating point values. It can’t take or return references to the host’s objects like, for example, a DOM node on a Web page or an open connection on a server. There are workarounds: for example, you can store the host objects in a side table and refere to them by index, but this adds an extra indirection and implementing the side table requires cooperating glue code on the host side. That glue code, in particular, is annoying because it is outside of the Wasm sandbox, diluting Wasm’s safety guarantees, and it is host-specific. If you want to use your Wasm module on the Web, in a Rust host, and in a Python host, you’ll need three separate glue code implementations. This makes WebAssembly less attractive as a universal binary format.

With the reference types proposal, you don’t need glue code to interact with host references. The proposal has three main parts:

1. A new externref type, representing an opaque, unforgable reference to a host object.
2. An extension to WebAssembly tables, allowing them to hold externrefs in addition to function references.
3. New instructions for manipulating tables and their entries.

With these new capabilities, Wasm modules can talk about host references directly, rather than requiring external glue code running in the host.

externrefs play nice with WebAssembly’s sandboxing properties:

• They are opaque: a Wasm module cannot observe an externref value’s bit pattern. Passing a reference to a host object into a Wasm module doesn’t reveal any information about the host’s address space and the layout of host objects within it.
• They are unforgable: a Wasm module can’t create a fake host reference out of thin air. It can only return either a reference you already gave it or the null reference. It cannot pretend like the integer value 0x1bad2bad is a valid host reference, return it to you, and trick you into dereferencing this invalid pointer.

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