Runtime for executing Rust procedural macros compiled as WebAssembly

14 releases

0.5.0 Sep 13, 2023
0.4.4 Jul 15, 2023
0.4.3 Mar 5, 2023
0.4.2 Aug 3, 2022
0.3.0 Nov 3, 2019

#46 in WebAssembly

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github crates.io docs.rs

Watt is a runtime for executing Rust procedural macros compiled as WebAssembly.

watt = "0.5"

Compiler support: requires rustc 1.42+


  • Faster compilation. By compiling macros ahead-of-time to Wasm, we save all downstream users of the macro from having to compile the macro logic or its dependencies themselves.

    Instead, what they compile is a small self-contained Wasm runtime (~3 seconds, shared by all macros) and a tiny proc macro shim for each macro crate to hand off Wasm bytecode into the Watt runtime (~0.3 seconds per proc-macro crate you depend on). This is much less than the 20+ seconds it can take to compile complex procedural macros and their dependencies.

  • Isolation. The Watt runtime is 100% safe code with zero dependencies. While running in this environment, a macro's only possible interaction with the world is limited to consuming tokens and producing tokens. This is true regardless of how much unsafe code the macro itself might contain! Modulo bugs in the Rust compiler or standard library, it is impossible for a macro to do anything other than shuffle tokens around.

  • Determinism. From a build system point of view, a macro backed by Wasm has the advantage that it can be treated as a purely deterministic function from input to output. There is no possibility of implicit dependencies, such as via the filesystem, which aren't visible to or taken into account by the build system.

Getting started

Start by implementing and testing your proc macro as you normally would, using whatever dependencies you want (syn, quote, etc). You will end up with something that looks like:

use proc_macro::TokenStream;

pub fn the_macro(input: TokenStream) -> TokenStream {
    /* ... */

#[proc_macro_derive] and #[proc_macro_attribute] are supported as well; everything is analogous to what will be shown here for #[proc_macro].

When your macro is ready, there are just a few changes we need to make to the signature and the Cargo.toml. In your lib.rs, change each of your macro entry points to a no_mangle extern "C" function, and change the TokenStream in the signature from proc_macro to proc_macro2.

It will look like:

use proc_macro2::TokenStream;

pub extern "C" fn the_macro(input: TokenStream) -> TokenStream {
    /* same as before */

Now in your macro's Cargo.toml which used to contain this:

# my_macros/Cargo.toml

proc-macro = true

change it instead to say:

crate-type = ["cdylib"]

proc-macro2 = { git = "https://github.com/dtolnay/watt" }

This crate will be the binary that we compile to Wasm. Compile it by running:

$ cargo build --release --target wasm32-unknown-unknown

Next we need to make a small proc-macro shim crate to hand off the compiled Wasm bytes into the Watt runtime. It's fine to give this the same crate name as the previous crate, since the other one won't be getting published to crates.io. In a new Cargo.toml, put:

proc-macro = true

watt = "0.5"

And in its src/lib.rs, define real proc macros corresponding to each of the ones previously defined as no_mangle extern "C" functions in the other crate:

use proc_macro::TokenStream;
use watt::WasmMacro;

static MACRO: WasmMacro = WasmMacro::new(WASM);
static WASM: &[u8] = include_bytes!("my_macros.wasm");

pub fn the_macro(input: TokenStream) -> TokenStream {
    MACRO.proc_macro("the_macro", input)

Finally, copy the compiled Wasm binary from target/wasm32-unknown-unknown/release/my_macros.wasm under your implementation crate, to the src directory of your shim crate, and it's ready to publish!

Remaining work

  • Performance. Watt compiles pretty fast, but so far I have not put any effort toward optimizing the runtime. That means macro expansion can potentially take longer than with a natively compiled proc macro.

    Note that the performance overhead of the Wasm environment is partially offset by the fact that our proc macros are compiled to Wasm in release mode, so downstream cargo build will be running a release-mode macro when it would have been running debug-mode for a traditional proc macro.

    A neat approach would be to provide some kind of cargo install watt-runtime which installs an optimized Wasm runtime locally, which the Watt crate can detect and hand off code to if available. That way we avoid running things in a debug-mode runtime altogether. The experimental beginnings of this can be found under the jit/ directory.

  • Tooling. The getting started section shows there are a lot of steps to building a macro for Watt, and a pretty hacky patching in of proc-macro2. Ideally this would all be more straightforward, including easy tooling for doing reproducible builds of the Wasm artifact for confirming that it was indeed compiled from the publicly available sources.

  • RFCs. The advantages of fast compile time, isolation, and determinism may make it worthwhile to build first-class support for Wasm proc macros into rustc and Cargo. The toolchain could ship its own high performance Wasm runtime, which is an even better outcome than Watt because that runtime can be heavily optimized and consumers of macros don't need to compile it.

This can't be real

To assist in convincing you that this is real, here is serde_derive compiled to Wasm. It was compiled from the commit serde-rs/serde@1afae183. Feel free to try it out as:

// [dependencies]
// serde = "1.0"
// serde_json = "1.0"
// wa-serde-derive = "0.1"

use wa_serde_derive::Serialize;

struct Watt {
    msg: &'static str,

fn main() {
    let w = Watt { msg: "hello from wasm!" };
    println!("{}", serde_json::to_string(&w).unwrap());


The current underlying Wasm runtime is a fork of the Rust-WASM project by Yoann Blein and Hugo Guiroux, a simple and spec-compliant WebAssembly interpreter.


Everything outside of the `runtime` directory is licensed under either of Apache License, Version 2.0 or MIT license at your option. The `runtime` directory is licensed under the ISC license.
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

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