dtolnay/watt — Runtime for executing procedural macros as WebAssembly

Description and use

**Brief summary**
dtolnay/watt is a Rust‑based runtime that lets you run procedural‑macro code compiled to WebAssembly, enabling macro execution without a full Rust compiler toolchain. With over 1.5 k stars and recent updates, it is a mature‑looking prototype that can simplify macro‑driven workflows, especially in CI pipelines or sandboxed environments.

**Value proposition**
Watt decouples procedural‑macro execution from the host compiler, allowing you to ship macro logic as a portable WASM module. This reduces build‑time dependencies, improves isolation (security‑wise and crash‑wise), and makes it easier to reuse macros across different Rust toolchains or even non‑Rust build systems.

**Practical adoption path**

1. **Evaluate the README and examples** – clone the repo, build the watt binary, and try the provided demo macro to confirm that your macro can be compiled to WASM and invoked via the runtime.
2. **Integrate into your workflow** – replace the usual rustc‑driven macro step with a call to watt run <wasm-file> <input> in your build script or CI job.
3. **Validate the toolchain** – ensure your CI environment can install the required Rust nightly (for proc-macro support) and the wasm32-unknown-unknown target; add a small wrapper script to automate this.
4. **Run a pilot** – apply the change to a low‑risk repository or a feature branch, monitor build times, and confirm that error reporting from the WASM runtime is acceptable.

**Production readiness**
The project sits at a medium readiness level: it is actively maintained (last commit 2026‑06‑24) and has a healthy community signal (≈1.5 k stars), but the integration documentation is sparse and the exact API surface for embedding the runtime is not fully described. For prototypes, internal tooling, or CI sandboxes, Watt can be adopted quickly after the manual validation steps above. For production use, you should perform a dedicated security review of the WASM sandbox, lock the runtime version, and set up automated tests to catch any breaking changes in future releases.

Problem

dtolnay/watt may be useful when its README and activity match a concrete workflow.

Outcome

Medium: useful for prototypes or internal workflows, with dependency and maintenance checks before production.

Evidence

Runtime for executing procedural macros as WebAssembly

Source