# dtolnay/watt

Runtime for executing procedural macros as WebAssembly

Repository: https://github.com/dtolnay/watt
Canonical: https://ross.abutalabs.com/products/watt
Language: Rust
License: Apache-2.0
License Family: permissive
Last push: 2026-07-19T00:26:44+00:00

## Health v2 (maintenance only)
Score: 65/100 (v2, computed 2026-09-03T02:20:16.233290+00:00)
- activity 93, release rhythm 8, longevity 100
- inputs: {"age_days": 2516, "days_push": 46, "days_rel": null, "gap_med": null, "n_releases_24m": 0}
- flags: none
- formula: round(0.45*activity + 0.35*rhythm + 0.20*longevity); archived -> min(score, 10)

## Adoption (not part of the score)
Stars 1523, forks 31 (observed 2026-08-28T04:04:58.076930+00:00)

## What it is
Watt is a Rust runtime for executing procedural macros that have been compiled ahead-of-time to WebAssembly. It lets downstream users run macros in a small, dependency-free Wasm interpreter instead of compiling the macro and its dependencies from source.

## Use cases
- speed up compilation of crates that depend on procedural macros
- run proc macros in a sandboxed, 100% safe-code environment
- make macro expansion deterministic for build systems
- distribute proc macros as precompiled Wasm bytecode
- reduce dependency compile times for syn/quote-heavy macro crates

## When to choose
- your proc macro and its dependencies are slow to compile and many crates depend on it
- you want strong isolation guarantees so macros can only transform tokens
- you need deterministic macro behavior visible to the build system

## When to avoid
- you need proc macro APIs or dependencies not supported by the Wasm runtime
- your project is small and macro compile time is not a bottleneck
- you need tight integration with the latest unstable proc_macro features

## Facets
- artifact type: library
- maturity: active
- function: compiler, wasm, developer-tools
- domain: developer-tools, compilers, programming-languages
- platform: rust, wasm, cross-platform
- tags: proc-macro, webassembly-runtime, build-time, rust-macros, sandboxing

## Member repositories
- dtolnay/watt (main) score 65

## Provenance
- Observed fields: from GitHub, fetched 2026-08-28T04:04:58.076930+00:00.
- Health v2: computed from the inputs above; adoption is never an input.
- Inferred fields (summary, facets, guidance): AI-extracted, prompt v1, taxonomy v1, on 2026-08-30T04:31:44.532254+00:00, confidence not recorded.
  - readme: https://github.com/dtolnay/watt (fetched 2026-08-28T04:04:58.076930+00:00, sha 973bcbfa7e82)
  - registry_crates: https://crates.io/api/v1/crates/watt (fetched 2026-08-29T11:34:29.481506+00:00, sha 1dda24cbf792)
- Data as of 2026-08-30T08:39:29.467469+00:00.