---

Why

• Byte-exact JS output — diffed against the reference compiler on 1500+ end-to-end cases (compiler + diagnostics).
• Drop-in — same compile() / compileModule() API as svelte/compiler.
• 30× faster (geomean across 24 benches, up to 52× on large components).
• Wired into a fork of vite-plugin-svelte, so you can try it in a real Vite app today.

Requirements

Node	^20.19 || ^22.12 || >=24
Platforms	macOS arm64/x64, Linux x64 glibc. Windows and Linux musl (Alpine) not yet.
Peer	[email protected] — Vite plugin falls back to svelte/compiler for unsupported options.

What Works So Far?

This is still a work in progress and is not yet at full feature parity with svelte/compiler. Parity target: svelte@5.55.5. Bugs may exist. Please check this list carefully before logging a new issue or assuming an intentional change.

Feature	Status	Notes
Svelte 5 syntax	done	Runes, template, bindings, directives, events, special elements, diagnostics, a11y. Byte-exact diff against svelte/compiler.
Svelte 4 legacy	in progress	export let, $:, beforeUpdate/afterUpdate, <svelte:component>, auto-mode detection done. <slot> / <svelte:self> still pending.
CSS pipeline	done	analyze + transform + codegen
TypeScript	done	Script stripping only — no type checking.
.svelte.js / .svelte.ts modules	done	—
Custom elements	in progress	Basic path works; some option combinations not covered.
Compiler options	in progress	Most common options honored; long tail (discloseVersion, etc.) still landing.
Dev mode (dev: true)	in progress	Runtime checks and ownership tracking land case-by-case.
Source maps	in progress	Partial implementation, not yet verified against reference.
Preprocessors	not ready	Not started.
SSR (generate: 'server')	not ready	Not started — large chunk of work.
HMR	not ready	Not started.

Definitions:

• done aka "believed done": we're not currently aware of any deficits or major left work to do. OK to log bugs.
• in progress: currently being worked on; some things may work, some may not. OK to log panics, but nothing else please.
• prototype: proof-of-concept only; do not log bugs.
• not ready: either haven't even started yet, or far enough from ready that you shouldn't bother messing with it yet.

Full breakdown: ROADMAP.md.

Known limitations

• ast option is not supported — passing it throws; the returned ast is always null.
• sourcemap and outputFilename options are not supported — passing them throws.
• modernAst: true is accepted but ignored (emits unsupported_option_ignored warning).
• js.map / css.map are always null (source maps are WIP).
• dev: true runtime checks land case-by-case — not all ownership / hydration diagnostics are emitted yet.

Install

As a compiler

npm i -D @mrwaip/svelte-rs2

import { compile } from '@mrwaip/svelte-rs2/compiler';

const { js } = compile(
  `<script>let { name } = $props();</script><h1>hello {name}</h1>`,
  { filename: 'Hello.svelte', generate: 'client' }
);

console.log(js.code);

API mirrors svelte/compiler. Unsupported options (ast, sourcemap, outputFilename) throw; see packages/svelte-rs2/compiler/index.d.ts.

Want to see real input → output? Browse tasks/compiler_tests/cases2/ — each folder has case.svelte (input), case-rust.js (our output) and case-svelte.js (reference). 1100+ cases, byte-diffed.

In a Vite app

Use the fork of vite-plugin-svelte — it routes compile / compileModule through @mrwaip/svelte-rs2 automatically, with fallback to svelte/compiler where the Rust side doesn't support an option yet.

npm i -D @mrwaip/vite-plugin-svelte

import { defineConfig } from 'vite';
import { svelte } from '@mrwaip/vite-plugin-svelte';

export default defineConfig({
  plugins: [svelte()],
});

Source: https://github.com/MrWaip/vite-plugin-svelte.

Benchmarks

Geomean: 30.18× across 24 cases (min 20.14×, max 51.97×). Walltime vs svelte/compiler, measured 2026-05-04 via just bench-compare-walltime. big_v6.svelte is a synthetic stress component (~5.8k LOC); the rest are real-world Svelte files.

Per-case table

case	rust med	js med	speedup
big_v6.svelte (compile, synthetic stress)	18.39 ms	955.9 ms	51.97×
big_v6.svelte (compile_dev, synthetic stress)	19.56 ms	944.7 ms	48.30×
module/case_02.svelte.js (real-world)	0.131 ms	5.283 ms	40.37×
template/case_01.svelte (real-world)	0.745 ms	21.65 ms	29.08×
snippets/case_01.svelte (real-world)	1.558 ms	35.62 ms	22.86×
css/case_01.svelte (real-world)	0.014 ms	0.290 ms	20.14×

Try it locally

Requires Rust, Node, and just (cargo install just or brew install just).

just playground               # build wasm + serve playground
just quick-check App.svelte   # diff one component against svelte/compiler
just test-compiler            # run the 1500+ e2e suite

Contributing

Standard PRs welcome — no AI required. Day-to-day work uses Claude Code; slash commands live in .claude/commands/ (/status, /port, /audit). Before opening a PR: just test-compiler && just test-diagnostics && just lint must be green.

Bugs and questions: https://github.com/MrWaip/svelte-rs-2/issues.

Acknowledgements

• Svelte — the compiler this project mirrors; reference output is the source of truth.
• OXC — JS parser, AST, and codegen.
• vite-plugin-svelte — base of the Vite integration fork.

License

MIT © Lobkov Constantine