rsvelte

rsvelte is a Rust port of the Svelte 5 compiler built on OXC. Mochi can route component compilation through it instead of the JavaScript svelte/compiler, which speeds up cold builds and dev rebuilds.

It’s opt-in. Install the adapter package:

bun add -d @mochi-framework/rsvelte

Then pass svelteCompiler:

// src/index.ts
await Mochi.serve({
  port: 3000,
  svelteCompiler: 'rsvelte',
  routes: {
    '/': Mochi.page('./src/Home.svelte'),
  },
});

The default is 'svelte' — without the option nothing changes.

Without touching code

MOCHI_SVELTE_COMPILER overrides the option, which is the easy way to A/B a build:

MOCHI_SVELTE_COMPILER=rsvelte bun run build
MOCHI_SVELTE_COMPILER=svelte bun run build

Mochi logs which compiler it resolved (Svelte compiler: rsvelte@0.2.8+svelte5.56.4 — the rsvelte release, then the Svelte version it targets) once at startup. Since the fallback is silent by design, that line is also how you assert a build really ran on rsvelte.

Benchmark

This benchmark can give you an idea of the gains in a small application (the demos preset you can access via the CLI when creating a new project).

(10 cold start runs, median value)

What runs on rsvelte

Only compile() and compileModule(). Mochi’s island preprocessor walks a real Svelte AST with zimmerframe, and rsvelte’s parse() returns a JSON string rather than an upstream-shaped AST — so parsing and preprocessing always stay on the official compiler. Islands, mochi:hydrate*, mochi:defer and user preprocessors behave identically either way.

If it can’t load

Prebuilt binaries cover macOS arm64/x64, Linux x64/arm64 (glibc) and Windows x64 (MSVC). There’s no musl build, so Alpine-based images aren’t supported. When the package is missing or its binary won’t load, Mochi logs a warning and compiles with svelte/compiler.

Known divergences

Production output should generally be byte-identical to svelte/compiler. Three differences remain in the configuration in relation to Mochi:

• cssHash and warningFilter in svelte.config.js are functions and can’t cross the native boundary. They’re stripped with a one-time warning; use rsvelte’s cssHashOverride: '<hash>' to force a fixed CSS hash.
• Dev-only instrumentation is not always reproduced — $derived(await …) loses its reactivity-loss warning, $effect differs in state-logging, and snippets skip two dev arg-validation guards. Development builds only; production output matches.
• compileModule() emits a vVERSION placeholder in its header comment and different printer whitespace. Semantically identical.