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 is 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 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) once at startup. That line is also how you confirm a build ran on rsvelte, since the fallback is silent.

Benchmark

This benchmark shows the gains on a small application (the demos preset from the CLI):

(10 cold-start runs, median value)

What runs on rsvelte

Only compile() and compileModule() run on rsvelte. Parsing and preprocessing always stay on the official compiler, so islands, mochi:hydrate*, mochi:defer, and user preprocessors behave identically either way.

If it cannot load

Prebuilt binaries cover macOS arm64/x64, Linux x64/arm64 (glibc), and Windows x64 (MSVC). There is no musl build, so Alpine-based images are unsupported. When the package is missing or its binary fails to 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:

• cssHash and warningFilter in svelte.config.js are functions and cannot cross the native boundary. They are stripped with a one-time warning. Use rsvelte’s cssHashOverride: '<hash>' to force a fixed CSS hash.
• Dev-only instrumentation is not always reproduced. This affects development builds only. Production output matches.
• compileModule() emits a vVERSION placeholder in its header comment and different printer whitespace. Semantically identical.