@mochi-framework/rsvelte

Opt-in rsvelte backend for mochi-framework. rsvelte is a Rust port of the Svelte 5 compiler built on OXC; this package adapts it to Mochi's compiler-backend contract so component compilation runs natively instead of through the JavaScript svelte/compiler.

Install

bun add -d @mochi-framework/rsvelte

Use

await Mochi.serve({
  svelteCompiler: 'rsvelte',
  routes,
});

Or without touching code — the env var wins over the option, which is handy for A/B timing an existing app:

MOCHI_SVELTE_COMPILER=rsvelte bun run build

If this package isn't installed (or its native binary won't load on the current platform), Mochi logs a warning and falls back to svelte/compiler. Builds never fail over it.

What is and isn't swapped

Only compile() and compileModule() run on rsvelte. 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. The output rsvelte emits is verified byte-identical to upstream by this package's test suite.

Caveats

• Pre-1.0. rsvelte's own README warns that APIs and behaviour may change without notice.
• Function-valued compiler options are ignored. cssHash and warningFilter cannot cross the native boundary. This package strips them and warns once. For cssHash, use rsvelte's cssHashOverride: '<hash>' to force a fixed value.
• $derived(await …) in dev client builds loses Svelte's dev-only reactivity-loss instrumentation. Server output and non-dev client output match upstream byte-for-byte.
• compileModule() output differs cosmetically — a vVERSION placeholder in the generated header comment and different printer whitespace. Semantically identical.
• Platforms. Prebuilt binaries cover macOS arm64/x64, Linux x64/arm64 (glibc) and Windows x64 (MSVC). There is no musl build, so Alpine-based images fall back to svelte/compiler.
• Switching backends invalidates compiled output. Mochi's in-memory compile cache is keyed on the backend, but a prebuilt manifest.json is not — run bun run clean and rebuild after changing svelteCompiler.