Question about Svelte v5 bundled code and rolldown

[webJose]

Hello, everyone!

I'm trying to set up some (integration - I guess) tests to test my routing library's "tree-shakability" (it is @wjfe/n-savant, if you're curious). My plan is as follows:

1. Create base project with only package.json.
2. Install dev dependencies: rollup, rolldown, rollup-plugin-svelte, plus any other packages to also test bundling with webpack, RsPack, etc.
3. Create configuration files for each of the bundlers.
4. Prepare test cases: One index.js file per test case, where I individually import and use each one of the exports of the library.
5. Create the special test cases base and full, where the former is an index.js with just a console.log() line, and the latter imports and uses every object from the routing library.
6. Run the test matrix (probably in GH Actions).
7. Collect file manifest and total bundle sizes.
8. At least for now, no pass/fail criteria. Just collect data in a nice table and commit a Markdown file.

This sounds like a solid plan to me but do tell me if I'm wrong/delusional. 😄

Anyway, I'm testing the project out with rolldown, and to my surprise the base test case creates a bundled index.js of almost 93kb. Examining the bundled file, it is almost all Svelte code.

Since I'm a noob at tree-shaking and rolldown, my question is: Why does this happen? Or is it even normal for this to happen?

In contrast, bundling the same project with rollup, the bundled JS file only carries the following before the code from my index.js file:

// generated during release, do not modify

const PUBLIC_VERSION = '5';

if (typeof window !== 'undefined') {
	// @ts-expect-error
	((window.__svelte ??= {}).v ??= new Set()).add(PUBLIC_VERSION);
}

Could it be that I'm making a mistake in my rolldown config? Here it is:

import { defineConfig } from "rolldown";
import svelte from "rollup-plugin-svelte"

export default defineConfig({
    input: "index.js",
    output: {
        dir: "bundle"
    },
    treeshake: true,
    plugins: [
        svelte({})
    ]
});