A simple component without script tag generates broken code when compiled

[kran6a]

Describe the bug

I have the following dumb svelte 5 (5.0.0-next.151) component (src/Index.svelte):

<h1>AAA</h1>

Along with the following vite.config.ts:

import type {RollupOptions} from "rollup";
import {defineConfig} from 'vite';
import {svelte} from '@sveltejs/vite-plugin-svelte';

const ROLLUP_OPTIONS = {
    output: {
        format: 'esm',
        preserveModules: false,
        entryFileNames: 'index.js',
        esModule: true
    },
} satisfies RollupOptions;

export default defineConfig({
    mode: 'development',
    build: {
        rollupOptions: ROLLUP_OPTIONS,
        lib: {
            formats: ['es'],
            entry: './src/Index.svelte'
        },
        target: 'esnext',
        emptyOutDir: true,
        outDir: './dist',
        minify: false,
    },
    plugins: [svelte()]
});

It produces the following code:

// generated during release, do not modify

const PUBLIC_VERSION = '5';

if (typeof window !== 'undefined')
	// @ts-ignore
	(window.__svelte ||= { v: new Set() }).v.add(PUBLIC_VERSION);

const TEMPLATE_USE_IMPORT_NODE = 1 << 1;

// Store the references to globals in case someone tries to monkey patch these, causing the below
// to de-opt (this occurs often when using popular extensions).
var is_array = Array.isArray;

/** @type {null | import('#client').Effect} */
let current_effect = null;

/** @param {string} html */
function create_fragment_from_html(html) {
	var elem = document.createElement('template');
	elem.innerHTML = html;
	return elem.content;
}

/**
 * @template {import("#client").TemplateNode | import("#client").TemplateNode[]} T
 * @param {T} dom
 * @param {import("#client").Effect} effect
 */
function push_template_node(
	dom,
	effect = /** @type {import('#client').Effect} */ (current_effect)
) {
	var current_dom = effect.dom;
	if (current_dom === null) {
		effect.dom = dom;
	} else {
		if (!is_array(current_dom)) {
			current_dom = effect.dom = [current_dom];
		}

		if (is_array(dom)) {
			current_dom.push(...dom);
		} else {
			current_dom.push(dom);
		}
	}
	return dom;
}

/**
 * @param {string} content
 * @param {number} flags
 * @returns {() => Node | Node[]}
 */
/*#__NO_SIDE_EFFECTS__*/
function template(content, flags) {
	var use_import_node = (flags & TEMPLATE_USE_IMPORT_NODE) !== 0;

	/** @type {Node} */
	var node;

	return () => {

		if (!node) {
			node = create_fragment_from_html(content);
			node = /** @type {Node} */ (node.firstChild);
		}

		var clone = use_import_node ? document.importNode(node, true) : node.cloneNode(true);

		push_template_node(
			/** @type {import('#client').TemplateNode} */ (clone)
		);

		return clone;
	};
}

/**
 * Assign the created (or in hydration mode, traversed) dom elements to the current block
 * and insert the elements into the dom (in client mode).
 * @param {Text | Comment | Element} anchor
 * @param {DocumentFragment | Element} dom
 */
function append(anchor, dom) {
	// We intentionally do not assign the `dom` property of the effect here because it's far too
	// late. If we try, we will capture additional DOM elements that we cannot control the lifecycle
	// for and will inevitably cause memory leaks. See https://github.com/sveltejs/svelte/pull/11832

	anchor.before(/** @type {Node} */ (dom));
}

var root = template(`<h1>AAA</h1>`);

function Index($$anchor) {
	var h1 = root();

	append($$anchor, h1);
}

export { Index as default };

When I import the generated js code and try to mount it on a DOM node it does not work as I get Uncaught (in promise) TypeError: Cannot read properties of null (reading 'dom')

This error happens because it does var current_dom = effect.dom; where effect is default parameter value (current_effect), which is assigned null at the top of the file and is never reassigned.

Reproduction

Create the Index.svelte and vite config described above and run vite build

Logs

No response

System Info

System:
    OS: Windows 11 10.0.22621
    CPU: (12) x64 AMD Ryzen 5 3600XT 6-Core Processor
    Memory: 20.82 GB / 63.93 GB
  Binaries:
    Node: 22.2.0 - C:\Program Files\nodejs\node.EXE
    npm: 10.7.0 - C:\Program Files\nodejs\npm.CMD
    pnpm: 9.1.1 - C:\Program Files\nodejs\pnpm.CMD
    bun: 1.1.12 - ~\.bun\bin\bun.EXE
  Browsers:
    Edge: Chromium (125.0.2535.85)
    Internet Explorer: 11.0.22621.3527
  npmPackages:
    rollup: ^4.18.0 => 4.18.0
    svelte: 5.0.0-next.151 => 5.0.0-next.151

Severity

annoyance