Svelte 5: Forcing runes mode causes issues with generated code.

[brunnerh]

Describe the bug

Setting compilerOptions: { runes: true } causes errors like this:

CompileError: Cannot use `export let` in runes mode — use $props instead
    at new CompileError (/home/projects/sveltejs-kit-template-default-alrzrc/node_modules/svelte/src/compiler/errors.js:544:3)
    at Module.error (/home/projects/sveltejs-kit-template-default-alrzrc/node_modules/svelte/src/compiler/errors.js:582:8)
    at ExportNamedDeclaration (/home/projects/sveltejs-kit-template-default-alrzrc/node_modules/svelte/src/compiler/phases/2-analyze/validation.js:646:14)
    at go (/home/projects/sveltejs-kit-template-default-alrzrc/node_modules/svelte/src/compiler/phases/visitors.js:62:5)
    at visitor (/home/projects/sveltejs-kit-template-default-alrzrc/node_modules/svelte/src/compiler/phases/visitors.js:77:4)
    at go (/home/projects/sveltejs-kit-template-default-alrzrc/node_modules/svelte/src/compiler/phases/visitors.js:62:5)
    at visitor (/home/projects/sveltejs-kit-template-default-alrzrc/node_modules/svelte/src/compiler/phases/visitors.js:77:4)
    at Object.next (/home/projects/sveltejs-kit-template-default-alrzrc/node_modules/zimmerframe/src/walk.js:119:21)
    at go (/home/projects/sveltejs-kit-template-default-alrzrc/node_modules/svelte/src/compiler/phases/visitors.js:58:29)
    at next (/home/projects/sveltejs-kit-template-default-alrzrc/node_modules/svelte/src/compiler/phases/visitors.js:68:7)

Found by Swarleys on Discord.

Reproduction

https://stackblitz.com/edit/sveltejs-kit-template-default-alrzrc?file=svelte.config.js

Logs

No response

System Info

System:
    OS: Windows 10 10.0.19045
    CPU: (12) x64 Intel(R) Core(TM) i7-8700K CPU @ 3.70GHz
    Memory: 5.07 GB / 31.92 GB
  Binaries:
    Node: 18.13.0 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.19 - ~\AppData\Roaming\npm\yarn.CMD
    npm: 9.2.0 - C:\Program Files\nodejs\npm.CMD
    pnpm: 8.6.9 - ~\AppData\Local\pnpm\pnpm.EXE
  Browsers:
    Chrome: 119.0.6045.105
    Edge: Spartan (44.19041.3570.0), Chromium (119.0.2151.58)
    Internet Explorer: 11.0.19041.3570

Severity

annoyance

Additional Information

No response

[Jerboas86]

I think it is the expected behavior. You ask the compiler to enforce runes syntax at the project level. Doing that, you are not allowed to use svelte 4 reactivity syntax

When you opt in to runes mode, the non-runes features listed in the 'What this replaces' sections are no longer available.

see docs

[brunnerh]

It's not the user code causing the problem and SvelteKit being a first-party framework probably should be able to take that flag into account.

If it causes an unreasonable amount of work to generate code in one format or the other (depending on flag), SvelteKit could at least throw an error that explains the situation, guiding the user.

Generated root.svelte file that has the prop exports:

<!-- This file is generated by @sveltejs/kit — do not edit it! -->
<script>
	import { setContext, afterUpdate, onMount, tick } from 'svelte';
	import { browser } from '$app/environment';

	// stores
	export let stores;
	export let page;

	export let constructors;
	export let components = [];
	export let form;
	export let data_0 = null;
	export let data_1 = null;

...