Svelte 5: Throw an error when importing svelte/internal/*

[dummdidumm]

Describe the problem

People have reached into svelte/internal too much in the past. We should discourage it as much as possible and throw a compiler error when someone imports from it. People who reaaaally wanna do it can hide it by importing it from a regular JS file and reexport it.

Describe the proposed solution

Error on svelte/internal/client and svelte/internal/server

Importance

nice to have

[dominikg]

warn in non-runes mode, error in runes mode 👍

The svelte compiler can only do this in .svelte and .svelte.js files, technically someone could be doing it in regular js files and reexport to fool it. There is an issue on v-p-s to do it on the resolve level sveltejs/vite-plugin-svelte#861

and iirc eslint-plugin-svelte also added a warning for this recently.

[trueadm]

I think we should error.

[paoloricciuti]

As a side note tho might be worth checking what they were using internals for? If we notice a pattern that is very useful for library authors might be worth exporting it for real?

[dominikg]

@JLarky please don't give people ideas. As the name says and our documentation and errors show, svelte/internal is for internal use only. You must not use it or provide guidance on how to do so. There are no guarantees whatsoever that these interfaces are stable or change at any point in time without notice. Your app or the apps of users using your library that contains this code are going to break and it will be all your fault.

TL;DR: don't use svelte/internal, period.