Reserve `config` export

Describe the problem

We’ve alluded to a vague plan to support route-level deployment config in the future — something like this…

export const config = {
  runtime: 'edge'
};

…where config is a freeform (but serializable) object that gets passed to adapters so that they can use it to customise the deployment.

At the moment, you can export anything from a +(layout|page)(.server)?.js file. This means that adding a new known export would be a breaking change, as someone might already be doing export const config = {...} for some other reason.

Describe the proposed solution

Reserve the config name, i.e. throw an error if a route file exports a binding called config.

Alternatives considered

Prevent any unknown exports. This is safer, but might feel unduly restrictive.

Importance

would make my life easier

Additional Information

No response

[AlecAivazis]

Just to reiterate - I dont think this conversation is super relevant to the original issue. Nor do I expect this to be resolved before 1.0 can get released. I understand the want to group as many breaking changes as possible before 1.0 and i would feel very bad if this conversation was blocking the release. To me, it seems like the _ prefix provides all of the future-proofing required and leaves the rest of the conversation for another time.

[gterras]

Some advanced (and very useful) plugins like https://github.com/HoudiniGraphql/houdini also take advantage of exporting stuff there. Overall if feels unnecessarily restricting and will close the door to many potential ingenious plugins later on (which is sad considering plugins may become very important for svelte-kit at some point).