[Lint Rule] Warn on files that export server-actions mixed with non server-actions

[Tbhesswebber]

Basic Request

Add a rule that checks to see if a file exports a variable defined with createServerFn and, if so, warn on any exports that aren't defined with createServerFn

Why

This rule attempts to mitigate (some of?) the issues about server dependencies being loaded in the browser. I don't know all of the edge cases for when this bundle bleed-through happens vs not, but even just a primitive rule that doesn't address the edge cases would be pretty helpful.

Additional Context

I got lost for quite a while the other day digging into why my database driver was being loaded into the browser. I only noticed that that was happening at all because some of the Clerk components stopped rendering (silent failures are fun!) and I opened the console to see what was going on. After a ton of searching the internet (especially looking at GitHub issues), I performed a git bisect to identify when the issue was created and finally identified the culprit by reverting that commit and slowly checking out files from the commit to see when things start to break. With this rule, I would have saved ~6 hours.

Examples

Note: I don't know for certain that all of these are actually passes and failures. Once this bit me, I only write actions in a files named actions.ts with no other exports just in case.

No error - exporting only server functions is fine

import { createServerFn } from "@tanstack/start";

export const createUser = createServerFn(/* code */);

export const deleteUser = createServerFn(/* code */);

Warn - mixing exports of server functions and with non-server functions can cause bugs

import { createServerFn } from "@tanstack/start";

export const createUser = createServerFn(/* code */);

export function isUser() {/* code */}

Warn - mixing exports of server functions and variables can cause bugs

import { createServerFn } from "@tanstack/start";

export const createUser = createServerFn(/* code */);

export const foo = 0;

No error(?) - server functions defined next to the components that use them are fine (I think?)

import { createServerFn } from "@tanstack/start";

export const createUser = createServerFn(/* code */);

export function CreateUserButton() {
  return <button onClick=(() => createUser())> Create </button>
}

No error(?) - server functions defined next abstractions around them are fine (I think?)

import { createServerFn } from "@tanstack/start";

const createUser = createServerFn(/* code */);

export function createUserAndCelebrate(data) {
  return createUser({ data })
    .then((user) => {
      setShowConfetti()
    })
}