Disallow client-only code in server components

TanStack Start defines React Server Component boundaries via RSC helpers like renderServerComponent() and createCompositeComponent().

renderServerComponent(<Element />) is a server boundary around the element.
createCompositeComponent((props) => <Element />) is a server boundary around the callback.

Inside these boundaries (and anything rendered from them, unless it crosses a 'use client' boundary) you must not use client-only code.

This rule detects common client-only patterns in server components:

React client hooks (eg useState, useEffect)
browser-only globals (eg window, document)
React event handlers (onClick, etc.)
function props (non-serializable across the server/client boundary)
class components

Rule Details

Examples of incorrect code for this rule:

tsx

/* eslint "@tanstack/start/no-client-code-in-server-component": "error" */

import { createCompositeComponent } from '@tanstack/react-start/rsc'
import { useState } from 'react'

export const Server = createCompositeComponent(() => {
  const [count] = useState(0)
  return <div>{count}</div>
})

tsx

/* eslint "@tanstack/start/no-client-code-in-server-component": "error" */

import { createCompositeComponent } from '@tanstack/react-start/rsc'

export const Server = createCompositeComponent(() => {
  return <div>{window.location.href}</div>
})

Examples of correct code for this rule:

tsx

/* eslint "@tanstack/start/no-client-code-in-server-component": "error" */

import { renderServerComponent } from '@tanstack/react-start/rsc'

export const Server = renderServerComponent(
  (async () => {
    const msg = await Promise.resolve('ok')
    return <div>{msg}</div>
  })(),
)