fix(nuxt): warn about legacy and invalid plugins

packages/nuxt/src/app/nuxt.ts

@@ -181,12 +181,39 @@ export async function applyPlugins (nuxtApp: NuxtApp, plugins: Plugin[]) {
 }
 
 export function normalizePlugins (_plugins: Plugin[]) {
+  const unwrappedPlugins = []
+  const legacyInjectPlugins = []
+  const invalidPlugins = []
+
   const plugins = _plugins.map((plugin) => {
     if (typeof plugin !== 'function') {
-      return () => {}
+      invalidPlugins.push(plugin)
+      return null
+    }
+    if (plugin.length > 1) {
+      legacyInjectPlugins.push(plugin)
+      // Allow usage without wrapper but warn
+      // TODO: Skip invalid in next releases
+      // @ts-ignore
+      return (nuxtApp: NuxtApp) => plugin(nuxtApp, nuxtApp.provide)
+      // return null
+    }
+    if (!isNuxtPlugin(plugin)) {
+      unwrappedPlugins.push(plugin)
+      // Allow usage without wrapper but warn
     }
     return plugin
-  })
+  }).filter(Boolean)
+
+  if (process.dev && legacyInjectPlugins.length) {
+    console.warn('[warn] [nuxt] You are using a plugin with legacy Nuxt 2 format (context, inject) which is likely to be broken. In the future they will be ignored:', legacyInjectPlugins.map(p => p.name || p).join(','))
+  }
+  if (process.dev && invalidPlugins.length) {
+    console.warn('[warn] [nuxt] Some plugins are not exposing a function and skipped:', invalidPlugins)
+  }
+  if (process.dev && unwrappedPlugins.length) {
+    console.warn('[warn] [nuxt] You are using a plugin that has not been wrapped in `defineNuxtPlugin`. It is advised to wrap your plugins as in the future this may enable enhancements:', unwrappedPlugins.map(p => p.name || p).join(','))
+  }
 
   return plugins as Plugin[]
 }
@@ -196,6 +223,10 @@ export function defineNuxtPlugin<T> (plugin: Plugin<T>) {
   return plugin
 }
 
+export function isNuxtPlugin (plugin: unknown) {
+  return typeof plugin === 'function' && NuxtPluginIndicator in plugin
+}
+
 /**
  * Ensures that the setup function passed in has access to the Nuxt instance via `useNuxt`.
  *