fix(suspense): warn when using asnyc setup when not inside a Supsense boundary.

Prior Art by https://github.com/HcySunYang in #3657

refactor: move warn code to more suitable location

Author: LinusBorg

packages/runtime-core/__tests__/components/Suspense.spec.ts

@@ -1232,4 +1232,25 @@ describe('Suspense', () => {
     await nextTick()
     expect(serializeInner(root)).toBe(`<div>parent<!----></div>`)
   })
+
+  test('warn if using async setup when not in a Suspense boundary', () => {
+    const Child = {
+      name: 'Child',
+      async setup() {
+        return () => h('div', 'child')
+      }
+    }
+    const Parent = {
+      setup() {
+        return () => h('div', [h(Child)])
+      }
+    }
+
+    const root = nodeOps.createElement('div')
+    render(h(Parent), root)
+
+    expect(
+      `Component <Child>: setup function returned a promise, but no <Suspense> boundary was found in the parent component tree. Therefore, this component can't render.`
+    ).toHaveBeenWarned()
+  })
 })

packages/runtime-core/src/component.ts

@@ -654,7 +654,6 @@ function setupStatefulComponent(
 
     if (isPromise(setupResult)) {
       setupResult.then(unsetCurrentInstance, unsetCurrentInstance)
-
       if (isSSR) {
         // return the promise so server-renderer can wait on it
         return setupResult
@@ -668,6 +667,12 @@ function setupStatefulComponent(
         // async setup returned Promise.
         // bail here and wait for re-entry.
         instance.asyncDep = setupResult
+        if (__DEV__ && !instance.suspense) {
+          const name = Component.name ?? 'Anonymous'
+          warn(
+            `Component <${name}>: setup function returned a promise, but no <Suspense> boundary was found in the parent component tree. Therefore, this component can't render.`
+          )
+        }
       } else if (__DEV__) {
         warn(
           `setup() returned a Promise, but the version of Vue you are using ` +