feat(ssr/suspense): suspense hydration

In order to support hydration of async components, server-rendered
fragments must be explicitly marked with comment nodes.

packages/server-renderer/src/helpers/ssrRenderSlot.ts

@@ -18,6 +18,8 @@ export function ssrRenderSlot(
   push: PushFn,
   parentComponent: ComponentInternalInstance
 ) {
+  // template-compiled slots are always rendered as fragments
+  push(`<!--1-->`)
   const slotFn = slots[slotName]
   if (slotFn) {
     if (slotFn.length > 1) {
@@ -31,4 +33,5 @@ export function ssrRenderSlot(
   } else if (fallbackRenderFn) {
     fallbackRenderFn()
   }
+  push(`<!--0-->`)
 }

packages/server-renderer/src/helpers/ssrRenderSuspense.ts

@@ -1,19 +1,30 @@
 import { PushFn, ResolvedSSRBuffer, createBuffer } from '../renderToString'
-import { NOOP } from '@vue/shared'
 
 type ContentRenderFn = (push: PushFn) => void
 
 export async function ssrRenderSuspense({
-  default: renderContent = NOOP,
-  fallback: renderFallback = NOOP
+  default: renderContent,
+  fallback: renderFallback
 }: Record<string, ContentRenderFn | undefined>): Promise<ResolvedSSRBuffer> {
   try {
-    const { push, getBuffer } = createBuffer()
-    renderContent(push)
-    return await getBuffer()
+    if (renderContent) {
+      const { push, getBuffer } = createBuffer()
+      push(`<!--1-->`)
+      renderContent(push)
+      push(`<!--0-->`)
+      return await getBuffer()
+    } else {
+      return []
+    }
   } catch {
-    const { push, getBuffer } = createBuffer()
-    renderFallback(push)
-    return getBuffer()
+    if (renderFallback) {
+      const { push, getBuffer } = createBuffer()
+      push(`<!--1-->`)
+      renderFallback(push)
+      push(`<!--0-->`)
+      return getBuffer()
+    } else {
+      return []
+    }
   }
 }

packages/server-renderer/src/renderToString.ts

@@ -256,7 +256,9 @@ function renderVNode(
       push(children ? `<!--${children}-->` : `<!---->`)
       break
     case Fragment:
+      push(`<!--1-->`) // open
       renderVNodeChildren(push, children as VNodeArrayChildren, parentComponent)
+      push(`<!--0-->`) // close
       break
     default:
       if (shapeFlag & ShapeFlags.ELEMENT) {