feat(ssr/suspense): suspense hydration

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

packages/compiler-ssr/src/ssrCodegenTransform.ts

@@ -10,7 +10,8 @@ import {
   createBlockStatement,
   CompilerOptions,
   IfStatement,
-  CallExpression
+  CallExpression,
+  isText
 } from '@vue/compiler-dom'
 import { isString, escapeHtml } from '@vue/shared'
 import { SSR_INTERPOLATE, ssrHelpers } from './runtimeHelpers'
@@ -28,7 +29,9 @@ import { ssrProcessElement } from './transforms/ssrTransformElement'
 
 export function ssrCodegenTransform(ast: RootNode, options: CompilerOptions) {
   const context = createSSRTransformContext(ast, options)
-  processChildren(ast.children, context)
+  const isFragment =
+    ast.children.length > 1 && ast.children.some(c => !isText(c))
+  processChildren(ast.children, context, isFragment)
   ast.codegenNode = createBlockStatement(context.body)
 
   // Finalize helpers.
@@ -104,8 +107,12 @@ function createChildContext(
 
 export function processChildren(
   children: TemplateChildNode[],
-  context: SSRTransformContext
+  context: SSRTransformContext,
+  asFragment = false
 ) {
+  if (asFragment) {
+    context.pushStringPart(`<!--1-->`)
+  }
   for (let i = 0; i < children.length; i++) {
     const child = children[i]
     if (child.type === NodeTypes.ELEMENT) {
@@ -128,14 +135,18 @@ export function processChildren(
       ssrProcessFor(child, context)
     }
   }
+  if (asFragment) {
+    context.pushStringPart(`<!--0-->`)
+  }
 }
 
 export function processChildrenAsStatement(
   children: TemplateChildNode[],
   parentContext: SSRTransformContext,
+  asFragment = false,
   withSlotScopeId = parentContext.withSlotScopeId
 ): BlockStatement {
   const childContext = createChildContext(parentContext, withSlotScopeId)
-  processChildren(children, childContext)
+  processChildren(children, childContext, asFragment)
   return createBlockStatement(childContext.body)
 }

packages/compiler-ssr/src/transforms/ssrTransformComponent.ts

@@ -30,7 +30,8 @@ import {
   traverseNode,
   ExpressionNode,
   TemplateNode,
-  SUSPENSE
+  SUSPENSE,
+  TRANSITION_GROUP
 } from '@vue/compiler-dom'
 import { SSR_RENDER_COMPONENT } from '../runtimeHelpers'
 import {
@@ -151,7 +152,7 @@ export function ssrProcessComponent(
       return ssrProcessSuspense(node, context)
     } else {
       // real fall-through (e.g. KeepAlive): just render its children.
-      processChildren(node.children, context)
+      processChildren(node.children, context, component === TRANSITION_GROUP)
     }
   } else {
     // finish up slot function expressions from the 1st pass.
@@ -167,6 +168,7 @@ export function ssrProcessComponent(
         processChildrenAsStatement(
           children,
           context,
+          false,
           true /* withSlotScopeId */
         ),
         vnodeBranch

packages/compiler-ssr/src/transforms/ssrVFor.ts

@@ -4,7 +4,8 @@ import {
   processFor,
   createCallExpression,
   createFunctionExpression,
-  createForLoopParams
+  createForLoopParams,
+  NodeTypes
 } from '@vue/compiler-dom'
 import {
   SSRTransformContext,
@@ -21,14 +22,23 @@ export const ssrTransformFor = createStructuralDirectiveTransform(
 // This is called during the 2nd transform pass to construct the SSR-sepcific
 // codegen nodes.
 export function ssrProcessFor(node: ForNode, context: SSRTransformContext) {
+  const needFragmentWrapper =
+    node.children.length !== 1 || node.children[0].type !== NodeTypes.ELEMENT
   const renderLoop = createFunctionExpression(
     createForLoopParams(node.parseResult)
   )
-  renderLoop.body = processChildrenAsStatement(node.children, context)
+  renderLoop.body = processChildrenAsStatement(
+    node.children,
+    context,
+    needFragmentWrapper
+  )
+  // v-for always renders a fragment
+  context.pushStringPart(`<!--1-->`)
   context.pushStatement(
     createCallExpression(context.helper(SSR_RENDER_LIST), [
       node.source,
       renderLoop
     ])
   )
+  context.pushStringPart(`<!--0-->`)
 }

packages/compiler-ssr/src/transforms/ssrVIf.ts

@@ -4,7 +4,10 @@ import {
   IfNode,
   createIfStatement,
   createBlockStatement,
-  createCallExpression
+  createCallExpression,
+  IfBranchNode,
+  BlockStatement,
+  NodeTypes
 } from '@vue/compiler-dom'
 import {
   SSRTransformContext,
@@ -23,17 +26,14 @@ export function ssrProcessIf(node: IfNode, context: SSRTransformContext) {
   const [rootBranch] = node.branches
   const ifStatement = createIfStatement(
     rootBranch.condition!,
-    processChildrenAsStatement(rootBranch.children, context)
+    processIfBranch(rootBranch, context)
   )
   context.pushStatement(ifStatement)
 
   let currentIf = ifStatement
   for (let i = 1; i < node.branches.length; i++) {
     const branch = node.branches[i]
-    const branchBlockStatement = processChildrenAsStatement(
-      branch.children,
-      context
-    )
+    const branchBlockStatement = processIfBranch(branch, context)
     if (branch.condition) {
       // else-if
       currentIf = currentIf.alternate = createIfStatement(
@@ -52,3 +52,15 @@ export function ssrProcessIf(node: IfNode, context: SSRTransformContext) {
     ])
   }
 }
+
+function processIfBranch(
+  branch: IfBranchNode,
+  context: SSRTransformContext
+): BlockStatement {
+  const { children } = branch
+  const needFragmentWrapper =
+    (children.length !== 1 || children[0].type !== NodeTypes.ELEMENT) &&
+    // optimize away nested fragments when the only child is a ForNode
+    !(children.length === 1 && children[0].type === NodeTypes.FOR)
+  return processChildrenAsStatement(children, context, needFragmentWrapper)
+}