perf: prevent renderer hot functions being inlined by minifiers

Terser will aggressively inline hot functions in renderer.ts in order
to reduce "function" declarations, but the inlining leads to performance
overhead (small, but noticeable in benchmarks).

Since we cannot control user's minifier options, we have to avoid the
deopt in the source code by using arrow functions in hot paths.

packages/runtime-core/src/hydration.ts

@@ -12,6 +12,7 @@ import { ComponentInternalInstance } from './component'
 import { invokeDirectiveHook } from './directives'
 import { warn } from './warning'
 import { PatchFlags, ShapeFlags, isReservedProp, isOn } from '@vue/shared'
+import { RendererOptions } from './renderer'
 
 // Note: hydration is DOM-specific
 // But we have to place it in core due to tight coupling with core - splitting
@@ -20,9 +21,9 @@ import { PatchFlags, ShapeFlags, isReservedProp, isOn } from '@vue/shared'
 // passed in via arguments.
 export function createHydrationFunctions(
   mountComponent: any, // TODO
-  patchProp: any // TODO
+  patchProp: RendererOptions['patchProp']
 ) {
-  function hydrate(vnode: VNode, container: Element) {
+  const hydrate = (vnode: VNode, container: Element) => {
     if (__DEV__ && !container.hasChildNodes()) {
       warn(`Attempting to hydrate existing markup but container is empty.`)
       return
@@ -34,11 +35,11 @@ export function createHydrationFunctions(
   // TODO handle mismatches
   // TODO SVG
   // TODO Suspense
-  function hydrateNode(
+  const hydrateNode = (
     node: Node,
     vnode: VNode,
     parentComponent: ComponentInternalInstance | null = null
-  ): Node | null | undefined {
+  ): Node | null | undefined => {
     const { type, shapeFlag } = vnode
     vnode.el = node
     switch (type) {
@@ -73,11 +74,11 @@ export function createHydrationFunctions(
     }
   }
 
-  function hydrateElement(
+  const hydrateElement = (
     el: Element,
     vnode: VNode,
     parentComponent: ComponentInternalInstance | null
-  ) {
+  ) => {
     const { props, patchFlag } = vnode
     // skip props & children if this is hoisted static nodes
     if (patchFlag !== PatchFlags.HOISTED) {
@@ -124,11 +125,11 @@ export function createHydrationFunctions(
     return el.nextSibling
   }
 
-  function hydrateChildren(
+  const hydrateChildren = (
     node: Node | null | undefined,
     vnodes: VNode[],
     parentComponent: ComponentInternalInstance | null
-  ): Node | null | undefined {
+  ): Node | null | undefined => {
     for (let i = 0; node != null && i < vnodes.length; i++) {
       // TODO can skip normalizeVNode in optimized mode
       // (need hint on rendered markup?)