wip: fix render fn compat detection

packages/runtime-core/src/compat/compatConfig.ts

@@ -30,6 +30,11 @@ export function isCompatEnabled(
   key: DeprecationTypes,
   instance: ComponentInternalInstance | null
 ): boolean {
+  // skip compat for built-in components
+  if (instance && instance.type.__isBuiltIn) {
+    return false
+  }
+
   const mode = getCompatConfigForKey('MODE', instance) || 2
   const val = getCompatConfigForKey(key, instance)
   if (mode === 2) {

packages/runtime-core/src/compat/component.ts

@@ -19,6 +19,10 @@ export function convertLegacyComponent(
   comp: any,
   instance: ComponentInternalInstance | null
 ): Component {
+  if (comp.__isBuiltIn) {
+    return comp
+  }
+
   // 2.x async component
   // since after disabling this, plain functions are still valid usage, do not
   // use softAssert here.

packages/runtime-core/src/compat/instance.ts

@@ -49,7 +49,11 @@ export function installCompatInstanceProperties(map: PublicPropertiesMap) {
 
     // overrides existing accessor
     $slots: i => {
-      if (isCompatEnabled(DeprecationTypes.RENDER_FUNCTION, i)) {
+      if (
+        isCompatEnabled(DeprecationTypes.RENDER_FUNCTION, i) &&
+        i.render &&
+        i.render._compatWrapped
+      ) {
         return new Proxy(i.slots, legacySlotProxyHandlers)
       }
       return __DEV__ ? shallowReadonly(i.slots) : i.slots

packages/runtime-core/src/compat/renderFn.ts

@@ -5,7 +5,13 @@ import {
   ShapeFlags,
   toHandlerKey
 } from '@vue/shared'
-import { Component, Data } from '../component'
+import {
+  Component,
+  ComponentInternalInstance,
+  ComponentOptions,
+  Data,
+  InternalRenderFunction
+} from '../component'
 import { DirectiveArguments, withDirectives } from '../directives'
 import {
   resolveDirective,
@@ -19,6 +25,35 @@ import {
   VNodeArrayChildren,
   VNodeProps
 } from '../vnode'
+import { checkCompatEnabled } from './compatConfig'
+import { DeprecationTypes } from './deprecations'
+
+export function convertLegacyRenderFn(instance: ComponentInternalInstance) {
+  const Component = instance.type as ComponentOptions
+  const render = Component.render as InternalRenderFunction | undefined
+
+  // v3 runtime compiled, or already checked / wrapped
+  if (!render || render._rc || render._compatChecked || render._compatWrapped) {
+    return
+  }
+
+  const string = render.toString()
+  if (string.startsWith('function render(_ctx') || string.startsWith('(_ctx')) {
+    // v3 pre-compiled function
+    render._compatChecked = true
+    return
+  }
+
+  // v2 render function, try to provide compat
+  if (checkCompatEnabled(DeprecationTypes.RENDER_FUNCTION, instance)) {
+    const wrapped = (Component.render = function compatRender() {
+      // @ts-ignore
+      return render.call(this, compatH)
+    })
+    // @ts-ignore
+    wrapped._compatWrapped = true
+  }
+}
 
 interface LegacyVNodeProps {
   key?: string | number