wip: add private api compat flag

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

@@ -58,7 +58,9 @@ export const enum DeprecationTypes {
 
   RENDER_FUNCTION = 'RENDER_FUNCTION',
 
-  FILTERS = 'FILTERS'
+  FILTERS = 'FILTERS',
+
+  PRIVATE_APIS = 'PRIVATE_APIS'
 }
 
 type DeprecationData = {
@@ -404,6 +406,13 @@ const deprecationData: Record<DeprecationTypes, DeprecationData> = {
       `The "|" symbol will be treated as native JavaScript bitwise OR operator. ` +
       `Use method calls or computed properties instead.`,
     link: `https://v3.vuejs.org/guide/migration/filters.html`
+  },
+
+  [DeprecationTypes.PRIVATE_APIS]: {
+    message: name =>
+      `"${name}" is a Vue 2 private API that no longer exists in Vue 3. ` +
+      `If you are seeing this warning only due to a dependency, you can ` +
+      `suppress this warning via { PRIVATE_APIS: 'supress-warning' }.`
   }
 }
 

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

@@ -91,8 +91,11 @@ export function installCompatInstanceProperties(map: PublicPropertiesMap) {
     $off: i => off.bind(null, i),
 
     $children: getCompatChildren,
-    $listeners: getCompatListeners,
+    $listeners: getCompatListeners
+  } as PublicPropertiesMap)
 
+  if (isCompatEnabled(DeprecationTypes.PRIVATE_APIS, null)) {
+    extend(map, {
       $vnode: i => i.vnode,
 
       // inject addtional properties into $options for compat
@@ -127,4 +130,5 @@ export function installCompatInstanceProperties(map: PublicPropertiesMap) {
       _d: () => legacyBindDynamicKeys,
       _p: () => legacyPrependModifier
     } as PublicPropertiesMap)
+  }
 }

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

@@ -310,7 +310,11 @@ function convertLegacySlots(vnode: VNode): VNode {
 
 export function defineLegacyVNodeProperties(vnode: VNode) {
   if (
-    isCompatEnabled(DeprecationTypes.RENDER_FUNCTION, currentRenderingInstance)
+    isCompatEnabled(
+      DeprecationTypes.RENDER_FUNCTION,
+      currentRenderingInstance
+    ) &&
+    isCompatEnabled(DeprecationTypes.PRIVATE_APIS, currentRenderingInstance)
   ) {
     const context = currentRenderingInstance
     const getInstance = () => vnode.component && vnode.component.proxy