feat(compiler): transform slot outlets

packages/runtime-core/src/componentSlots.ts

@@ -1,10 +1,16 @@
 import { ComponentInternalInstance, currentInstance } from './component'
-import { VNode, NormalizedChildren, normalizeVNode, VNodeChild } from './vnode'
+import {
+  VNode,
+  NormalizedChildren,
+  normalizeVNode,
+  VNodeChild,
+  VNodeChildren
+} from './vnode'
 import { isArray, isFunction } from '@vue/shared'
 import { ShapeFlags } from './shapeFlags'
 import { warn } from './warning'
 
-export type Slot = (...args: any[]) => VNode[]
+export type Slot = (...args: any[]) => VNodeChildren
 export type Slots = Readonly<{
   [name: string]: Slot
 }>

packages/runtime-core/src/helpers/renderSlot.ts

@@ -0,0 +1,12 @@
+import { Slot } from '../componentSlots'
+import { VNodeChildren } from '../vnode'
+
+export function renderSlot(
+  slot: Slot | undefined,
+  props: any = {},
+  // this is not a user-facing function, so the fallback is always generated by
+  // the compiler.
+  fallback?: string | VNodeChildren
+): string | VNodeChildren | null {
+  return slot ? slot() : fallback || null
+}

packages/runtime-core/src/index.ts

@@ -42,6 +42,7 @@ export { resolveComponent, resolveDirective } from './helpers/resolveAssets'
 export { renderList } from './helpers/renderList'
 export { toString } from './helpers/toString'
 export { toHandlers } from './helpers/toHandlers'
+export { renderSlot } from './helpers/renderSlot'
 export { capitalize, camelize } from '@vue/shared'
 
 // Internal, for integration with runtime compiler