feat: v-once

Note: only compiler transform is tested - integration with runtime
still needs to be tested.

packages/compiler-core/src/index.ts

@@ -13,6 +13,7 @@ import { transformBind } from './transforms/vBind'
 import { defaultOnError, createCompilerError, ErrorCodes } from './errors'
 import { trackSlotScopes, trackVForSlotScopes } from './transforms/vSlot'
 import { optimizeText } from './transforms/optimizeText'
+import { transformOnce } from './transforms/vOnce'
 
 export type CompilerOptions = ParserOptions & TransformOptions & CodegenOptions
 
@@ -60,6 +61,7 @@ export function baseCompile(
     directiveTransforms: {
       on: transformOn,
       bind: transformBind,
+      once: transformOnce,
       ...(options.directiveTransforms || {}) // user transforms
     }
   })

packages/compiler-core/src/transforms/vOnce.ts

@@ -0,0 +1,15 @@
+import {
+  DirectiveTransform,
+  createObjectProperty,
+  createSimpleExpression
+} from '@vue/compiler-core'
+
+export const transformOnce: DirectiveTransform = dir => {
+  return {
+    props: createObjectProperty(
+      createSimpleExpression(`$once`, true, dir.loc),
+      createSimpleExpression('true', false)
+    ),
+    needRuntime: false
+  }
+}

packages/compiler-core/src/utils.ts

@@ -63,7 +63,7 @@ export const walkJS: typeof walk = (ast, walker) => {
 }
 
 export const isSimpleIdentifier = (name: string): boolean =>
-  !/^\d|[^\w]/.test(name)
+  !/^\d|[^\$\w]/.test(name)
 
 export function getInnerRange(
   loc: SourceLocation,