feat: v-once

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

packages/compiler-core/__tests__/transforms/vOnce.spec.ts

@@ -0,0 +1,30 @@
+import { parse, transform, ElementNode, CallExpression } from '../../src'
+import { transformOnce } from '../../src/transforms/vOnce'
+import { transformElement } from '../../src/transforms/transformElement'
+import { createObjectMatcher } from '../testUtils'
+
+function transformWithCloak(template: string) {
+  const ast = parse(template)
+  transform(ast, {
+    nodeTransforms: [transformElement],
+    directiveTransforms: {
+      once: transformOnce
+    }
+  })
+  return ast.children[0] as ElementNode
+}
+
+describe('compiler: v-once transform', () => {
+  test('should add no props to DOM', () => {
+    const node = transformWithCloak(`<div v-once />`)
+    const codegenArgs = (node.codegenNode as CallExpression).arguments
+
+    // As v-cloak adds no properties the codegen should be identical to
+    // rendering a div with no props or reactive data (so just the tag as the arg)
+    expect(codegenArgs[1]).toMatchObject(
+      createObjectMatcher({
+        $once: `[true]`
+      })
+    )
+  })
+})