Add placeholders support to @babel/types and @babel/generator

packages/babel-generator/src/generators/base.js

@@ -82,3 +82,13 @@ export function DirectiveLiteral(node: Object) {
 export function InterpreterDirective(node: Object) {
   this.token(`#!${node.value}\n`);
 }
+
+export function Placeholder(node: Object) {
+  this.token("%%");
+  this.print(node.name);
+  this.token("%%");
+
+  if (node.expectedNode === "Statement") {
+    this.semicolon();
+  }
+}

packages/babel-generator/test/fixtures/misc/placeholders/input.js

@@ -0,0 +1,8 @@
+var %%a%% = %%b%%
+
+%%c%%
+
+class %%d%% {}
+class A %%e%%
+
+function %%f%%(...%%g%%) %%h%%

packages/babel-generator/test/fixtures/misc/placeholders/options.json

@@ -0,0 +1,3 @@
+{
+  "plugins": ["placeholders"]
+}

packages/babel-generator/test/fixtures/misc/placeholders/output.js

@@ -0,0 +1,8 @@
+var %%a%% = %%b%%;
+%%c%%;
+
+class %%d%% {}
+
+class A %%e%%
+
+function %%f%%(...%%g%%) %%h%%

packages/babel-types/scripts/generators/generateValidators.js

@@ -1,22 +1,45 @@
 "use strict";
 const definitions = require("../../lib/definitions");
 
+const has = Function.call.bind(Object.prototype.hasOwnProperty);
+
+function joinComparisons(leftArr, right) {
+  return (
+    leftArr.map(JSON.stringify).join(` === ${right} || `) + ` === ${right}`
+  );
+}
+
 function addIsHelper(type, aliasKeys, deprecated) {
   const targetType = JSON.stringify(type);
   let aliasSource = "";
   if (aliasKeys) {
-    aliasSource =
-      " || " +
-      aliasKeys.map(JSON.stringify).join(" === nodeType || ") +
-      " === nodeType";
+    aliasSource = " || " + joinComparisons(aliasKeys, "nodeType");
+  }
+
+  let placeholderSource = "";
+  const placeholderTypes = [];
+  if (
+    definitions.PLACEHOLDERS.includes(type) &&
+    has(definitions.FLIPPED_ALIAS_KEYS, type)
+  ) {
+    placeholderTypes.push(type);
+  }
+  if (has(definitions.PLACEHOLDERS_FLIPPED_ALIAS, type)) {
+    placeholderTypes.push(...definitions.PLACEHOLDERS_FLIPPED_ALIAS[type]);
+  }
+  if (placeholderTypes.length > 0) {
+    placeholderSource =
+      ' || nodeType === "Placeholder" && (' +
+      joinComparisons(placeholderTypes, "node.expectedNode") +
+      ")";
   }
 
   return `export function is${type}(node: ?Object, opts?: Object): boolean {
     ${deprecated || ""}
     if (!node) return false;
 
     const nodeType = node.type;
-    if (nodeType === ${targetType}${aliasSource}) {
+    if (nodeType === ${targetType}${aliasSource}${placeholderSource}) {
       if (typeof opts === "undefined") {
         return true;
       } else {

packages/babel-types/src/asserts/generated/index.js

@@ -660,6 +660,9 @@ export function assertJSXClosingFragment(
 export function assertNoop(node: Object, opts?: Object = {}): void {
   assert("Noop", node, opts);
 }
+export function assertPlaceholder(node: Object, opts?: Object = {}): void {
+  assert("Placeholder", node, opts);
+}
 export function assertArgumentPlaceholder(
   node: Object,
   opts?: Object = {},

packages/babel-types/src/builders/generated/index.js

@@ -596,6 +596,10 @@ export function Noop(...args: Array<any>): Object {
   return builder("Noop", ...args);
 }
 export { Noop as noop };
+export function Placeholder(...args: Array<any>): Object {
+  return builder("Placeholder", ...args);
+}
+export { Placeholder as placeholder };
 export function ArgumentPlaceholder(...args: Array<any>): Object {
   return builder("ArgumentPlaceholder", ...args);
 }

packages/babel-types/src/definitions/index.js

@@ -15,6 +15,11 @@ import {
   BUILDER_KEYS,
   DEPRECATED_KEYS,
 } from "./utils";
+import {
+  PLACEHOLDERS,
+  PLACEHOLDERS_ALIAS,
+  PLACEHOLDERS_FLIPPED_ALIAS,
+} from "./placeholders";
 
 // We do this here, because at this point the visitor keys should be ready and setup
 toFastProperties(VISITOR_KEYS);
@@ -24,6 +29,9 @@ toFastProperties(NODE_FIELDS);
 toFastProperties(BUILDER_KEYS);
 toFastProperties(DEPRECATED_KEYS);
 
+toFastProperties(PLACEHOLDERS_ALIAS);
+toFastProperties(PLACEHOLDERS_FLIPPED_ALIAS);
+
 const TYPES: Array<string> = Object.keys(VISITOR_KEYS)
   .concat(Object.keys(FLIPPED_ALIAS_KEYS))
   .concat(Object.keys(DEPRECATED_KEYS));
@@ -35,5 +43,8 @@ export {
   NODE_FIELDS,
   BUILDER_KEYS,
   DEPRECATED_KEYS,
+  PLACEHOLDERS,
+  PLACEHOLDERS_ALIAS,
+  PLACEHOLDERS_FLIPPED_ALIAS,
   TYPES,
 };

packages/babel-types/src/definitions/misc.js

@@ -1,6 +1,21 @@
 // @flow
-import defineType from "./utils";
+import defineType, { assertNodeType, assertOneOf } from "./utils";
+import { PLACEHOLDERS } from "./placeholders";
 
 defineType("Noop", {
   visitor: [],
 });
+
+defineType("Placeholder", {
+  visitor: [],
+  builder: ["expectedNode", "name"],
+  // aliases: [], defined in placeholders.js
+  fields: {
+    name: {
+      validate: assertNodeType("Identifier"),
+    },
+    expectedNode: {
+      validate: assertOneOf(...PLACEHOLDERS),
+    },
+  },
+});

packages/babel-types/src/definitions/placeholders.js

@@ -0,0 +1,33 @@
+import { ALIAS_KEYS } from "./utils";
+
+export const PLACEHOLDERS = [
+  "Identifier",
+  "StringLiteral",
+  "Expression",
+  "Statement",
+  "Declaration",
+  "BlockStatement",
+  "ClassBody",
+  "Pattern",
+];
+
+export const PLACEHOLDERS_ALIAS: { [string]: Array<string> } = {
+  Declaration: ["Statement"],
+  Pattern: ["PatternLike", "LVal"],
+};
+
+for (const type of PLACEHOLDERS) {
+  const alias = ALIAS_KEYS[type];
+  if (alias && alias.length) PLACEHOLDERS_ALIAS[type] = alias;
+}
+
+export const PLACEHOLDERS_FLIPPED_ALIAS: { [string]: Array<string> } = {};
+
+Object.keys(PLACEHOLDERS_ALIAS).forEach(type => {
+  PLACEHOLDERS_ALIAS[type].forEach(alias => {
+    if (!Object.hasOwnProperty.call(PLACEHOLDERS_FLIPPED_ALIAS, alias)) {
+      PLACEHOLDERS_FLIPPED_ALIAS[alias] = [];
+    }
+    PLACEHOLDERS_FLIPPED_ALIAS[alias].push(type);
+  });
+});

packages/babel-types/src/index.js

@@ -100,6 +100,7 @@ export { default as isImmutable } from "./validators/isImmutable";
 export { default as isLet } from "./validators/isLet";
 export { default as isNode } from "./validators/isNode";
 export { default as isNodesEquivalent } from "./validators/isNodesEquivalent";
+export { default as isPlaceholderType } from "./validators/isPlaceholderType";
 export { default as isReferenced } from "./validators/isReferenced";
 export { default as isScope } from "./validators/isScope";
 export { default as isSpecifierDefault } from "./validators/isSpecifierDefault";

packages/babel-types/src/validators/generated/index.js

@@ -2093,6 +2093,20 @@ export function isNoop(node: ?Object, opts?: Object): boolean {
 
   return false;
 }
+export function isPlaceholder(node: ?Object, opts?: Object): boolean {
+  if (!node) return false;
+
+  const nodeType = node.type;
+  if (nodeType === "Placeholder") {
+    if (typeof opts === "undefined") {
+      return true;
+    } else {
+      return shallowEqual(node, opts);
+    }
+  }
+
+  return false;
+}
 export function isArgumentPlaceholder(node: ?Object, opts?: Object): boolean {
   if (!node) return false;
 
@@ -3280,7 +3294,11 @@ export function isExpression(node: ?Object, opts?: Object): boolean {
     "BigIntLiteral" === nodeType ||
     "TSAsExpression" === nodeType ||
     "TSTypeAssertion" === nodeType ||
-    "TSNonNullExpression" === nodeType
+    "TSNonNullExpression" === nodeType ||
+    (nodeType === "Placeholder" &&
+      ("Expression" === node.expectedNode ||
+        "Identifier" === node.expectedNode ||
+        "StringLiteral" === node.expectedNode))
   ) {
     if (typeof opts === "undefined") {
       return true;
@@ -3331,7 +3349,8 @@ export function isScopable(node: ?Object, opts?: Object): boolean {
     "ClassExpression" === nodeType ||
     "ForOfStatement" === nodeType ||
     "ClassMethod" === nodeType ||
-    "ClassPrivateMethod" === nodeType
+    "ClassPrivateMethod" === nodeType ||
+    (nodeType === "Placeholder" && "BlockStatement" === node.expectedNode)
   ) {
     if (typeof opts === "undefined") {
       return true;
@@ -3362,7 +3381,8 @@ export function isBlockParent(node: ?Object, opts?: Object): boolean {
     "ArrowFunctionExpression" === nodeType ||
     "ForOfStatement" === nodeType ||
     "ClassMethod" === nodeType ||
-    "ClassPrivateMethod" === nodeType
+    "ClassPrivateMethod" === nodeType ||
+    (nodeType === "Placeholder" && "BlockStatement" === node.expectedNode)
   ) {
     if (typeof opts === "undefined") {
       return true;
@@ -3380,7 +3400,8 @@ export function isBlock(node: ?Object, opts?: Object): boolean {
   if (
     nodeType === "Block" ||
     "BlockStatement" === nodeType ||
-    "Program" === nodeType
+    "Program" === nodeType ||
+    (nodeType === "Placeholder" && "BlockStatement" === node.expectedNode)
   ) {
     if (typeof opts === "undefined") {
       return true;
@@ -3442,7 +3463,11 @@ export function isStatement(node: ?Object, opts?: Object): boolean {
     "TSModuleDeclaration" === nodeType ||
     "TSImportEqualsDeclaration" === nodeType ||
     "TSExportAssignment" === nodeType ||
-    "TSNamespaceExportDeclaration" === nodeType
+    "TSNamespaceExportDeclaration" === nodeType ||
+    (nodeType === "Placeholder" &&
+      ("Statement" === node.expectedNode ||
+        "Declaration" === node.expectedNode ||
+        "BlockStatement" === node.expectedNode))
   ) {
     if (typeof opts === "undefined") {
       return true;
@@ -3667,7 +3692,8 @@ export function isPureish(node: ?Object, opts?: Object): boolean {
     "ArrowFunctionExpression" === nodeType ||
     "ClassDeclaration" === nodeType ||
     "ClassExpression" === nodeType ||
-    "BigIntLiteral" === nodeType
+    "BigIntLiteral" === nodeType ||
+    (nodeType === "Placeholder" && "StringLiteral" === node.expectedNode)
   ) {
     if (typeof opts === "undefined") {
       return true;
@@ -3708,7 +3734,8 @@ export function isDeclaration(node: ?Object, opts?: Object): boolean {
     "TSInterfaceDeclaration" === nodeType ||
     "TSTypeAliasDeclaration" === nodeType ||
     "TSEnumDeclaration" === nodeType ||
-    "TSModuleDeclaration" === nodeType
+    "TSModuleDeclaration" === nodeType ||
+    (nodeType === "Placeholder" && "Declaration" === node.expectedNode)
   ) {
     if (typeof opts === "undefined") {
       return true;
@@ -3729,7 +3756,9 @@ export function isPatternLike(node: ?Object, opts?: Object): boolean {
     "RestElement" === nodeType ||
     "AssignmentPattern" === nodeType ||
     "ArrayPattern" === nodeType ||
-    "ObjectPattern" === nodeType
+    "ObjectPattern" === nodeType ||
+    (nodeType === "Placeholder" &&
+      ("Pattern" === node.expectedNode || "Identifier" === node.expectedNode))
   ) {
     if (typeof opts === "undefined") {
       return true;
@@ -3752,7 +3781,9 @@ export function isLVal(node: ?Object, opts?: Object): boolean {
     "AssignmentPattern" === nodeType ||
     "ArrayPattern" === nodeType ||
     "ObjectPattern" === nodeType ||
-    "TSParameterProperty" === nodeType
+    "TSParameterProperty" === nodeType ||
+    (nodeType === "Placeholder" &&
+      ("Pattern" === node.expectedNode || "Identifier" === node.expectedNode))
   ) {
     if (typeof opts === "undefined") {
       return true;
@@ -3770,7 +3801,8 @@ export function isTSEntityName(node: ?Object, opts?: Object): boolean {
   if (
     nodeType === "TSEntityName" ||
     "Identifier" === nodeType ||
-    "TSQualifiedName" === nodeType
+    "TSQualifiedName" === nodeType ||
+    (nodeType === "Placeholder" && "Identifier" === node.expectedNode)
   ) {
     if (typeof opts === "undefined") {
       return true;
@@ -3793,7 +3825,8 @@ export function isLiteral(node: ?Object, opts?: Object): boolean {
     "BooleanLiteral" === nodeType ||
     "RegExpLiteral" === nodeType ||
     "TemplateLiteral" === nodeType ||
-    "BigIntLiteral" === nodeType
+    "BigIntLiteral" === nodeType ||
+    (nodeType === "Placeholder" && "StringLiteral" === node.expectedNode)
   ) {
     if (typeof opts === "undefined") {
       return true;
@@ -3824,7 +3857,8 @@ export function isImmutable(node: ?Object, opts?: Object): boolean {
     "JSXFragment" === nodeType ||
     "JSXOpeningFragment" === nodeType ||
     "JSXClosingFragment" === nodeType ||
-    "BigIntLiteral" === nodeType
+    "BigIntLiteral" === nodeType ||
+    (nodeType === "Placeholder" && "StringLiteral" === node.expectedNode)
   ) {
     if (typeof opts === "undefined") {
       return true;
@@ -3940,7 +3974,8 @@ export function isPattern(node: ?Object, opts?: Object): boolean {
     nodeType === "Pattern" ||
     "AssignmentPattern" === nodeType ||
     "ArrayPattern" === nodeType ||
-    "ObjectPattern" === nodeType
+    "ObjectPattern" === nodeType ||
+    (nodeType === "Placeholder" && "Pattern" === node.expectedNode)
   ) {
     if (typeof opts === "undefined") {
       return true;

packages/babel-types/src/validators/is.js

@@ -1,6 +1,8 @@
 // @flow
 import shallowEqual from "../utils/shallowEqual";
 import isType from "./isType";
+import isPlaceholderType from "./isPlaceholderType";
+import { FLIPPED_ALIAS_KEYS } from "../definitions";
 
 /**
  * Returns whether `node` is of given `type`.
@@ -11,7 +13,21 @@ export default function is(type: string, node: Object, opts?: Object): boolean {
   if (!node) return false;
 
   const matches = isType(node.type, type);
-  if (!matches) return false;
+  if (!matches) {
+    if (!opts && node.type === "Placeholder" && type in FLIPPED_ALIAS_KEYS) {
+      // We can only return true if the placeholder doesn't replace a real node,
+      // but it replaces a category of nodes (an alias).
+      //
+      // t.is("Identifier", node) gives some guarantees about node's shape, so we
+      // can't say that Placeholder(expectedNode: "Identifier") is an identifier
+      // because it doesn't have the same properties.
+      // On the other hand, t.is("Expression", node) doesn't say anything about
+      // the shape of node because Expression can be many different nodes: we can,
+      // and should, safely report expression placeholders as Expressions.
+      return isPlaceholderType(node.expectedNode, type);
+    }
+    return false;
+  }
 
   if (typeof opts === "undefined") {
     return true;