From d0ca19157e490bbe19d0331fffb86e9d62f40421 Mon Sep 17 00:00:00 2001
From: Jason Miller <developit@users.noreply.github.com>
Date: Fri, 8 May 2020 19:51:25 +0000
Subject: [PATCH] Add fast rest parameters transform

---
 src/lib/babel-custom.js        | 26 ++++++++--
 src/lib/transform-fast-rest.js | 90 ++++++++++++++++++++++++++++++++++
 2 files changed, 113 insertions(+), 3 deletions(-)
 create mode 100644 src/lib/transform-fast-rest.js

diff --git a/src/lib/babel-custom.js b/src/lib/babel-custom.js
index d9af3ee..f593982 100644
--- a/src/lib/babel-custom.js
+++ b/src/lib/babel-custom.js
@@ -1,6 +1,7 @@
 import { createConfigItem } from '@babel/core';
 import babelPlugin from 'rollup-plugin-babel';
 import merge from 'lodash.merge';
+import transformFastRest from './transform-fast-rest';
 import { isTruthy } from '../utils';
 
 const ESMODULES_TARGET = {
@@ -13,7 +14,9 @@ const mergeConfigItems = (type, ...configItemsToMerge) => {
 	configItemsToMerge.forEach(configItemToMerge => {
 		configItemToMerge.forEach(item => {
 			const itemToMergeWithIndex = mergedItems.findIndex(
-				mergedItem => mergedItem.file.resolved === item.file.resolved,
+				mergedItem =>
+					(mergedItem.name || mergedItem.file.resolved) ===
+					(item.name || item.file.resolved),
 			);
 
 			if (itemToMergeWithIndex === -1) {
@@ -37,8 +40,10 @@ const mergeConfigItems = (type, ...configItemsToMerge) => {
 };
 
 const createConfigItems = (type, items) => {
-	return items.map(({ name, ...options }) => {
-		return createConfigItem([require.resolve(name), options], { type });
+	return items.map(item => {
+		let { name, value, ...options } = item;
+		value = value || [require.resolve(name), options];
+		return createConfigItem(value, { type });
 	});
 };
 
@@ -59,6 +64,9 @@ export default () => {
 			},
 
 			config(config, { customOptions }) {
+				const targets = customOptions.targets;
+				const isNodeTarget = targets && targets.node != null;
+
 				const defaultPlugins = createConfigItems(
 					'plugin',
 					[
@@ -80,6 +88,18 @@ export default () => {
 							externalHelpers: false,
 							minify: true,
 						},
+						!customOptions.modern &&
+							!isNodeTarget && {
+								value: [
+									transformFastRest,
+									{
+										// Use inline [].slice.call(arguments)
+										helper: false,
+										literal: true,
+									},
+									'transform-fast-rest',
+								],
+							},
 						{
 							name: '@babel/plugin-proposal-class-properties',
 							loose: true,
diff --git a/src/lib/transform-fast-rest.js b/src/lib/transform-fast-rest.js
new file mode 100644
index 0000000..9b36758
--- /dev/null
+++ b/src/lib/transform-fast-rest.js
@@ -0,0 +1,90 @@
+/**
+ * @type {import('@babel/core')}
+ */
+
+/**
+ * Transform ...rest parameters to [].slice.call(arguments,offset).
+ * Demo: https://astexplorer.net/#/gist/70aaa0306db9a642171ef3e2f35df2e0/576c150f647e4936fa6960e0453a11cdc5d81f21
+ * Benchmark: https://jsperf.com/rest-arguments-babel-pr-9152/4
+ * @param {object} opts
+ * @param {babel.template} opts.template
+ * @param {babel.types} opts.types
+ * @returns {babel.PluginObj}
+ */
+export default function fastRestTransform({ template, types: t }) {
+	const slice = template`var IDENT = Array.prototype.slice;`;
+
+	const VISITOR = {
+		RestElement(path, state) {
+			if (path.parentKey !== 'params') return;
+
+			// Create a global _slice alias
+			let slice = state.get('slice');
+			if (!slice) {
+				slice = path.scope.generateUidIdentifier('slice');
+				state.set('slice', slice);
+			}
+
+			// _slice.call(arguments) or _slice.call(arguments, 1)
+			const args = [t.identifier('arguments')];
+			if (path.key) args.push(t.numericLiteral(path.key));
+			const sliced = t.callExpression(
+				t.memberExpression(t.clone(slice), t.identifier('call')),
+				args,
+			);
+
+			const ident = path.node.argument;
+			const binding = path.scope.getBinding(ident.name);
+
+			if (binding.referencePaths.length !== 0) {
+				// arguments access requires a non-Arrow function:
+				const func = path.parentPath;
+				if (t.isArrowFunctionExpression(func)) {
+					func.arrowFunctionToExpression();
+				}
+
+				if (binding.constant && binding.referencePaths.length === 1) {
+					// one usage, never assigned - replace usage inline
+					binding.referencePaths[0].replaceWith(sliced);
+				} else {
+					// unknown usage, create a binding
+					const decl = t.variableDeclaration('var', [
+						t.variableDeclarator(t.clone(ident), sliced),
+					]);
+					func.get('body').unshiftContainer('body', decl);
+				}
+			}
+
+			path.remove();
+		},
+	};
+
+	return {
+		name: 'transform-fast-rest',
+		visitor: {
+			Program(path, state) {
+				const childState = new Map();
+				const useHelper = state.opts.helper === true; // defaults to false
+
+				if (!useHelper) {
+					let inlineHelper;
+					if (state.opts.literal === false) {
+						inlineHelper = template.expression.ast`Array.prototype.slice`;
+					} else {
+						inlineHelper = template.expression.ast`[].slice`;
+					}
+					childState.set('slice', inlineHelper);
+				}
+
+				path.traverse(VISITOR, childState);
+
+				const name = childState.get('slice');
+				if (name && useHelper) {
+					const helper = slice({ IDENT: name });
+					t.addComment(helper.declarations[0].init, 'leading', '#__PURE__');
+					path.unshiftContainer('body', helper);
+				}
+			},
+		},
+	};
+}