Add experimental transformer settings: compactOutput, disableNormaliz…

…ePseudoGlobals

Summary:
Adds two experimental flags to Metro's built-in JS transformer. These are:

* `unstable_compactOutput`: generate compact output from Babel. This is a little faster than the default behaviour of generating pretty-printed JS. The change in output is only noticeable when not subsequently running a minifier (per the normal flags).
* `unstable_disableNormalizePseudoGlobals`: disables the normalizePseudoGlobals pass, a pre-processing step Metro runs before the minifier to make the resulting module headers more consistent and therefore compressible.

NOTE: These flags may change or stop working at any point, even in a patch version of Metro.

Reviewed By: MichaReiser, feedthejim

Differential Revision: D28442418

fbshipit-source-id: c692af6ebda1cfd1284d59701a168bb56689da60

packages/metro-config/src/__tests__/__snapshots__/loadConfig-test.js.snap

@@ -129,7 +129,9 @@ Object {
       "default": Object {},
     },
     "unstable_collectDependenciesPath": "metro/src/ModuleGraph/worker/collectDependencies.js",
+    "unstable_compactOutput": false,
     "unstable_dependencyMapReservedName": null,
+    "unstable_disableNormalizePseudoGlobals": false,
     "workerPath": "metro/src/DeltaBundler/Worker",
   },
   "transformerPath": "",
@@ -268,7 +270,9 @@ Object {
       "default": Object {},
     },
     "unstable_collectDependenciesPath": "metro/src/ModuleGraph/worker/collectDependencies.js",
+    "unstable_compactOutput": false,
     "unstable_dependencyMapReservedName": null,
+    "unstable_disableNormalizePseudoGlobals": false,
     "workerPath": "metro/src/DeltaBundler/Worker",
   },
   "transformerPath": "",
@@ -407,7 +411,9 @@ Object {
       "default": Object {},
     },
     "unstable_collectDependenciesPath": "metro/src/ModuleGraph/worker/collectDependencies.js",
+    "unstable_compactOutput": false,
     "unstable_dependencyMapReservedName": null,
+    "unstable_disableNormalizePseudoGlobals": false,
     "workerPath": "metro/src/DeltaBundler/Worker",
   },
   "transformerPath": "",
@@ -546,7 +552,9 @@ Object {
       "default": Object {},
     },
     "unstable_collectDependenciesPath": "metro/src/ModuleGraph/worker/collectDependencies.js",
+    "unstable_compactOutput": false,
     "unstable_dependencyMapReservedName": null,
+    "unstable_disableNormalizePseudoGlobals": false,
     "workerPath": "metro/src/DeltaBundler/Worker",
   },
   "transformerPath": "",

packages/metro-config/src/defaults/index.js

@@ -119,6 +119,8 @@ const getDefaultValues = (projectRoot: ?string): ConfigT => ({
     unstable_collectDependenciesPath:
       'metro/src/ModuleGraph/worker/collectDependencies.js',
     unstable_dependencyMapReservedName: null,
+    unstable_disableNormalizePseudoGlobals: false,
+    unstable_compactOutput: false,
   },
   cacheStores: [
     new FileStore({

packages/metro-transform-worker/src/__tests__/index-test.js

@@ -61,6 +61,8 @@ const baseConfig: JsTransformerConfig = {
   unstable_collectDependenciesPath:
     'metro/src/ModuleGraph/worker/collectDependencies',
   unstable_dependencyMapReservedName: null,
+  unstable_compactOutput: false,
+  unstable_disableNormalizePseudoGlobals: false,
 };
 
 beforeEach(() => {
@@ -480,3 +482,39 @@ it('throws if the reserved dependency map name appears in the input', async () =
     `"Source code contains the reserved string \`THE_DEP_MAP\` at character offset 55"`,
   );
 });
+
+it('allows disabling the normalizePseudoGlobals pass when minifying', async () => {
+  const result = await Transformer.transform(
+    {...baseConfig, unstable_disableNormalizePseudoGlobals: true},
+    '/root',
+    'local/file.js',
+    'arbitrary(code);',
+    {
+      dev: false,
+      minify: true,
+      type: 'module',
+    },
+  );
+  expect(result.output[0].data.code).toMatchInlineSnapshot(`
+    "__d(function (global, _$$_REQUIRE, _$$_IMPORT_DEFAULT, _$$_IMPORT_ALL, module, exports, _dependencyMap) {
+      minified(code);
+    });"
+  `);
+});
+
+it('allows emitting compact code when not minifying', async () => {
+  const result = await Transformer.transform(
+    {...baseConfig, unstable_compactOutput: true},
+    '/root',
+    'local/file.js',
+    'arbitrary(code);',
+    {
+      dev: false,
+      minify: false,
+      type: 'module',
+    },
+  );
+  expect(result.output[0].data.code).toMatchInlineSnapshot(
+    `"__d(function(global,_$$_REQUIRE,_$$_IMPORT_DEFAULT,_$$_IMPORT_ALL,module,exports,_dependencyMap){arbitrary(code);});"`,
+  );
+});

packages/metro-transform-worker/src/index.js

@@ -93,6 +93,8 @@ export type JsTransformerConfig = $ReadOnly<{|
   allowOptionalDependencies: AllowOptionalDependencies,
   unstable_collectDependenciesPath: string,
   unstable_dependencyMapReservedName: ?string,
+  unstable_disableNormalizePseudoGlobals: boolean,
+  unstable_compactOutput: boolean,
 |}>;
 
 export type {CustomTransformOptions} from 'metro-babel-transformer';
@@ -329,7 +331,6 @@ async function transformJS(
       code: false,
       configFile: false,
       comments: false,
-      compact: false,
       filename: file.filename,
       plugins,
       sourceMaps: false,
@@ -389,7 +390,11 @@ async function transformJS(
   if (config.unstable_dependencyMapReservedName != null) {
     reserved.push(config.unstable_dependencyMapReservedName);
   }
-  if (options.minify && file.inputFileSize <= config.optimizationSizeLimit) {
+  if (
+    options.minify &&
+    file.inputFileSize <= config.optimizationSizeLimit &&
+    !config.unstable_disableNormalizePseudoGlobals
+  ) {
     reserved.push(
       ...metroTransformPlugins.normalizePseudoGlobals(wrappedAst, {
         reservedNames: reserved,
@@ -401,7 +406,7 @@ async function transformJS(
     wrappedAst,
     {
       comments: false,
-      compact: false,
+      compact: config.unstable_compactOutput,
       filename: file.filename,
       retainLines: false,
       sourceFileName: file.filename,