Build: Switch form Terser to SWC for JS minification (#5286)

Also, as part of this, fix the `file` & `sources` properties of the source map
file.

Fixes gh-5285
Closes gh-5286
Ref gh-5258

5 files changed, 88 insertions, 19 deletions

diff --git a/Gruntfile.js b/Gruntfile.js
index 65aec9648..b5cbc1bf3 100644
--- a/Gruntfile.js
+++ b/Gruntfile.js
@@ -312,27 +312,41 @@ module.exports = function( grunt ) {
 			files: [ "<%= eslint.dev.src %>" ],
 			tasks: [ "dev" ]
 		},
-		terser: {
+		minify: {
 			all: {
 				files: {
 					"dist/<%= grunt.option('filename').replace('.js', '.min.js') %>":
 						"dist/<%= grunt.option('filename') %>"
 				},
 				options: {
-					ecma: 5,
 					sourceMap: {
-						filename: "dist/<%= grunt.option('filename').replace('.js', '.min.map') %>"
-					},
-					format: {
-						ascii_only: true,
-						comments: false,
-						preamble: "/*! jQuery v<%= pkg.version %> | " +
-							"(c) OpenJS Foundation and other contributors | " +
-							"jquery.org/license */"
+						filename: "dist/<%= grunt.option('filename').replace('.js', '.min.map') %>",
+
+						// The map's `files` & `sources` property are set incorrectly, fix
+						// them via overrides from the task config.
+						// See https://github.com/swc-project/swc/issues/7588#issuecomment-1624345254
+						overrides: {
+							file: "jquery.min.js",
+							sources: [
+								"jquery.js"
+							]
+						}
 					},
-					compress: {
-						hoist_funs: false,
-						loops: false
+					swc: {
+						format: {
+							ecma: 5,
+							asciiOnly: true,
+							comments: false,
+							preamble: "/*! jQuery v4.0.0-pre | " +
+								"(c) OpenJS Foundation and other contributors | " +
+								"jquery.org/license */\n"
+						},
+						compress: {
+							ecma: 5,
+							hoist_funs: false,
+							loops: false
+						},
+						mangle: true
 					}
 				}
 			}
@@ -400,7 +414,7 @@ module.exports = function( grunt ) {
 	grunt.registerTask( "dev", [
 		"build:*:*",
 		runIfNewNode( "newer:eslint:dev" ),
-		"newer:terser",
+		"newer:minify",
 		"remove_map_comment",
 		"dist:*",
 		"qunit_fixture",
@@ -410,7 +424,7 @@ module.exports = function( grunt ) {
 	grunt.registerTask( "default", [
 		runIfNewNode( "eslint:dev" ),
 		"build:*:*",
-		"terser",
+		"minify",
 		"remove_map_comment",
 		"dist:*",
 		"test:prepare",
diff --git a/build/tasks/build.js b/build/tasks/build.js
index 98d95cbe5..e1dd39993 100644
--- a/build/tasks/build.js
+++ b/build/tasks/build.js
@@ -339,6 +339,6 @@ module.exports = function( grunt ) {
 			"";
 
 		grunt.log.writeln( "Creating custom build...\n" );
-		grunt.task.run( [ "build:*:*" + ( modules ? ":" + modules : "" ), "terser", "dist" ] );
+		grunt.task.run( [ "build:*:*" + ( modules ? ":" + modules : "" ), "minify", "dist" ] );
 	} );
 };
diff --git a/build/tasks/minify.js b/build/tasks/minify.js
new file mode 100644
index 000000000..1a6ddbd4a
--- /dev/null
+++ b/build/tasks/minify.js
@@ -0,0 +1,56 @@
+/**
+ * Minify JavaScript using SWC.
+ */
+
+"use strict";
+
+module.exports = ( grunt ) => {
+	const swc = require( "@swc/core" );
+
+	grunt.registerMultiTask(
+		"minify",
+		"Minify JavaScript using SWC",
+		async function() {
+			const done = this.async();
+			const options = this.options();
+			const sourceMapFilename = options.sourceMap && options.sourceMap.filename;
+			const sourceMapOverrides = options.sourceMap && options.sourceMap.overrides || {};
+
+			await Promise.all( this.files.map( async( { src, dest } ) => {
+				if ( src.length !== 1 ) {
+					grunt.fatal( "The minify task requires a single source per destination" );
+				}
+
+				const { code, map: incompleteMap } = await swc.minify(
+					grunt.file.read( src[ 0 ] ),
+					{
+						...options.swc,
+						inlineSourcesContent: false,
+						sourceMap: sourceMapFilename ?
+							{
+								filename: sourceMapFilename
+							} :
+							false
+					}
+				);
+
+				grunt.file.write( dest, code );
+
+				if ( sourceMapFilename ) {
+
+					// Apply map overrides if needed. See the task config description
+					// for more details.
+					const mapObject = {
+						...JSON.parse( incompleteMap ),
+						...sourceMapOverrides
+					};
+					const map = JSON.stringify( mapObject );
+
+					grunt.file.write( sourceMapFilename, map );
+				}
+			} ) );
+
+			done();
+		}
+	);
+};
diff --git a/build/tasks/sourcemap.js b/build/tasks/sourcemap.js
index f6ef5698e..8b9c248e0 100644
--- a/build/tasks/sourcemap.js
+++ b/build/tasks/sourcemap.js
@@ -3,7 +3,7 @@
 var fs = require( "fs" );
 
 module.exports = function( grunt ) {
-	var config = grunt.config( "terser.all.files" );
+	var config = grunt.config( "minify.all.files" );
 	grunt.registerTask( "remove_map_comment", function() {
 		var minLoc = grunt.config.process( Object.keys( config )[ 0 ] );
 
diff --git a/package.json b/package.json
index dd92954d3..e02aa9b80 100644
--- a/package.json
+++ b/package.json
@@ -26,6 +26,7 @@
   "devDependencies": {
     "@babel/core": "7.10.5",
     "@babel/plugin-transform-for-of": "7.10.4",
+    "@swc/core": "1.3.66",
     "colors": "1.4.0",
     "commitplease": "3.2.0",
     "core-js-bundle": "3.6.5",
@@ -42,7 +43,6 @@
     "grunt-karma": "4.0.2",
     "grunt-newer": "1.3.0",
     "grunt-npmcopy": "0.2.0",
-    "grunt-terser": "2.0.0",
     "gzip-js": "0.3.2",
     "husky": "4.2.5",
     "jsdom": "19.0.0",
@@ -67,7 +67,6 @@
     "rollup": "2.21.0",
     "sinon": "7.3.1",
     "strip-json-comments": "3.1.1",
-    "terser": "5.17.6",
     "testswarm": "1.1.2"
   },
   "scripts": {