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

3 files changed, 58 insertions, 2 deletions

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 ] );