Adds support for Promises returned in config, fixes #1503

bin/src/run/loadConfigFile.js

@@ -37,17 +37,18 @@ export default function loadConfigFile (configFile, silent) {
 			};
 
 			delete require.cache[configFile];
-			const configs = require( configFile );
-			if ( Object.keys( configs ).length === 0 ) {
-				handleError({
-					code: 'MISSING_CONFIG',
-					message: 'Config file must export an options object, or an array of options objects',
-					url: 'https://github.com/rollup/rollup/wiki/Command-Line-Interface#using-a-config-file'
-				});
-			}
+			return Promise.resolve(require( configFile )).then(configs => {
+				if ( Object.keys( configs ).length === 0 ) {
+					handleError({
+						code: 'MISSING_CONFIG',
+						message: 'Config file must export an options object, or an array of options objects',
+						url: 'https://github.com/rollup/rollup/wiki/Command-Line-Interface#using-a-config-file'
+					});
+				}
 
-			require.extensions[ '.js' ] = defaultLoader;
+				require.extensions[ '.js' ] = defaultLoader;
 
-			return Array.isArray( configs ) ? configs : [configs];
+				return Array.isArray( configs ) ? configs : [configs];
+			});
 		});
 }

test/cli/samples/config-promise/_config.js

@@ -0,0 +1,5 @@
+module.exports = {
+	description: 'uses config file which returns a Promise',
+	command: 'rollup --config rollup.config.js',
+	execute: true
+};

test/cli/samples/config-promise/main.js

@@ -0,0 +1 @@
+assert.equal( ANSWER, 42 );

test/cli/samples/config-promise/rollup.config.js

@@ -0,0 +1,9 @@
+var replace = require( 'rollup-plugin-replace' );
+
+module.exports = Promise.resolve({
+	input: 'main.js',
+	format: 'cjs',
+	plugins: [
+		replace({ 'ANSWER': 42 })
+	]
+});