clone ASTs, for fast incremental rebuilds

src/Module.js

@@ -1,6 +1,6 @@
 import { parse } from 'acorn/src/index.js';
 import MagicString from 'magic-string';
-import { assign, blank, keys } from './utils/object.js';
+import { assign, blank, deepClone, keys } from './utils/object.js';
 import { basename, extname } from './utils/path.js';
 import getLocation from './utils/getLocation.js';
 import makeLegalIdentifier from './utils/makeLegalIdentifier.js';
@@ -35,6 +35,7 @@ export default class Module {
 
 		this.comments = [];
 		this.ast = ast || tryParse( code, this.comments, bundle.acornOptions, id ); // TODO what happens to comments if AST is provided?
+		this.astClone = deepClone( this.ast );
 
 		this.bundle = bundle;
 		this.id = id;
@@ -316,8 +317,7 @@ export default class Module {
 			id: this.id,
 			code: this.code,
 			originalCode: this.originalCode,
-			// TODO reinstate AST caching (rewrite broke it, because AST is enhanced)
-			// ast: this.ast,
+			ast: this.astClone,
 			sourceMapChain: this.sourceMapChain,
 			resolvedIds: this.resolvedIds
 		};

src/utils/object.js

@@ -17,3 +17,24 @@ export function assign ( target, ...sources ) {
 
 	return target;
 }
+
+const isArray = Array.isArray;
+
+// used for cloning ASTs. Not for use with cyclical structures!
+export function deepClone ( obj ) {
+	if ( !obj ) return obj;
+	if ( typeof obj !== 'object' ) return obj;
+
+	if ( isArray( obj ) ) {
+		const clone = new Array( obj.length );
+		for ( let i = 0; i < obj.length; i += 1 ) clone[i] = deepClone( obj[i] );
+		return clone;
+	}
+
+	const clone = {};
+	for ( const key in obj ) {
+		clone[ key ] = deepClone( obj[ key ] );
+	}
+
+	return clone;
+}

test/test.js

@@ -6,6 +6,7 @@ const sander = require( 'sander' );
 const assert = require( 'assert' );
 const { exec } = require( 'child_process' );
 const buble = require( 'buble' );
+const acorn = require( 'acorn' );
 const rollup = require( '../dist/rollup' );
 
 const FUNCTION = path.resolve( __dirname, 'function' );
@@ -587,6 +588,21 @@ describe( 'rollup', function () {
 				assert.equal( executeBundle( bundle ), 21 );
 			});
 		});
+
+		it( 'keeps ASTs between runs', () => {
+			return rollup.rollup({
+				entry: 'entry',
+				plugins: [ plugin ]
+			}).then( bundle => {
+				const asts = {};
+				bundle.modules.forEach( module => {
+					asts[ module.id ] = module.ast;
+				});
+
+				assert.deepEqual( asts.entry, acorn.parse( modules.entry, { sourceType: 'module' }) );
+				assert.deepEqual( asts.foo, acorn.parse( modules.foo, { sourceType: 'module' }) );
+			});
+		});
 	});
 
 	describe( 'hooks', () => {