Preserve empty for...of loops (#3132)

* Add test based on #3126

* Placeholder fix

* comment out options

* remove solo

* get rid of comments

* leap of faith on export let;

* "Fix" two for...of tests

* fix for-of-scpes "fix"

* fix effect-in-for-of-loop test

* fix for effect-in-for-of-loop-in-functions

src/ast/nodes/ForOfStatement.ts

@@ -1,6 +1,5 @@
 import MagicString from 'magic-string';
 import { NO_SEMICOLON, RenderOptions } from '../../utils/renderHelpers';
-import { ExecutionPathOptions } from '../ExecutionPathOptions';
 import BlockScope from '../scopes/BlockScope';
 import Scope from '../scopes/Scope';
 import { EMPTY_PATH } from '../values';
@@ -27,14 +26,9 @@ export default class ForOfStatement extends StatementBase {
 		this.scope = new BlockScope(parentScope);
 	}
 
-	hasEffects(options: ExecutionPathOptions): boolean {
-		return (
-			(this.left &&
-				(this.left.hasEffects(options) ||
-					this.left.hasEffectsWhenAssignedAtPath(EMPTY_PATH, options))) ||
-			(this.right && this.right.hasEffects(options)) ||
-			this.body.hasEffects(options.setIgnoreBreakStatements())
-		);
+	hasEffects(): boolean {
+		// Placeholder until proper Symbol.Iterator support
+		return true;
 	}
 
 	include(includeChildrenRecursively: IncludeChildren) {

test/form/samples/effect-in-for-of-loop-in-functions/main.js

@@ -8,14 +8,6 @@ function a () {
 
 a();
 
-function b () {
-	for ( const item of items.children ) {
-		// do nothing
-	}
-}
-
-b();
-
 function c () {
 	let item;
 	for ( item of items.children ) {
@@ -25,15 +17,6 @@ function c () {
 
 c();
 
-function d () {
-	let item;
-	for ( item of items.children ) {
-		// do nothing
-	}
-}
-
-d();
-
 assert.deepEqual( items, [
 	{ foo: 'a', bar: 'c' },
 	{ foo: 'a', bar: 'c' },

test/form/samples/effect-in-for-of-loop/main.js

@@ -4,30 +4,16 @@ for ( const a of items ) {
 	a.foo = 'a';
 }
 
-for ( const b of items ) {
-	// do nothing
-}
-
 let c;
 for ( c of items ) {
 	c.bar = 'c';
 }
 
-let d;
-for ( d of items ) {
-	// do nothing
-}
-
 for ( e of items ) {
 	e.baz = 'e';
 }
 var e;
 
-for ( f of items ) {
-	// do nothing
-}
-var f;
-
 assert.deepEqual( items, [
 	{ foo: 'a', bar: 'c', baz: 'e' },
 	{ foo: 'a', bar: 'c', baz: 'e' },

test/form/samples/for-of-scopes/_expected.js

@@ -3,6 +3,9 @@ var associated = () => {};
 for ( var associated of [ effect1 ] ) {}
 associated();
 
+var effect2 = () => console.log( 'effect' );
+for ( let shadowed of [ effect2 ] ) {}
+
 var effect3 = () => console.log( 'effect' ); // Must not be removed!
 for ( const foo of [ effect3 ] ) {
 	foo(); // Must not be removed!

test/form/samples/side-effects-break-statements/main.js

@@ -16,10 +16,6 @@ for ( const val in { x: 1, y: 2 } ) {
 	break;
 }
 
-for ( const val of { x: 1, y: 2 } ) {
-	break;
-}
-
 for ( const val of { x: 1, y: 2 } ) {
 	console.log( 'effect' );
 	break;

test/function/samples/preserve-for-of-iterable/_config.js

@@ -0,0 +1,3 @@
+module.exports = {
+	description: 'preserves for...of loops'
+};

test/function/samples/preserve-for-of-iterable/iterables.js

@@ -0,0 +1,20 @@
+export let dirty;
+
+export const zeroToFive = {
+    [Symbol.iterator]() {
+        return {
+            current: 0,
+            last: 5,
+            next() {
+                const ret = this.current < this.last
+                    ? { done: false, value: this.current++ }
+                    : { done: true };
+
+                // assert later
+                dirty = this.current;
+
+                return ret;
+            }
+        };
+    }
+};

test/function/samples/preserve-for-of-iterable/loops.js

@@ -0,0 +1,19 @@
+export const awaitable = async (iterable) => {
+	for await (const value of iterable) {
+	}
+}
+
+// This is equivalent to the above 'awaitable' function.
+export const equivalent = async (iterable) => {
+	const iterator = iterable[Symbol.asyncIterator]()
+  let { done } = await iterator.next()
+  while (!done) {
+    ({ done } = await iterator.next())
+  }
+
+}
+
+export const iterate = iterable => {
+	for (const value of iterable) {
+	}
+}

test/function/samples/preserve-for-of-iterable/main.js

@@ -0,0 +1,6 @@
+import { zeroToFive, dirty } from './iterables';
+import { iterate } from './loops';
+
+assert.equal(dirty, undefined);
+iterate(zeroToFive);
+assert.equal(dirty, 5);