tree shaking: handle destructuring of an array

internal/bundler_tests/bundler_dce_test.go

@@ -4335,6 +4335,34 @@ func TestDCEOfIIFE(t *testing.T) {
 	})
 }
 
+func TestDCEOfDestructuring(t *testing.T) {
+	dce_suite.expectBundled(t, bundled{
+		files: map[string]string{
+			"/entry.js": `
+				// Identifier bindings
+				var remove1
+				var remove2 = null
+				var KEEP1 = x
+
+				// Array patterns
+				var [remove3] = []
+				var [remove4, ...remove5] = [...[1, 2], 3]
+				var [, , remove6] = [, , 3]
+				var [KEEP2] = [x]
+				var [KEEP3] = [...{}]
+
+				// Object patterns (not handled right now)
+				var { KEEP4 } = {}
+			`,
+		},
+		entryPaths: []string{"/entry.js"},
+		options: config.Options{
+			Mode:         config.ModeBundle,
+			AbsOutputDir: "/out",
+		},
+	})
+}
+
 func TestDCEOfDecorators(t *testing.T) {
 	dce_suite.expectBundled(t, bundled{
 		files: map[string]string{

internal/bundler_tests/bundler_default_test.go

@@ -8187,9 +8187,8 @@ func TestCommentPreservation(t *testing.T) {
 		},
 		entryPaths: []string{"/entry.js"},
 		options: config.Options{
-			Mode:         config.ModeBundle,
+			Mode:         config.ModePassThrough,
 			AbsOutputDir: "/out",
-			OutputFormat: config.FormatCommonJS,
 			ExternalSettings: config.ExternalSettings{
 				PreResolve: config.ExternalMatchers{
 					Exact: map[string]bool{"foo": true},

internal/bundler_tests/snapshots/snapshots_dce.txt

@@ -438,6 +438,15 @@ var StaticAccessor = class {
   static accessor accessor;
 };
 
+================================================================================
+TestDCEOfDestructuring
+---------- /out/entry.js ----------
+// entry.js
+var KEEP1 = x;
+var [KEEP2] = [x];
+var [KEEP3] = [...{}];
+var { KEEP4 } = {};
+
 ================================================================================
 TestDCEOfExperimentalDecorators
 ---------- /out/keep-these.js ----------

internal/bundler_tests/snapshots/snapshots_default.txt

@@ -300,7 +300,6 @@ export {
 ================================================================================
 TestCommentPreservation
 ---------- /out/entry.js ----------
-// entry.js
 console.log(
   import(
     /* before */
@@ -393,54 +392,54 @@ console.log(
     /* after */
   )
 );
-var [
+let [
   /* foo */
 ] = [
   /* bar */
 ];
-var [
+let [
   // foo
 ] = [
   // bar
 ];
-var [
+let [
   /*before*/
   ...s
 ] = [
   /*before*/
   ...s
 ];
-var [.../*before*/
+let [.../*before*/
 s2] = [.../*before*/
 s2];
-var {
+let {
   /* foo */
 } = {
   /* bar */
 };
-var {
+let {
   // foo
 } = {
   // bar
 };
-var {
+let {
   /*before*/
   ...s3
 } = {
   /*before*/
   ...s3
 };
-var { .../*before*/
+let { .../*before*/
 s4 } = { .../*before*/
 s4 };
-var [
+let [
   /* before */
   x
 ] = [
   /* before */
   x
 ];
-var [
+let [
   /* before */
   x2
   /* after */
@@ -449,7 +448,7 @@ var [
   x2
   /* after */
 ];
-var [
+let [
   // before
   x3
   // after
@@ -458,14 +457,14 @@ var [
   x3
   // after
 ];
-var {
+let {
   /* before */
   y
 } = {
   /* before */
   y
 };
-var {
+let {
   /* before */
   y2
   /* after */
@@ -474,7 +473,7 @@ var {
   y2
   /* after */
 };
-var {
+let {
   // before
   y3
   // after
@@ -483,21 +482,21 @@ var {
   y3
   // after
 };
-var {
+let {
   /* before */
   [y4]: y4
 } = {
   /* before */
   [y4]: y4
 };
-var { [
+let { [
   /* before */
   y5
 ]: y5 } = { [
   /* before */
   y5
 ]: y5 };
-var { [
+let { [
   y6
   /* after */
 ]: y6 } = { [

internal/js_ast/js_ast_helpers.go

@@ -2035,14 +2035,46 @@ func StmtsCanBeRemovedIfUnused(stmts []Stmt, flags StmtsCanBeRemovedIfUnusedFlag
 			}
 
 			for _, decl := range s.Decls {
-				if _, ok := decl.Binding.Data.(*BIdentifier); !ok {
+				// Check that the bindings are side-effect free
+				switch binding := decl.Binding.Data.(type) {
+				case *BIdentifier:
+					// An identifier binding has no side effects
+
+				case *BArray:
+					// Destructuring the initializer has no side effects if the
+					// initializer is an array, since we assume the iterator is then
+					// the built-in side-effect free array iterator.
+					if _, ok := decl.ValueOrNil.Data.(*EArray); ok {
+						for _, item := range binding.Items {
+							if item.DefaultValueOrNil.Data != nil && !ExprCanBeRemovedIfUnused(item.DefaultValueOrNil, isUnbound) {
+								return false
+							}
+
+							switch item.Binding.Data.(type) {
+							case *BIdentifier, *BMissing:
+								// Right now we only handle an array pattern with identifier
+								// bindings or with empty holes (i.e. "missing" elements)
+							default:
+								return false
+							}
+						}
+						break
+					}
+					return false
+
+				default:
+					// Consider anything else to potentially have side effects
 					return false
 				}
+
+				// Check that the initializer is side-effect free
 				if decl.ValueOrNil.Data != nil {
 					if !ExprCanBeRemovedIfUnused(decl.ValueOrNil, isUnbound) {
 						return false
-					} else if s.Kind.IsUsing() {
-						// "using" declarations are only side-effect free if they are initialized to null or undefined
+					}
+
+					// "using" declarations are only side-effect free if they are initialized to null or undefined
+					if s.Kind.IsUsing() {
 						if t := KnownPrimitiveType(decl.ValueOrNil.Data); t != PrimitiveNull && t != PrimitiveUndefined {
 							return false
 						}
@@ -2261,6 +2293,13 @@ func ExprCanBeRemovedIfUnused(expr Expr, isUnbound func(ast.Ref) bool) bool {
 
 	case *EArray:
 		for _, item := range e.Items {
+			if spread, ok := item.Data.(*ESpread); ok {
+				if _, ok := spread.Value.Data.(*EArray); ok {
+					// Spread of an inline array such as "[...[x]]" is side-effect free
+					item = spread.Value
+				}
+			}
+
 			if !ExprCanBeRemovedIfUnused(item, isUnbound) {
 				return false
 			}