From 79187160bcf3ca0e24d0371cdaf0defcd2fc6824 Mon Sep 17 00:00:00 2001
From: Evan Wallace <evan.exe@gmail.com>
Date: Tue, 14 Dec 2021 00:36:15 +0000
Subject: [PATCH] automatically add __PURE__ to WeakMap and WeakSet

---
 internal/js_parser/js_parser.go      | 29 ++++++++++++++++++++++++++++
 internal/js_parser/js_parser_test.go | 28 +++++++++++++++++++++++++++
 2 files changed, 57 insertions(+)

diff --git a/internal/js_parser/js_parser.go b/internal/js_parser/js_parser.go
index 03141a7352d..6203a2f6758 100644
--- a/internal/js_parser/js_parser.go
+++ b/internal/js_parser/js_parser.go
@@ -13103,6 +13103,35 @@ func (p *parser) maybeMarkKnownGlobalConstructorAsPure(e *js_ast.ENew) {
 	if id, ok := e.Target.Data.(*js_ast.EIdentifier); ok {
 		if symbol := p.symbols[id.Ref.InnerIndex]; symbol.Kind == js_ast.SymbolUnbound {
 			switch symbol.OriginalName {
+			case "WeakSet", "WeakMap":
+				n := len(e.Args)
+
+				if n == 0 {
+					// "new WeakSet()" is pure
+					e.CanBeUnwrappedIfUnused = true
+					break
+				}
+
+				if n == 1 {
+					switch arg := e.Args[0].Data.(type) {
+					case *js_ast.ENull, *js_ast.EUndefined:
+						// "new WeakSet(null)" is pure
+						// "new WeakSet(void 0)" is pure
+						e.CanBeUnwrappedIfUnused = true
+
+					case *js_ast.EArray:
+						if len(arg.Items) == 0 {
+							// "new WeakSet([])" is pure
+							e.CanBeUnwrappedIfUnused = true
+						} else {
+							// "new WeakSet([x])" is impure because an exception is thrown if "x" is not an object
+						}
+
+					default:
+						// "new WeakSet(x)" is impure because the iterator for "x" could have side effects
+					}
+				}
+
 			case "Set":
 				n := len(e.Args)
 
diff --git a/internal/js_parser/js_parser_test.go b/internal/js_parser/js_parser_test.go
index 961ede1403f..f48036fc5c6 100644
--- a/internal/js_parser/js_parser_test.go
+++ b/internal/js_parser/js_parser_test.go
@@ -4911,3 +4911,31 @@ func TestAutoPureForMap(t *testing.T) {
 	expectPrinted(t, "new Map([x, []])", "new Map([x, []]);\n")
 	expectPrinted(t, "new Map([[], x])", "new Map([[], x]);\n")
 }
+
+func TestAutoPureForWeakSet(t *testing.T) {
+	expectPrinted(t, "new WeakSet", "/* @__PURE__ */ new WeakSet();\n")
+	expectPrinted(t, "new WeakSet(null)", "/* @__PURE__ */ new WeakSet(null);\n")
+	expectPrinted(t, "new WeakSet(undefined)", "/* @__PURE__ */ new WeakSet(void 0);\n")
+	expectPrinted(t, "new WeakSet([])", "/* @__PURE__ */ new WeakSet([]);\n")
+
+	expectPrinted(t, "new WeakSet([x])", "new WeakSet([x]);\n")
+	expectPrinted(t, "new WeakSet(x)", "new WeakSet(x);\n")
+	expectPrinted(t, "new WeakSet(false)", "new WeakSet(false);\n")
+	expectPrinted(t, "new WeakSet({})", "new WeakSet({});\n")
+	expectPrinted(t, "new WeakSet({ x })", "new WeakSet({ x });\n")
+}
+
+func TestAutoPureForWeakMap(t *testing.T) {
+	expectPrinted(t, "new WeakMap", "/* @__PURE__ */ new WeakMap();\n")
+	expectPrinted(t, "new WeakMap(null)", "/* @__PURE__ */ new WeakMap(null);\n")
+	expectPrinted(t, "new WeakMap(undefined)", "/* @__PURE__ */ new WeakMap(void 0);\n")
+	expectPrinted(t, "new WeakMap([])", "/* @__PURE__ */ new WeakMap([]);\n")
+
+	expectPrinted(t, "new WeakMap([[]])", "new WeakMap([[]]);\n")
+	expectPrinted(t, "new WeakMap([[], []])", "new WeakMap([[], []]);\n")
+	expectPrinted(t, "new WeakMap(x)", "new WeakMap(x);\n")
+	expectPrinted(t, "new WeakMap(false)", "new WeakMap(false);\n")
+	expectPrinted(t, "new WeakMap([x])", "new WeakMap([x]);\n")
+	expectPrinted(t, "new WeakMap([x, []])", "new WeakMap([x, []]);\n")
+	expectPrinted(t, "new WeakMap([[], x])", "new WeakMap([[], x]);\n")
+}