From 0ce529584225c87ff7e93c85c8eca3ffaff78224 Mon Sep 17 00:00:00 2001
From: Lukas Taegert-Atkinson <lukas.taegert-atkinson@tngtech.com>
Date: Tue, 5 Jan 2021 08:39:48 +0100
Subject: [PATCH] Treeshake chained assignment expressions

---
 src/ast/nodes/AssignmentExpression.ts         | 20 ++++++++++++++++---
 .../samples/chained-assignments/_config.js    |  3 +++
 .../samples/chained-assignments/_expected.js  |  8 ++++++++
 test/form/samples/chained-assignments/main.js |  8 ++++++++
 .../samples/nested-member-access/_expected.js |  4 +---
 .../form/samples/nested-member-access/main.js |  4 ++--
 6 files changed, 39 insertions(+), 8 deletions(-)
 create mode 100644 test/form/samples/chained-assignments/_config.js
 create mode 100644 test/form/samples/chained-assignments/_expected.js
 create mode 100644 test/form/samples/chained-assignments/main.js

diff --git a/src/ast/nodes/AssignmentExpression.ts b/src/ast/nodes/AssignmentExpression.ts
index 1bf17272651..44bbc3de7d2 100644
--- a/src/ast/nodes/AssignmentExpression.ts
+++ b/src/ast/nodes/AssignmentExpression.ts
@@ -5,7 +5,7 @@ import {
 	RenderOptions
 } from '../../utils/renderHelpers';
 import { getSystemExportFunctionLeft } from '../../utils/systemJsRendering';
-import { HasEffectsContext, InclusionContext } from '../ExecutionContext';
+import { createHasEffectsContext, HasEffectsContext, InclusionContext } from '../ExecutionContext';
 import { EMPTY_PATH, ObjectPath, UNKNOWN_PATH } from '../utils/PathTracker';
 import Variable from '../variables/Variable';
 import * as NodeType from './NodeType';
@@ -48,13 +48,27 @@ export default class AssignmentExpression extends NodeBase {
 	include(context: InclusionContext, includeChildrenRecursively: IncludeChildren): void {
 		if (!this.deoptimized) this.applyDeoptimizations();
 		this.included = true;
-		this.left.include(context, includeChildrenRecursively);
+		let hasEffectsContext;
+		if (
+			includeChildrenRecursively ||
+			this.operator !== '=' ||
+			this.left.included ||
+			((hasEffectsContext = createHasEffectsContext()),
+			this.left.hasEffects(hasEffectsContext) ||
+				this.left.hasEffectsWhenAssignedAtPath(EMPTY_PATH, hasEffectsContext))
+		) {
+			this.left.include(context, includeChildrenRecursively);
+		}
 		this.right.include(context, includeChildrenRecursively);
 	}
 
 	render(code: MagicString, options: RenderOptions) {
-		this.left.render(code, options);
 		this.right.render(code, options);
+		if (this.left.included) {
+			this.left.render(code, options);
+		} else {
+			code.remove(this.start, this.right.start);
+		}
 		if (options.format === 'system') {
 			const exportNames =
 				this.left.variable && options.exportNamesByVariable.get(this.left.variable);
diff --git a/test/form/samples/chained-assignments/_config.js b/test/form/samples/chained-assignments/_config.js
new file mode 100644
index 00000000000..ddbf4e78e64
--- /dev/null
+++ b/test/form/samples/chained-assignments/_config.js
@@ -0,0 +1,3 @@
+module.exports = {
+	description: 'treeshakes chained assignments'
+};
diff --git a/test/form/samples/chained-assignments/_expected.js b/test/form/samples/chained-assignments/_expected.js
new file mode 100644
index 00000000000..e27cbb566aa
--- /dev/null
+++ b/test/form/samples/chained-assignments/_expected.js
@@ -0,0 +1,8 @@
+let a, d;
+a = 1;
+d = 2;
+console.log(a, d);
+
+let e = 'first', f = 'second';
+e += f += 'third';
+console.log(e);
diff --git a/test/form/samples/chained-assignments/main.js b/test/form/samples/chained-assignments/main.js
new file mode 100644
index 00000000000..9ed53d07748
--- /dev/null
+++ b/test/form/samples/chained-assignments/main.js
@@ -0,0 +1,8 @@
+let a, b, c, d;
+a = b = 1;
+c = d = 2;
+console.log(a, d);
+
+let e = 'first', f = 'second';
+e += f += 'third';
+console.log(e);
diff --git a/test/form/samples/nested-member-access/_expected.js b/test/form/samples/nested-member-access/_expected.js
index efa8afcd2c8..cf136c54870 100644
--- a/test/form/samples/nested-member-access/_expected.js
+++ b/test/form/samples/nested-member-access/_expected.js
@@ -6,9 +6,7 @@ const retainedResult2 = retained2.foo.bar;
 
 const retained3 = void {};
 const retainedResult3 = retained3.foo;
-
-let retained4a;
-const retained4b = retained4a = undefined;
+const retained4b = undefined;
 const retainedResult4 = retained4b.foo;
 
 const retained5 = 1 + 2;
diff --git a/test/form/samples/nested-member-access/main.js b/test/form/samples/nested-member-access/main.js
index 8b42d3c5ad3..323ca9e7885 100644
--- a/test/form/samples/nested-member-access/main.js
+++ b/test/form/samples/nested-member-access/main.js
@@ -32,8 +32,8 @@ const retainedResult2 = retained2.foo.bar;
 const retained3 = void {};
 const retainedResult3 = retained3.foo;
 
-let retained4a;
-const retained4b = retained4a = undefined;
+let removed4a;
+const retained4b = removed4a = undefined;
 const retainedResult4 = retained4b.foo;
 
 const retained5 = 1 + 2;