Do not partially tree-shake unused declarations in for loops (#3943)

src/ast/nodes/ForStatement.ts

@@ -40,7 +40,7 @@ export default class ForStatement extends StatementBase {
 
 	include(context: InclusionContext, includeChildrenRecursively: IncludeChildren) {
 		this.included = true;
-		if (this.init) this.init.include(context, includeChildrenRecursively);
+		if (this.init) this.init.includeAllDeclaredVariables(context, includeChildrenRecursively);
 		if (this.test) this.test.include(context, includeChildrenRecursively);
 		const { brokenFlow } = context;
 		if (this.update) this.update.include(context, includeChildrenRecursively);

src/ast/nodes/VariableDeclaration.ts

@@ -87,6 +87,7 @@ export default class VariableDeclaration extends NodeBase {
 
 	render(code: MagicString, options: RenderOptions, nodeRenderOptions: NodeRenderOptions = BLANK) {
 		if (
+			nodeRenderOptions.isNoStatement ||
 			areAllDeclarationsIncludedAndNotExported(this.declarations, options.exportNamesByVariable)
 		) {
 			for (const declarator of this.declarations) {

src/ast/nodes/VariableDeclarator.ts

@@ -45,6 +45,9 @@ export default class VariableDeclarator extends NodeBase {
 	): void {
 		this.included = true;
 		this.id.include(context, includeChildrenRecursively);
+		if (this.init) {
+			this.init.include(context, includeChildrenRecursively);
+		}
 	}
 
 	render(code: MagicString, options: RenderOptions) {

test/function/samples/unused-for-loop-declaration/_config.js

@@ -0,0 +1,3 @@
+module.exports = {
+	description: 'does not partially tree-shake unused declarations with side-effects in for loops'
+};

test/function/samples/unused-for-loop-declaration/main.js

@@ -0,0 +1,9 @@
+let result;
+let reassigned;
+
+for (var a = (reassigned = 'reassigned'), b = 0; b < 2; b++) {
+	result = b;
+}
+
+assert.strictEqual(result, 1);
+assert.strictEqual(reassigned, 'reassigned');