feat: better destructuring assignments (#12872)

* preserve existing destructuring assignments where appropriate

* add test

* changeset

Author: Rich-Harris

.changeset/rich-elephants-relax.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: better destructuring assignments

packages/svelte/src/compiler/phases/3-transform/client/visitors/AssignmentExpression.js

@@ -11,7 +11,13 @@ import { visit_assignment_expression } from '../../shared/assignments.js';
  * @param {Context} context
  */
 export function AssignmentExpression(node, context) {
-	return visit_assignment_expression(node, context, build_assignment);
+	const expression = /** @type {Expression} */ (
+		visit_assignment_expression(node, context, build_assignment) ?? context.next()
+	);
+
+	return is_ignored(node, 'ownership_invalid_mutation')
+		? b.call('$.skip_ownership_validation', b.thunk(expression))
+		: expression;
 }
 
 /**
@@ -109,19 +115,17 @@ function build_assignment(operator, left, right, context) {
 		return transform.assign(object, value);
 	}
 
-	/** @type {Expression} */
-	let mutation = b.assignment(
-		operator,
-		/** @type {Pattern} */ (context.visit(left)),
-		/** @type {Expression} */ (context.visit(right))
-	);
-
 	// mutation
 	if (transform?.mutate) {
-		mutation = transform.mutate(object, mutation);
+		return transform.mutate(
+			object,
+			b.assignment(
+				operator,
+				/** @type {Pattern} */ (context.visit(left)),
+				/** @type {Expression} */ (context.visit(right))
+			)
+		);
 	}
 
-	return is_ignored(left, 'ownership_invalid_mutation')
-		? b.call('$.skip_ownership_validation', b.thunk(mutation))
-		: mutation;
+	return null;
 }

packages/svelte/src/compiler/phases/3-transform/server/visitors/AssignmentExpression.js

@@ -10,7 +10,7 @@ import { visit_assignment_expression } from '../../shared/assignments.js';
  * @param {Context} context
  */
 export function AssignmentExpression(node, context) {
-	return visit_assignment_expression(node, context, build_assignment);
+	return visit_assignment_expression(node, context, build_assignment) ?? context.next();
 }
 
 /**

packages/svelte/src/compiler/phases/3-transform/shared/assignments.js

@@ -41,7 +41,7 @@ export function visit_assignment_expression(node, context, build_assignment) {
 
 		if (!changed) {
 			// No change to output -> nothing to transform -> we can keep the original assignment
-			return context.next();
+			return null;
 		}
 
 		const is_standalone = /** @type {Node} */ (context.path.at(-1)).type.endsWith('Statement');
@@ -70,8 +70,5 @@ export function visit_assignment_expression(node, context, build_assignment) {
 		throw new Error(`Unexpected assignment type ${node.left.type}`);
 	}
 
-	return (
-		build_assignment(node.operator, node.left, node.right, context) ??
-		/** @type {Expression} */ (context.next())
-	);
+	return build_assignment(node.operator, node.left, node.right, context);
 }

packages/svelte/tests/snapshot/samples/destructured-assignments/_expected/client/index.svelte.js

@@ -3,10 +3,14 @@ import * as $ from "svelte/internal/client";
 
 let a = $.source(1);
 let b = $.source(2);
+let c = 3;
+let d = 4;
 
 export function update(array) {
 	(
 		$.set(a, $.proxy(array[0])),
 		$.set(b, $.proxy(array[1]))
 	);
+
+	[c, d] = array;
 }

packages/svelte/tests/snapshot/samples/destructured-assignments/_expected/server/index.svelte.js

@@ -3,7 +3,10 @@ import * as $ from "svelte/internal/server";
 
 let a = 1;
 let b = 2;
+let c = 3;
+let d = 4;
 
 export function update(array) {
 	[a, b] = array;
+	[c, d] = array;
 }

packages/svelte/tests/snapshot/samples/destructured-assignments/index.svelte.js

@@ -1,6 +1,9 @@
 let a = $state(1);
 let b = $state(2);
+let c = 3;
+let d = 4;
 
 export function update(array) {
 	[a, b] = array;
+	[c, d] = array;
 }