perf: optimize simple object destructuring in @const tags (#18390)

This PR resolves a `TODO` in `ConstTag.js` regarding the optimization of
simple object pattern matching cases like `{@const { x } = y}`.

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/optimize-const-tag.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+perf: optimize simple object destructuring in `@const` tags

packages/svelte/src/compiler/phases/1-parse/acorn.js

@@ -59,7 +59,7 @@ export function parse(source, comments, typescript, is_script) {
 
 		return /** @type {Program} */ (ast);
 	} catch (err) {
-		// TODO the `return` in necessary for TS<7 due to a bug; otherwise
+		// TODO the `return` is necessary for TS<7 due to a bug; otherwise
 		// the `finally` block is regarded as unreachable
 		return handle_parse_error(err);
 	} finally {

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

@@ -45,18 +45,29 @@ export function ConstTag(node, context) {
 			transform
 		});
 
-		// TODO optimise the simple `{ x } = y` case — we can just return `y`
+		const is_simple_object_pattern =
-		// instead of destructuring it only to return a new object
+			declaration.id.type === 'ObjectPattern' &&
+			declaration.id.properties.every(
+				(p) =>
+					p.type === 'Property' &&
+					!p.computed &&
+					p.key.type === 'Identifier' &&
+					p.value.type === 'Identifier' &&
+					p.key.name === p.value.name
+			);
+
 		const init = build_expression(
 			{ ...context, state: child_state },
 			declaration.init,
 			node.metadata.expression
 		);
 
-		const block = b.block([
+		const block = is_simple_object_pattern
-			b.const(/** @type {Pattern} */ (context.visit(declaration.id, child_state)), init),
+			? b.block([b.return(init)])
-			b.return(b.object(identifiers.map((node) => b.prop('init', node, node))))
+			: b.block([
-		]);
+					b.const(/** @type {Pattern} */ (context.visit(declaration.id, child_state)), init),
+					b.return(b.object(identifiers.map((node) => b.prop('init', node, node))))
+				]);
 
 		let expression = create_derived(context.state, block, node.metadata.expression.has_await);