fix: add labels to @const tags and props (#16454)

* fix: add labels to `@const` tags and props

* changeset

Author: Rich-Harris

.changeset/four-spiders-type.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: add labels to `@const` tags and props

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

@@ -17,7 +17,13 @@ export function ConstTag(node, context) {
 	// TODO we can almost certainly share some code with $derived(...)
 	if (declaration.id.type === 'Identifier') {
 		const init = build_expression(context, declaration.init, node.metadata.expression);
-		context.state.init.push(b.const(declaration.id, create_derived(context.state, b.thunk(init))));
+		let expression = create_derived(context.state, b.thunk(init));
+
+		if (dev) {
+			expression = b.call('$.tag', expression, b.literal(declaration.id.name));
+		}
+
+		context.state.init.push(b.const(declaration.id, expression));
 
 		context.state.transform[declaration.id.name] = { read: get_value };
 
@@ -55,7 +61,13 @@ export function ConstTag(node, context) {
 			])
 		);
 
-		context.state.init.push(b.const(tmp, create_derived(context.state, fn)));
+		let expression = create_derived(context.state, fn);
+
+		if (dev) {
+			expression = b.call('$.tag', expression, b.literal('[@const]'));
+		}
+
+		context.state.init.push(b.const(tmp, expression));
 
 		// we need to eagerly evaluate the expression in order to hit any
 		// 'Cannot access x before initialization' errors

packages/svelte/src/internal/client/reactivity/props.js

@@ -393,6 +393,10 @@ export function prop(props, key, flags, fallback) {
 		return getter();
 	});
 
+	if (DEV) {
+		d.label = key;
+	}
+
 	// Capture the initial value if it's bindable
 	if (bindable) get(d);