fix: disallow $state/$derived in const tags (#15115)

Author: Rich-Harris

.changeset/strong-shoes-whisper.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: disallow $state/$derived in const tags

packages/svelte/src/compiler/phases/2-analyze/visitors/CallExpression.js

@@ -79,7 +79,8 @@ export function CallExpression(node, context) {
 		case '$derived':
 		case '$derived.by':
 			if (
-				parent.type !== 'VariableDeclarator' &&
+				(parent.type !== 'VariableDeclarator' ||
+					get_parent(context.path, -3).type === 'ConstTag') &&
 				!(parent.type === 'PropertyDefinition' && !parent.static && !parent.computed)
 			) {
 				e.state_invalid_placement(node, rune);

packages/svelte/tests/validator/samples/const-tag-invalid-rune-usage/errors.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "state_invalid_placement",
+		"message": "`$derived(...)` can only be used as a variable declaration initializer or a class field",
+		"start": {
+			"line": 2,
+			"column": 15
+		},
+		"end": {
+			"line": 2,
+			"column": 26
+		}
+	}
+]

packages/svelte/tests/validator/samples/const-tag-invalid-rune-usage/input.svelte

@@ -0,0 +1,3 @@
+{#snippet test()}
+	{@const der = $derived(0)}
+{/snippet}