Warn on improper use of reactive declarations

src/compile/Component.ts

@@ -541,9 +541,21 @@ export default class Component {
 	}
 
 	walk_module_js() {
+		const component = this;
 		const script = this.ast.module;
 		if (!script) return;
 
+		walk(script.content, {
+			enter(node) {
+				if (node.type === 'LabeledStatement' && node.label.name === '$') {
+					component.warn(node, {
+						code: 'module-script-reactive-declaration',
+						message: '$: has no effect in a module script'
+					});
+				}
+			}
+		});
+
 		this.add_sourcemap_locations(script.content);
 
 		let { scope, globals } = create_scopes(script.content);
@@ -735,6 +747,13 @@ export default class Component {
 					scope = map.get(node);
 				}
 
+				if (node.type === 'LabeledStatement' && node.label.name === '$' && parent.type !== 'Program') {
+					component.warn(node, {
+						code: 'non-top-level-reactive-declaration',
+						message: '$: has no effect outside of the top-level'
+					});
+				}
+
 				if (is_reference(node, parent)) {
 					const object = get_object(node);
 					const { name } = object;

test/validator/samples/module-script-reactive-declaration/input.svelte

@@ -0,0 +1,5 @@
+<script context="module">
+	let num = 2;
+	let square;
+	$: square = num * num;
+</script>

test/validator/samples/module-script-reactive-declaration/warnings.json

@@ -0,0 +1,7 @@
+[{
+	"message": "$: has no effect in a module script",
+	"code": "module-script-reactive-declaration",
+	"start": { "line": 4, "column": 1, "character": 54 },
+	"end": { "line": 4, "column": 23, "character": 76 },
+	"pos": 54
+}]

test/validator/samples/reactive-declaration-non-top-level/input.svelte

@@ -0,0 +1,8 @@
+<script>
+	let num = 2;
+	$: square = num * num;
+
+	function myFunc() {
+		$: double = num * 2;
+	}
+</script>

test/validator/samples/reactive-declaration-non-top-level/warnings.json

@@ -0,0 +1,7 @@
+[{
+	"message": "$: has no effect outside of the top-level",
+	"code": "non-top-level-reactive-declaration",
+	"start": { "line": 6, "column": 2, "character": 71 },
+	"end": { "line": 6, "column": 22, "character": 91 },
+	"pos": 71
+}]