fix: don't warn on writes to `$state`

fixes #10905

.changeset/calm-buses-clap.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: don't warn on writes to `$state`

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -1242,6 +1242,7 @@ const common_visitors = {
 			if (
 				context.state.analysis.runes &&
 				node !== binding.node &&
+				context.state.function_depth === binding.scope.function_depth &&
 				// If we have $state that can be proxied or frozen and isn't re-assigned, then that means
 				// it's likely not using a primitive value and thus this warning isn't that helpful.
 				((binding.kind === 'state' &&
@@ -1252,7 +1253,12 @@ const common_visitors = {
 							!should_proxy_or_freeze(binding.initial.arguments[0], context.state.scope)))) ||
 					binding.kind === 'frozen_state' ||
 					binding.kind === 'derived') &&
-				context.state.function_depth === binding.scope.function_depth
+				// We're only concerned with reads here
+				parent.type !== 'AssignmentExpression' &&
+				parent.type !== 'UpdateExpression' &&
+				(parent.type !== 'MemberExpression' ||
+					(context.path.at(-2)?.type !== 'AssignmentExpression' &&
+						context.path.at(-2)?.type !== 'UpdateExpression'))
 			) {
 				w.state_referenced_locally(node);
 			}

packages/svelte/tests/validator/samples/static-state-reference/input.svelte

@@ -6,6 +6,11 @@
 	console.log(obj);
 	console.log(count);
 	console.log(doubled);
+	// these are ok because they're writes
+	count++;
+	count = 1;
+	obj.a++;
+	obj.a = 1;
 </script>
 
 <button onclick={() => count += 1}>