deprecate immutable

packages/svelte/src/compiler/validate-options.js

@@ -72,7 +72,10 @@ export const validate_component_options =
 
 			discloseVersion: boolean(true),
 
-			immutable: boolean(false),
+			immutable: deprecate(
+				'The immutable option has been deprecated. It has no effect in runes mode.',
+				boolean(false)
+			),
 
 			legacy: object({
 				componentApi: boolean(false)
@@ -166,6 +169,18 @@ function warn_removed(message) {
 	};
 }
 
+/**
+ * @param {string} message
+ * @param {Validator} validator
+ * @returns {Validator}
+ */
+function deprecate(message, validator) {
+	return (input, keypath) => {
+		if (input !== undefined) warn(message);
+		return validator(input, keypath);
+	};
+}
+
 /**
  * @param {Record<string, Validator>} children
  * @param {boolean} [allow_unknown]

sites/svelte-5-preview/src/routes/docs/content/03-appendix/03-deprecations.md

@@ -39,3 +39,7 @@ These functions run indiscriminately when _anything_ changes. By using `$effect.
 ```
 
 Note that using `$effect` and `$effect.pre` will put you in [runes mode](/docs/runes) — be sure to update your props and state accordingly.
+
+## `immutable`
+
+The `immutable` compiler option is deprecated. Use runes mode instead, where all state is immutable (which means that assigning to `object.property` won't cause updates for anything that is observing `object` itself, or a different property of it).