breaking: overhaul proxies, remove $state.is (#12916)

* chore: use closures for state proxies

* use variables

* early return

* tidy up

* move ownership stuff into separate object

* put original value directly on STATE_SYMBOL

* rename

* tidy up

* tidy

* tweak

* fix

* remove is_frozen check

* remove `$state.is`

* avoid mutations

* tweak

* changesets

* changeset

* changeset

* regenerate

* add comment

* add note

* add test

Author: Rich-Harris

.changeset/fifty-actors-agree.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+breaking: disallow `Object.defineProperty` on state proxies with non-basic descriptors

.changeset/gorgeous-pans-sort.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+breaking: allow frozen objects to be proxied

.changeset/heavy-houses-pay.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+breaking: avoid mutations to underlying proxied object with $state

.changeset/short-starfishes-beg.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+breaking: remove $state.is rune

documentation/docs/03-runes/01-state.md

@@ -101,28 +101,6 @@ To take a static snapshot of a deeply reactive `$state` proxy, use `$state.snaps
 
 This is handy when you want to pass some state to an external library or API that doesn't expect a proxy, such as `structuredClone`.
 
-## `$state.is`
-
-Sometimes you might need to compare two values, one of which is a reactive `$state(...)` proxy but the other is not. For this you can use `$state.is(a, b)`:
-
-```svelte
-<script>
-	let foo = $state({});
-	let bar = {};
-
-	foo.bar = bar;
-
-	console.log(foo.bar === bar); // false — `foo.bar` is a reactive proxy
-	console.log($state.is(foo.bar, bar)); // true
-</script>
-```
-
-This is handy when you might want to check if the object exists within a deeply reactive object/array.
-
-Under the hood, `$state.is` uses [`Object.is`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/is) for comparing the values.
-
-> Use this as an escape hatch - most of the time you don't need this. Svelte will warn you at dev time if you happen to run into this problem
-
 ## `$derived`
 
 Derived state is declared with the `$derived` rune:

packages/svelte/messages/client-errors/errors.md

@@ -64,6 +64,10 @@
 
 > The `%rune%` rune is only available inside `.svelte` and `.svelte.js/ts` files
 
+## state_descriptors_fixed
+
+> Property descriptors defined on `$state` objects must contain `value` and always be `enumerable`, `configurable` and `writable`.
+
 ## state_prototype_fixed
 
 > Cannot set prototype of `$state` object

packages/svelte/messages/client-warnings/warnings.md

@@ -44,7 +44,7 @@
 
 ## state_proxy_equality_mismatch
 
-> Reactive `$state(...)` proxies and the values they proxy have different identities. Because of this, comparisons with `%operator%` will produce unexpected results. Consider using `$state.is(a, b)` instead%details%
+> Reactive `$state(...)` proxies and the values they proxy have different identities. Because of this, comparisons with `%operator%` will produce unexpected results
 
 `$state(...)` creates a [proxy](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy) of the value it is passed. The proxy and the value have different identities, meaning equality checks will always return `false`:
 
@@ -57,15 +57,4 @@
 </script>
 ```
 
-In the rare case that you need to compare them, you can use `$state.is`, which unwraps proxies:
-
-```svelte
-<script>
-	let value = { foo: 'bar' };
-	let proxy = $state(value);
-
-	$state.is(value, proxy); // true
-</script>
-```
-
-During development, Svelte will warn you when comparing values with proxies.
+To resolve this, ensure you're comparing values where both values were created with `$state(...)`, or neither were. Note that `$state.raw(...)` will _not_ create a state proxy.

packages/svelte/messages/compile-errors/script.md

@@ -122,6 +122,10 @@
 
 > Cannot use rune without parentheses
 
+## rune_removed
+
+> The `%name%` rune has been removed
+
 ## rune_renamed
 
 > `%name%` is now `%replacement%`

packages/svelte/src/ambient.d.ts

@@ -147,27 +147,6 @@ declare namespace $state {
 	 */
 	export function snapshot<T>(state: T): Snapshot<T>;
 
-	/**
-	 * Compare two values, one or both of which is a reactive `$state(...)` proxy.
-	 *
-	 * Example:
-	 * ```ts
-	 * <script>
-	 *	 let foo = $state({});
-	 *	 let bar = {};
-	 *
-	 *	 foo.bar = bar;
-	 *
-	 *	 console.log(foo.bar === bar); // false — `foo.bar` is a reactive proxy
-	 *   console.log($state.is(foo.bar, bar)); // true
-	 * </script>
-	 * ```
-	 *
-	 * https://svelte-5-preview.vercel.app/docs/runes#$state.is
-	 *
-	 */
-	export function is(a: any, b: any): boolean;
-
 	// prevent intellisense from being unhelpful
 	/** @deprecated */
 	export const apply: never;

packages/svelte/src/compiler/errors.js

@@ -348,6 +348,16 @@ export function rune_missing_parentheses(node) {
 	e(node, "rune_missing_parentheses", "Cannot use rune without parentheses");
 }
 
+/**
+ * The `%name%` rune has been removed
+ * @param {null | number | NodeLike} node
+ * @param {string} name
+ * @returns {never}
+ */
+export function rune_removed(node, name) {
+	e(node, "rune_removed", `The \`${name}\` rune has been removed`);
+}
+
 /**
  * `%name%` is now `%replacement%`
  * @param {null | number | NodeLike} node