breaking: use structuredClone inside $state.snapshot (#12413)

* move cloning logic into new file, use structuredClone, add tests

* changeset

* breaking

* tweak

* use same cloning approach between server and client

* get types mostly working

* fix type error that popped up

* cheeky hack

* we no longer need deep_snapshot

* shallow copy state when freezing

* throw if argument is a state proxy

* docs

* regenerate

Author: Rich-Harris

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

@@ -90,9 +90,7 @@ person = {
 
 This can improve performance with large arrays and objects that you weren't planning to mutate anyway, since it avoids the cost of making them reactive. Note that frozen state can _contain_ reactive state (for example, a frozen array of reactive objects).
 
-In development mode, the argument to `$state.frozen` will be shallowly frozen with `Object.freeze()`, to make it obvious if you accidentally mutate it.
-
-> Objects and arrays passed to `$state.frozen` will have a `Symbol` property added to them to signal to Svelte that they are frozen. If you don't want this, pass in a clone of the object or array instead. The argument cannot be an existing state proxy created with `$state(...)`.
+> Objects and arrays passed to `$state.frozen` will be shallowly frozen using `Object.freeze()`. If you don't want this, pass in a clone of the object or array instead. The argument cannot be an existing state proxy created with `$state(...)`.
 
 ## `$state.snapshot`
 

packages/svelte/src/internal/client/errors.js

@@ -294,6 +294,22 @@ export function snippet_missing_mount() {
 	}
 }
 
+/**
+ * The argument to `$state.frozen(...)` cannot be an object created with `$state(...)`. You should create a copy of it first, for example with `$state.snapshot`
+ * @returns {never}
+ */
+export function state_frozen_invalid_argument() {
+	if (DEV) {
+		const error = new Error(`state_frozen_invalid_argument\nThe argument to \`$state.frozen(...)\` cannot be an object created with \`$state(...)\`. You should create a copy of it first, for example with \`$state.snapshot\``);
+
+		error.name = 'Svelte error';
+		throw error;
+	} else {
+		// TODO print a link to the documentation
+		throw new Error("state_frozen_invalid_argument");
+	}
+}
+
 /**
  * Cannot set prototype of `$state` object
  * @returns {never}

packages/svelte/src/internal/shared/types.d.ts

@@ -12,3 +12,5 @@ export type Snapshot<T> = ReturnType<typeof $state.snapshot<T>>;
 export type Getters<T> = {
 	[K in keyof T]: () => T[K];
 };
+
+export type Snapshot<T> = ReturnType<typeof $state.snapshot<T>>;