add warning in dev, add docs

documentation/docs/02-runes/02-$state.md

@@ -65,6 +65,19 @@ let { done, text } = todos[0];
 todos[0].done = !todos[0].done;
 ```
 
+You can also use `$state` in return statements to proxy their argument:
+
+```js
+function createCounter() {
+	return $state({
+		count: 0,
+		increment() {
+			this.count++;
+		}
+	});
+}
+```
+
 ### Classes
 
 You can also use `$state` in class fields (whether public or private):

documentation/docs/98-reference/.generated/client-warnings.md

@@ -219,6 +219,12 @@ Reactive `$state(...)` proxies and the values they proxy have different identiti
 
 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.
 
+### state_return_not_proxyable
+
+```
+The argument passed to a `$state` in a return statement must be a plain object or array. Otherwise, the `$state` call will have no effect
+```
+
 ### transition_slide_display
 
 ```

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

@@ -185,6 +185,10 @@ To fix it, either create callback props to communicate changes, or mark `person`
 
 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.
 
+## state_return_not_proxyable
+
+> The argument passed to a `$state` call in a return statement must be a plain object or array. Otherwise, the `$state` call will have no effect
+
 ## transition_slide_display
 
 > The `slide` transition does not work correctly for elements with `display: %value%`

packages/svelte/src/compiler/phases/3-transform/client/visitors/CallExpression.js

@@ -40,16 +40,11 @@ export function CallExpression(node, context) {
 				parent?.type === 'ReturnStatement' ||
 				(parent?.type === 'ArrowFunctionExpression' && parent.body === node)
 			) {
-				if (
-					node.arguments[0] &&
-					should_proxy(
-						/** @type {Expression} */ (context.visit(node.arguments[0])),
-						context.state.scope
-					)
-				) {
-					return b.call('$.proxy', node.arguments[0]);
-				} else {
-					return node.arguments[0] ?? b.void0;
+				if (node.arguments[0]) {
+					return b.call(
+						'$.return_proxy',
+						/** @type {Expression} */ (context.visit(node.arguments[0] ?? b.void0))
+					);
 				}
 			}
 	}

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

@@ -12,6 +12,7 @@ import { state as source, set } from './reactivity/sources.js';
 import { STATE_SYMBOL } from '#client/constants';
 import { UNINITIALIZED } from '../../constants.js';
 import * as e from './errors.js';
+import * as w from './warnings.js';
 import { get_stack } from './dev/tracing.js';
 import { tracing_mode_flag } from '../flags/index.js';
 
@@ -282,6 +283,21 @@ export function proxy(value) {
 	});
 }
 
+/**
+ * @template T
+ * @param {T} value
+ * @param {Source<T>} [prev]
+ * @returns {T | void}
+ */
+export function return_proxy(value, prev) {
+	const res = proxy(value, prev);
+	if (res !== value || (typeof value === 'object' && value !== null && STATE_SYMBOL in value)) {
+		// if the argument passed was already a proxy, we don't warn
+		return res;
+	}
+	w.state_return_not_proxyable();
+}
+
 /**
  * @param {Source<number>} signal
  * @param {1 | -1} [d]

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

@@ -170,6 +170,17 @@ export function state_proxy_equality_mismatch(operator) {
 	}
 }
 
+/**
+ * The argument passed to a `$state` in a return statement must be a plain object or array. Otherwise, the `$state` call will have no effect
+ */
+export function state_return_not_proxyable() {
+	if (DEV) {
+		console.warn(`%c[svelte] state_return_not_proxyable\n%cThe argument passed to a \`$state\` in a return statement must be a plain object or array. Otherwise, the \`$state\` call will have no effect\nhttps://svelte.dev/e/state_return_not_proxyable`, bold, normal);
+	} else {
+		console.warn(`https://svelte.dev/e/state_return_not_proxyable`);
+	}
+}
+
 /**
  * The `slide` transition does not work correctly for elements with `display: %value%`
  * @param {string} value

packages/svelte/tests/snapshot/samples/state-in-return/_expected/client/index.svelte.js

@@ -2,7 +2,7 @@
 import * as $ from 'svelte/internal/client';
 
 export default function proxy(object) {
-	return $.proxy(object);
+	return $.return_proxy(object);
 }
 
 export function createCounter() {
@@ -11,4 +11,4 @@ export function createCounter() {
 	$.update(count);
 }
 
-export const proxy_in_arrow = (object) => $.proxy(object);
+export const proxy_in_arrow = (object) => $.return_proxy(object);