fix: don't turn component instances stored in $state into state proxies (#18646)

Fixes #18416.

We previously said that we don't want to handle component instances specifically when they're wrapped with state in  #16747 - though the use case presented back then was much more arcane than the one in #18416. Therefore we now don't proxify component instances anymore, which also makes the dev time proxy warning obsolete.

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>
Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/olive-crabs-refuse.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't turn component instances stored in `$state` into state proxies

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

@@ -339,27 +339,6 @@ 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_proxy_unmount
-
-```
-Tried to unmount a state proxy, rather than a component
-```
-
-`unmount` was called with a state proxy:
-
-```js
-import { mount, unmount } from 'svelte';
-import Component from './Component.svelte';
-let target = document.body;
-// ---cut---
-let component = $state(mount(Component, { target }));
-
-// later...
-unmount(component);
-```
-
-Avoid using `$state` here. If `component` _does_ need to be reactive for some reason, use `$state.raw` instead.
-
 ### svelte_boundary_reset_noop
 
 ```

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

@@ -295,25 +295,6 @@ To silence the warning, ensure that `value`:
 
 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_proxy_unmount
-
-> Tried to unmount a state proxy, rather than a component
-
-`unmount` was called with a state proxy:
-
-```js
-import { mount, unmount } from 'svelte';
-import Component from './Component.svelte';
-let target = document.body;
-// ---cut---
-let component = $state(mount(Component, { target }));
-
-// later...
-unmount(component);
-```
-
-Avoid using `$state` here. If `component` _does_ need to be reactive for some reason, use `$state.raw` instead.
-
 ## svelte_boundary_reset_noop
 
 > A `<svelte:boundary>` `reset` function only resets the boundary the first time it is called

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

@@ -60,6 +60,8 @@ export const ASYNC = 1 << 22;
 export const ERROR_VALUE = 1 << 23;
 
 export const STATE_SYMBOL = Symbol('$state');
+/** Marks component export objects, so that `proxy(...)` leaves them untouched */
+export const COMPONENT_SYMBOL = Symbol('component');
 export const LEGACY_PROPS = Symbol('legacy props');
 export const LOADING_ATTR_SYMBOL = Symbol('');
 export const PROXY_PATH_SYMBOL = Symbol('proxy path');

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

@@ -5,7 +5,8 @@ import { active_effect, active_reaction } from './runtime.js';
 import { create_user_effect } from './reactivity/effects.js';
 import { async_mode_flag, legacy_mode_flag } from '../flags/index.js';
 import { FILENAME } from '../../constants.js';
-import { BRANCH_EFFECT } from './constants.js';
+import { BRANCH_EFFECT, COMPONENT_SYMBOL } from './constants.js';
+import { define_property } from '../shared/utils.js';
 import { create_context, get_or_init_context_map } from '../shared/context.js';
 
 /** @type {ComponentContext | null} */
@@ -217,7 +218,16 @@ export function pop(component) {
 		dev_current_component_function = component_context?.function ?? null;
 	}
 
-	return component ?? /** @type {T} */ ({});
+	return mark_as_component(component);
+}
+
+/**
+ * Add a symbol to the object (or create one if undefined) to mark it as a component so it isn't proxified.
+ * @param {any} component
+ */
+export function mark_as_component(component = {}) {
+	define_property(component, COMPONENT_SYMBOL, { value: true });
+	return component;
 }
 
 /** @returns {boolean} */

packages/svelte/src/internal/client/dom/elements/bindings/this.js

@@ -1,6 +1,6 @@
 /** @import { ComponentContext, Effect } from '#client' */
 import { DESTROYING, STATE_SYMBOL } from '#client/constants';
-import { component_context } from '../../../context.js';
+import { component_context, mark_as_component } from '../../../context.js';
 import { effect, render_effect } from '../../../reactivity/effects.js';
 import { active_effect, untrack } from '../../../runtime.js';
 
@@ -23,7 +23,12 @@ function is_bound_this(bound_value, element_or_component) {
  * 										returns all the parts of the each block context that are used in the expression
  * @returns {void}
  */
-export function bind_this(element_or_component = {}, update, get_value, get_parts) {
+export function bind_this(
+	element_or_component = mark_as_component(),
+	update,
+	get_value,
+	get_parts
+) {
 	var component_effect = /** @type {ComponentContext} */ (component_context).r;
 	var parent = /** @type {Effect} */ (active_effect);
 

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

@@ -22,7 +22,7 @@ import {
 	flush_eager_effects,
 	set_eager_effects_deferred
 } from './reactivity/sources.js';
-import { PROXY_PATH_SYMBOL, STATE_SYMBOL } from '#client/constants';
+import { COMPONENT_SYMBOL, PROXY_PATH_SYMBOL, STATE_SYMBOL } from '#client/constants';
 import { UNINITIALIZED } from '../../constants.js';
 import * as e from './errors.js';
 import { tag } from './dev/tracing.js';
@@ -38,8 +38,13 @@ const regex_is_valid_identifier = /^[a-zA-Z_$][a-zA-Z_$0-9]*$/;
  * @returns {T}
  */
 export function proxy(value) {
-	// if non-proxyable, or is already a proxy, return `value`
-	if (typeof value !== 'object' || value === null || STATE_SYMBOL in value) {
+	// if non-proxyable, a component instance, or already a proxy, return `value`
+	if (
+		typeof value !== 'object' ||
+		value === null ||
+		STATE_SYMBOL in value ||
+		COMPONENT_SYMBOL in value
+	) {
 		return value;
 	}
 

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

@@ -10,7 +10,7 @@ import {
 } from './dom/operations.js';
 import { HYDRATION_END, HYDRATION_ERROR, HYDRATION_START } from '../../constants.js';
 import { active_effect } from './runtime.js';
-import { push, pop, component_context } from './context.js';
+import { push, pop, component_context, mark_as_component } from './context.js';
 import { component_root } from './reactivity/effects.js';
 import { hydrate_node, hydrating, set_hydrate_node, set_hydrating } from './dom/hydration.js';
 import { array_from } from '../shared/utils.js';
@@ -23,7 +23,7 @@ import * as w from './warnings.js';
 import * as e from './errors.js';
 import { assign_nodes } from './dom/template.js';
 import { is_passive_event } from '../../utils.js';
-import { COMMENT_NODE, STATE_SYMBOL, TEXT_CACHE } from './constants.js';
+import { COMMENT_NODE, TEXT_CACHE } from './constants.js';
 import { boundary } from './dom/blocks/boundary.js';
 
 /**
@@ -193,7 +193,7 @@ function _mount(
 
 				should_intro = intro;
 				// @ts-expect-error the public typings are not what the actual function looks like
-				component = Component(anchor_node, props) || {};
+				component = Component(anchor_node, props) || mark_as_component();
 				should_intro = true;
 
 				if (hydrating) {
@@ -323,11 +323,7 @@ export function unmount(component, options) {
 	}
 
 	if (DEV) {
-		if (STATE_SYMBOL in component) {
-			w.state_proxy_unmount();
-		} else {
-			w.lifecycle_double_unmount();
-		}
+		w.lifecycle_double_unmount();
 	}
 
 	return Promise.resolve();

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

@@ -247,17 +247,6 @@ export function state_proxy_equality_mismatch(operator) {
 	}
 }
 
-/**
- * Tried to unmount a state proxy, rather than a component
- */
-export function state_proxy_unmount() {
-	if (DEV) {
-		console.warn(`%c[svelte] state_proxy_unmount\n%cTried to unmount a state proxy, rather than a component\nhttps://svelte.dev/e/state_proxy_unmount`, bold, normal);
-	} else {
-		console.warn(`https://svelte.dev/e/state_proxy_unmount`);
-	}
-}
-
 /**
  * A `<svelte:boundary>` `reset` function only resets the boundary the first time it is called
  */

packages/svelte/tests/runtime-runes/samples/bind-this-component-in-state-dev/Child.svelte

@@ -0,0 +1,7 @@
+<script>
+	import { items } from './data.js';
+
+	export const myArr = items;
+</script>
+
+<p>child</p>

packages/svelte/tests/runtime-runes/samples/bind-this-component-in-state-dev/_config.js

@@ -0,0 +1,15 @@
+import { test } from '../../test';
+import { items } from './data.js';
+
+export default test({
+	compileOptions: {
+		dev: true
+	},
+
+	html: `<p>child</p>`,
+
+	test({ assert, instance }) {
+		// ensure component instance doesn't get proxified (https://github.com/sveltejs/svelte/issues/18416)
+		assert.ok(instance.get_first().myArr === items);
+	}
+});

packages/svelte/tests/runtime-runes/samples/bind-this-component-in-state-dev/data.js

@@ -0,0 +1 @@
+export const items = [{ id: 5, name: 'John' }];

packages/svelte/tests/runtime-runes/samples/bind-this-component-in-state-dev/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	import Child from './Child.svelte';
+
+	const components = $state({});
+
+	export function get_first() {
+		return components.first;
+	}
+</script>
+
+<Child bind:this={components.first} />

packages/svelte/tests/runtime-runes/samples/bind-this-component-in-state/Child.svelte

@@ -0,0 +1,7 @@
+<script>
+	import { items } from './data.js';
+
+	export const myArr = items;
+</script>
+
+<p>child</p>

packages/svelte/tests/runtime-runes/samples/bind-this-component-in-state/_config.js

@@ -0,0 +1,11 @@
+import { test } from '../../test';
+import { items } from './data.js';
+
+export default test({
+	html: `<p>child</p>`,
+
+	test({ assert, instance }) {
+		// ensure component instance doesn't get proxified (https://github.com/sveltejs/svelte/issues/18416)
+		assert.ok(instance.get_first().myArr === items);
+	}
+});

packages/svelte/tests/runtime-runes/samples/bind-this-component-in-state/data.js

@@ -0,0 +1 @@
+export const items = [{ id: 5, name: 'John' }];

packages/svelte/tests/runtime-runes/samples/bind-this-component-in-state/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	import Child from './Child.svelte';
+
+	const components = $state({});
+
+	export function get_first() {
+		return components.first;
+	}
+</script>
+
+<Child bind:this={components.first} />