feat: adds legacy mode flag

.changeset/warm-snakes-remain.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: adds legacy mode flag reducing bundle size in runes mode only apps

packages/svelte/package.json

@@ -53,6 +53,9 @@
     "./internal/disclose-version": {
       "default": "./src/internal/disclose-version.js"
     },
+    "./internal/flags/legacy": {
+      "default": "./src/internal/flags/legacy.js"
+    },
     "./internal/server": {
       "default": "./src/internal/server/index.js"
     },

packages/svelte/scripts/check-treeshakeability.js

@@ -57,6 +57,7 @@ for (const key in pkg.exports) {
 	if (key === './compiler') continue;
 	if (key === './internal') continue;
 	if (key === './internal/disclose-version') continue;
+	if (key === './internal/flags/legacy') continue;
 
 	for (const type of ['browser', 'default']) {
 		if (!pkg.exports[key][type]) continue;
@@ -83,6 +84,7 @@ const bundle = await bundle_code(
 	// Use all features which contain hydration code to ensure it's treeshakeable
 	compile(
 		`
+<svelte:options runes />
 <script>
 	import { mount } from ${JSON.stringify(client_main)}; mount();
 	let foo;
@@ -118,12 +120,23 @@ if (!bundle.includes('hydrate_node') && !bundle.includes('hydrate_next')) {
 	// eslint-disable-next-line no-console
 	console.error(`✅ Hydration code treeshakeable`);
 } else {
-	// eslint-disable-next-line no-console
-	console.error(bundle);
+	failed = true;
 	// eslint-disable-next-line no-console
 	console.error(`❌ Hydration code not treeshakeable`);
+}
+
+if (!bundle.includes('component_context.l')) {
+	// eslint-disable-next-line no-console
+	console.error(`✅ Legacy code treeshakeable`);
+} else {
 	failed = true;
+	// eslint-disable-next-line no-console
+	console.error(`❌ Legacy code not treeshakeable`);
+}
 
+if (failed) {
+	// eslint-disable-next-line no-console
+	console.error(bundle);
 	fs.writeFileSync('scripts/_bundle.js', bundle);
 }
 

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

@@ -531,6 +531,10 @@ export function client_component(analysis, options) {
 		body.push(b.stmt(b.call(b.id('$.mark_module_end'), b.id(analysis.name))));
 	}
 
+	if (!analysis.runes) {
+		body.unshift(b.imports([], 'svelte/internal/flags/legacy'));
+	}
+
 	if (options.discloseVersion) {
 		body.unshift(b.imports([], 'svelte/internal/disclose-version'));
 	}

packages/svelte/src/index-client.js

@@ -6,6 +6,7 @@ import { is_array } from './internal/shared/utils.js';
 import { user_effect } from './internal/client/index.js';
 import * as e from './internal/client/errors.js';
 import { lifecycle_outside_component } from './internal/shared/errors.js';
+import { legacy_mode_flag } from './internal/flags/index.js';
 
 /**
  * The `onMount` function schedules a callback to run as soon as the component has been mounted to the DOM.
@@ -25,7 +26,7 @@ export function onMount(fn) {
 		lifecycle_outside_component('onMount');
 	}
 
-	if (component_context.l !== null) {
+	if (legacy_mode_flag && component_context.l !== null) {
 		init_update_callbacks(component_context).m.push(fn);
 	} else {
 		user_effect(() => {

packages/svelte/src/internal/client/dom/blocks/each.js

@@ -490,27 +490,27 @@ function update_item(item, value, index, type) {
  */
 function create_item(anchor, state, prev, next, value, key, index, render_fn, flags) {
 	var previous_each_item = current_each_item;
+	var reactive = (flags & EACH_ITEM_REACTIVE) !== 0;
+	var mutable = (flags & EACH_ITEM_IMMUTABLE) === 0;
+
+	var v = reactive ? (mutable ? mutable_source(value) : source(value)) : value;
+	var i = (flags & EACH_INDEX_REACTIVE) === 0 ? index : source(index);
+
+	/** @type {EachItem} */
+	var item = {
+		i,
+		v,
+		k: key,
+		a: null,
+		// @ts-expect-error
+		e: null,
+		prev,
+		next
+	};
+
+	current_each_item = item;
 
 	try {
-		var reactive = (flags & EACH_ITEM_REACTIVE) !== 0;
-		var mutable = (flags & EACH_ITEM_IMMUTABLE) === 0;
-
-		var v = reactive ? (mutable ? mutable_source(value) : source(value)) : value;
-		var i = (flags & EACH_INDEX_REACTIVE) === 0 ? index : source(index);
-
-		/** @type {EachItem} */
-		var item = {
-			i,
-			v,
-			k: key,
-			a: null,
-			// @ts-expect-error
-			e: null,
-			prev,
-			next
-		};
-
-		current_each_item = item;
 		item.e = branch(() => render_fn(anchor, v, i), hydrating);
 
 		item.e.prev = prev && prev.e;

packages/svelte/src/internal/client/reactivity/props.js

@@ -23,6 +23,7 @@ import * as e from '../errors.js';
 import { BRANCH_EFFECT, LEGACY_DERIVED_PROP, ROOT_EFFECT } from '../constants.js';
 import { proxy } from '../proxy.js';
 import { capture_store_binding } from './store.js';
+import { legacy_mode_flag } from '../../flags/index.js';
 
 /**
  * @param {((value?: number) => number)} fn
@@ -270,7 +271,7 @@ function with_parent_branch(fn) {
  */
 export function prop(props, key, flags, fallback) {
 	var immutable = (flags & PROPS_IS_IMMUTABLE) !== 0;
-	var runes = (flags & PROPS_IS_RUNES) !== 0;
+	var runes = !legacy_mode_flag || (flags & PROPS_IS_RUNES) !== 0;
 	var bindable = (flags & PROPS_IS_BINDABLE) !== 0;
 	var lazy = (flags & PROPS_IS_LAZY_INITIAL) !== 0;
 	var is_store_sub = false;

packages/svelte/src/internal/client/reactivity/sources.js

@@ -32,6 +32,7 @@ import {
 	BLOCK_EFFECT
 } from '../constants.js';
 import * as e from '../errors.js';
+import { legacy_mode_flag } from '../../flags/index.js';
 
 export let inspect_effects = new Set();
 
@@ -80,7 +81,7 @@ export function mutable_source(initial_value, immutable = false) {
 
 	// bind the signal to the component context, in case we need to
 	// track updates to trigger beforeUpdate/afterUpdate callbacks
-	if (component_context !== null && component_context.l !== null) {
+	if (legacy_mode_flag && component_context !== null && component_context.l !== null) {
 		(component_context.l.s ??= []).push(s);
 	}
 

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

@@ -33,6 +33,7 @@ import { destroy_derived, execute_derived, update_derived } from './reactivity/d
 import * as e from './errors.js';
 import { lifecycle_outside_component } from '../shared/errors.js';
 import { FILENAME } from '../../constants.js';
+import { legacy_mode_flag } from '../flags/index.js';
 
 const FLUSH_MICROTASK = 0;
 const FLUSH_SYNC = 1;
@@ -162,7 +163,7 @@ export function increment_version() {
 
 /** @returns {boolean} */
 export function is_runes() {
-	return component_context !== null && component_context.l === null;
+	return !legacy_mode_flag || (component_context !== null && component_context.l === null);
 }
 
 /**
@@ -1025,7 +1026,7 @@ export function push(props, runes = false, fn) {
 		l: null
 	};
 
-	if (!runes) {
+	if (legacy_mode_flag && !runes) {
 		component_context.l = {
 			s: null,
 			u: null,

packages/svelte/src/internal/flags/index.js

@@ -0,0 +1,5 @@
+export let legacy_mode_flag = false;
+
+export function enable_legacy_mode_flag() {
+	legacy_mode_flag = true;
+}

packages/svelte/src/internal/flags/legacy.js

@@ -0,0 +1,3 @@
+import { enable_legacy_mode_flag } from './index.js';
+
+enable_legacy_mode_flag();

packages/svelte/tests/migrate/samples/derivations/output.svelte

@@ -13,4 +13,4 @@
 
 </script>
 
-{count} / {doubled} / {quadrupled} / {time_8} / {time_16}
+{count} / {doubled} / {quadrupled} / {time_8} / {time_16}

packages/svelte/tests/migrate/samples/props-ts/output.svelte

@@ -39,4 +39,4 @@
 {readonly}
 {optional}
 <input bind:value={binding} />
-<input bind:value={bindingOptional} />
+<input bind:value={bindingOptional} />

packages/svelte/tests/snapshot/samples/bind-this/_expected/client/index.svelte.js

@@ -1,4 +1,5 @@
 import "svelte/internal/disclose-version";
+import "svelte/internal/flags/legacy";
 import * as $ from "svelte/internal/client";
 
 export default function Bind_this($$anchor) {

packages/svelte/tests/snapshot/samples/each-string-template/_expected/client/index.svelte.js

@@ -1,4 +1,5 @@
 import "svelte/internal/disclose-version";
+import "svelte/internal/flags/legacy";
 import * as $ from "svelte/internal/client";
 
 export default function Each_string_template($$anchor) {

packages/svelte/tests/snapshot/samples/hello-world/_expected/client/index.svelte.js

@@ -1,4 +1,5 @@
 import "svelte/internal/disclose-version";
+import "svelte/internal/flags/legacy";
 import * as $ from "svelte/internal/client";
 
 var root = $.template(`<h1>hello world</h1>`);

packages/svelte/tests/snapshot/samples/hmr/_expected/client/index.svelte.js

@@ -1,4 +1,5 @@
 import "svelte/internal/disclose-version";
+import "svelte/internal/flags/legacy";
 import * as $ from "svelte/internal/client";
 
 var root = $.template(`<h1>hello world</h1>`);

packages/svelte/tests/snapshot/samples/imports-in-modules/_expected/client/index.svelte.js

@@ -1,4 +1,5 @@
 import "svelte/internal/disclose-version";
+import "svelte/internal/flags/legacy";
 import * as $ from "svelte/internal/client";
 import { random } from './module.svelte';
 

packages/svelte/tests/snapshot/samples/purity/_expected/client/index.svelte.js

@@ -1,4 +1,5 @@
 import "svelte/internal/disclose-version";
+import "svelte/internal/flags/legacy";
 import * as $ from "svelte/internal/client";
 
 var root = $.template(`<p></p> <p></p> <!>`, 1);