From f9fe9f042a8fc07e9558e657d2312e1ad5b716b4 Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <dg@domgan.com>
Date: Wed, 6 Nov 2024 12:17:00 +0000
Subject: [PATCH 1/8] feat: adds legacy mode flag

---
 .changeset/warm-snakes-remain.md                             | 5 +++++
 packages/svelte/package.json                                 | 3 +++
 .../compiler/phases/3-transform/client/transform-client.js   | 4 ++++
 packages/svelte/src/internal/client/reactivity/props.js      | 3 ++-
 packages/svelte/src/internal/client/runtime.js               | 5 +++--
 packages/svelte/src/internal/feature-flags.js                | 5 +++++
 packages/svelte/src/internal/legacy-component.js             | 3 +++
 .../svelte/tests/migrate/samples/derivations/output.svelte   | 2 +-
 packages/svelte/tests/migrate/samples/props-ts/output.svelte | 2 +-
 .../samples/bind-this/_expected/client/index.svelte.js       | 1 +
 .../each-string-template/_expected/client/index.svelte.js    | 1 +
 .../samples/hello-world/_expected/client/index.svelte.js     | 1 +
 .../snapshot/samples/hmr/_expected/client/index.svelte.js    | 1 +
 .../imports-in-modules/_expected/client/index.svelte.js      | 1 +
 .../snapshot/samples/purity/_expected/client/index.svelte.js | 1 +
 15 files changed, 33 insertions(+), 5 deletions(-)
 create mode 100644 .changeset/warm-snakes-remain.md
 create mode 100644 packages/svelte/src/internal/feature-flags.js
 create mode 100644 packages/svelte/src/internal/legacy-component.js

diff --git a/.changeset/warm-snakes-remain.md b/.changeset/warm-snakes-remain.md
new file mode 100644
index 000000000000..5d7ef6e5f5cc
--- /dev/null
+++ b/.changeset/warm-snakes-remain.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: adds legacy mode flag
diff --git a/packages/svelte/package.json b/packages/svelte/package.json
index 5234e334b681..94b5d8baf093 100644
--- a/packages/svelte/package.json
+++ b/packages/svelte/package.json
@@ -53,6 +53,9 @@
     "./internal/disclose-version": {
       "default": "./src/internal/disclose-version.js"
     },
+    "./internal/legacy-component": {
+      "default": "./src/internal/legacy-component.js"
+    },
     "./internal/server": {
       "default": "./src/internal/server/index.js"
     },
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
index 4daff53efb0f..c0d2694b0b6b 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
+++ b/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/legacy-component'));
+	}
+
 	if (options.discloseVersion) {
 		body.unshift(b.imports([], 'svelte/internal/disclose-version'));
 	}
diff --git a/packages/svelte/src/internal/client/reactivity/props.js b/packages/svelte/src/internal/client/reactivity/props.js
index e24deb5a20c2..43a8182527e6 100644
--- a/packages/svelte/src/internal/client/reactivity/props.js
+++ b/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 '../../feature-flags.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;
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index bd4d23dd15b6..797d97403d65 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/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 '../feature-flags.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,
diff --git a/packages/svelte/src/internal/feature-flags.js b/packages/svelte/src/internal/feature-flags.js
new file mode 100644
index 000000000000..767a40a76543
--- /dev/null
+++ b/packages/svelte/src/internal/feature-flags.js
@@ -0,0 +1,5 @@
+export let legacy_mode_flag = false;
+
+export function enable_legacy_mode_flag() {
+	legacy_mode_flag = true;
+}
diff --git a/packages/svelte/src/internal/legacy-component.js b/packages/svelte/src/internal/legacy-component.js
new file mode 100644
index 000000000000..7a02002f763c
--- /dev/null
+++ b/packages/svelte/src/internal/legacy-component.js
@@ -0,0 +1,3 @@
+import { enable_legacy_mode_flag } from './feature-flags.js';
+
+enable_legacy_mode_flag();
diff --git a/packages/svelte/tests/migrate/samples/derivations/output.svelte b/packages/svelte/tests/migrate/samples/derivations/output.svelte
index 79ff089018f8..5200b7f8a733 100644
--- a/packages/svelte/tests/migrate/samples/derivations/output.svelte
+++ b/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}
\ No newline at end of file
diff --git a/packages/svelte/tests/migrate/samples/props-ts/output.svelte b/packages/svelte/tests/migrate/samples/props-ts/output.svelte
index e9deea13846d..b75f4383f52f 100644
--- a/packages/svelte/tests/migrate/samples/props-ts/output.svelte
+++ b/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} />
\ No newline at end of file
diff --git a/packages/svelte/tests/snapshot/samples/bind-this/_expected/client/index.svelte.js b/packages/svelte/tests/snapshot/samples/bind-this/_expected/client/index.svelte.js
index bd24eca96227..3ba3c9a0cfbb 100644
--- a/packages/svelte/tests/snapshot/samples/bind-this/_expected/client/index.svelte.js
+++ b/packages/svelte/tests/snapshot/samples/bind-this/_expected/client/index.svelte.js
@@ -1,4 +1,5 @@
 import "svelte/internal/disclose-version";
+import "svelte/internal/legacy-component";
 import * as $ from "svelte/internal/client";
 
 export default function Bind_this($$anchor) {
diff --git a/packages/svelte/tests/snapshot/samples/each-string-template/_expected/client/index.svelte.js b/packages/svelte/tests/snapshot/samples/each-string-template/_expected/client/index.svelte.js
index 81c06f23ec3a..b7fa8fa6cf33 100644
--- a/packages/svelte/tests/snapshot/samples/each-string-template/_expected/client/index.svelte.js
+++ b/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/legacy-component";
 import * as $ from "svelte/internal/client";
 
 export default function Each_string_template($$anchor) {
diff --git a/packages/svelte/tests/snapshot/samples/hello-world/_expected/client/index.svelte.js b/packages/svelte/tests/snapshot/samples/hello-world/_expected/client/index.svelte.js
index 92354d8f1483..5d80754ad243 100644
--- a/packages/svelte/tests/snapshot/samples/hello-world/_expected/client/index.svelte.js
+++ b/packages/svelte/tests/snapshot/samples/hello-world/_expected/client/index.svelte.js
@@ -1,4 +1,5 @@
 import "svelte/internal/disclose-version";
+import "svelte/internal/legacy-component";
 import * as $ from "svelte/internal/client";
 
 var root = $.template(`<h1>hello world</h1>`);
diff --git a/packages/svelte/tests/snapshot/samples/hmr/_expected/client/index.svelte.js b/packages/svelte/tests/snapshot/samples/hmr/_expected/client/index.svelte.js
index 01daada7acd0..d21e6c3ff8f2 100644
--- a/packages/svelte/tests/snapshot/samples/hmr/_expected/client/index.svelte.js
+++ b/packages/svelte/tests/snapshot/samples/hmr/_expected/client/index.svelte.js
@@ -1,4 +1,5 @@
 import "svelte/internal/disclose-version";
+import "svelte/internal/legacy-component";
 import * as $ from "svelte/internal/client";
 
 var root = $.template(`<h1>hello world</h1>`);
diff --git a/packages/svelte/tests/snapshot/samples/imports-in-modules/_expected/client/index.svelte.js b/packages/svelte/tests/snapshot/samples/imports-in-modules/_expected/client/index.svelte.js
index 464ef519afa0..6a0a38a702cc 100644
--- a/packages/svelte/tests/snapshot/samples/imports-in-modules/_expected/client/index.svelte.js
+++ b/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/legacy-component";
 import * as $ from "svelte/internal/client";
 import { random } from './module.svelte';
 
diff --git a/packages/svelte/tests/snapshot/samples/purity/_expected/client/index.svelte.js b/packages/svelte/tests/snapshot/samples/purity/_expected/client/index.svelte.js
index 305e9aa0d772..09f4e61e4be9 100644
--- a/packages/svelte/tests/snapshot/samples/purity/_expected/client/index.svelte.js
+++ b/packages/svelte/tests/snapshot/samples/purity/_expected/client/index.svelte.js
@@ -1,4 +1,5 @@
 import "svelte/internal/disclose-version";
+import "svelte/internal/legacy-component";
 import * as $ from "svelte/internal/client";
 
 var root = $.template(`<p></p> <p></p> <!>`, 1);

From 4cd06e223855c6bbf212ef119468c8eaa75979a1 Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <dg@domgan.com>
Date: Wed, 6 Nov 2024 12:43:05 +0000
Subject: [PATCH 2/8] feedback

---
 packages/svelte/package.json                                  | 4 ++--
 .../compiler/phases/3-transform/client/transform-client.js    | 2 +-
 packages/svelte/src/internal/client/reactivity/props.js       | 2 +-
 packages/svelte/src/internal/client/runtime.js                | 2 +-
 .../svelte/src/internal/{feature-flags.js => flags/index.js}  | 0
 packages/svelte/src/internal/flags/legacy.js                  | 3 +++
 packages/svelte/src/internal/legacy-component.js              | 3 ---
 .../samples/bind-this/_expected/client/index.svelte.js        | 2 +-
 .../each-string-template/_expected/client/index.svelte.js     | 2 +-
 .../samples/hello-world/_expected/client/index.svelte.js      | 2 +-
 .../snapshot/samples/hmr/_expected/client/index.svelte.js     | 2 +-
 .../imports-in-modules/_expected/client/index.svelte.js       | 2 +-
 .../snapshot/samples/purity/_expected/client/index.svelte.js  | 2 +-
 13 files changed, 14 insertions(+), 14 deletions(-)
 rename packages/svelte/src/internal/{feature-flags.js => flags/index.js} (100%)
 create mode 100644 packages/svelte/src/internal/flags/legacy.js
 delete mode 100644 packages/svelte/src/internal/legacy-component.js

diff --git a/packages/svelte/package.json b/packages/svelte/package.json
index 94b5d8baf093..144726436ffd 100644
--- a/packages/svelte/package.json
+++ b/packages/svelte/package.json
@@ -53,8 +53,8 @@
     "./internal/disclose-version": {
       "default": "./src/internal/disclose-version.js"
     },
-    "./internal/legacy-component": {
-      "default": "./src/internal/legacy-component.js"
+    "./internal/flags/legacy": {
+      "default": "./src/internal/flags/legacy.js"
     },
     "./internal/server": {
       "default": "./src/internal/server/index.js"
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
index c0d2694b0b6b..5349f6025533 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/transform-client.js
@@ -532,7 +532,7 @@ export function client_component(analysis, options) {
 	}
 
 	if (!analysis.runes) {
-		body.unshift(b.imports([], 'svelte/internal/legacy-component'));
+		body.unshift(b.imports([], 'svelte/internal/flags/legacy'));
 	}
 
 	if (options.discloseVersion) {
diff --git a/packages/svelte/src/internal/client/reactivity/props.js b/packages/svelte/src/internal/client/reactivity/props.js
index 43a8182527e6..ae934c01a561 100644
--- a/packages/svelte/src/internal/client/reactivity/props.js
+++ b/packages/svelte/src/internal/client/reactivity/props.js
@@ -23,7 +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 '../../feature-flags.js';
+import { legacy_mode_flag } from '../../flags/index.js';
 
 /**
  * @param {((value?: number) => number)} fn
diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index 797d97403d65..e9f1c773e689 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -33,7 +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 '../feature-flags.js';
+import { legacy_mode_flag } from '../flags/index.js';
 
 const FLUSH_MICROTASK = 0;
 const FLUSH_SYNC = 1;
diff --git a/packages/svelte/src/internal/feature-flags.js b/packages/svelte/src/internal/flags/index.js
similarity index 100%
rename from packages/svelte/src/internal/feature-flags.js
rename to packages/svelte/src/internal/flags/index.js
diff --git a/packages/svelte/src/internal/flags/legacy.js b/packages/svelte/src/internal/flags/legacy.js
new file mode 100644
index 000000000000..cab3c010635f
--- /dev/null
+++ b/packages/svelte/src/internal/flags/legacy.js
@@ -0,0 +1,3 @@
+import { enable_legacy_mode_flag } from './index.js';
+
+enable_legacy_mode_flag();
diff --git a/packages/svelte/src/internal/legacy-component.js b/packages/svelte/src/internal/legacy-component.js
deleted file mode 100644
index 7a02002f763c..000000000000
--- a/packages/svelte/src/internal/legacy-component.js
+++ /dev/null
@@ -1,3 +0,0 @@
-import { enable_legacy_mode_flag } from './feature-flags.js';
-
-enable_legacy_mode_flag();
diff --git a/packages/svelte/tests/snapshot/samples/bind-this/_expected/client/index.svelte.js b/packages/svelte/tests/snapshot/samples/bind-this/_expected/client/index.svelte.js
index 3ba3c9a0cfbb..fedcc87696cf 100644
--- a/packages/svelte/tests/snapshot/samples/bind-this/_expected/client/index.svelte.js
+++ b/packages/svelte/tests/snapshot/samples/bind-this/_expected/client/index.svelte.js
@@ -1,5 +1,5 @@
 import "svelte/internal/disclose-version";
-import "svelte/internal/legacy-component";
+import "svelte/internal/flags/legacy";
 import * as $ from "svelte/internal/client";
 
 export default function Bind_this($$anchor) {
diff --git a/packages/svelte/tests/snapshot/samples/each-string-template/_expected/client/index.svelte.js b/packages/svelte/tests/snapshot/samples/each-string-template/_expected/client/index.svelte.js
index b7fa8fa6cf33..80f2da11a2a1 100644
--- a/packages/svelte/tests/snapshot/samples/each-string-template/_expected/client/index.svelte.js
+++ b/packages/svelte/tests/snapshot/samples/each-string-template/_expected/client/index.svelte.js
@@ -1,5 +1,5 @@
 import "svelte/internal/disclose-version";
-import "svelte/internal/legacy-component";
+import "svelte/internal/flags/legacy";
 import * as $ from "svelte/internal/client";
 
 export default function Each_string_template($$anchor) {
diff --git a/packages/svelte/tests/snapshot/samples/hello-world/_expected/client/index.svelte.js b/packages/svelte/tests/snapshot/samples/hello-world/_expected/client/index.svelte.js
index 5d80754ad243..9f6f29166975 100644
--- a/packages/svelte/tests/snapshot/samples/hello-world/_expected/client/index.svelte.js
+++ b/packages/svelte/tests/snapshot/samples/hello-world/_expected/client/index.svelte.js
@@ -1,5 +1,5 @@
 import "svelte/internal/disclose-version";
-import "svelte/internal/legacy-component";
+import "svelte/internal/flags/legacy";
 import * as $ from "svelte/internal/client";
 
 var root = $.template(`<h1>hello world</h1>`);
diff --git a/packages/svelte/tests/snapshot/samples/hmr/_expected/client/index.svelte.js b/packages/svelte/tests/snapshot/samples/hmr/_expected/client/index.svelte.js
index d21e6c3ff8f2..86c2880abc8f 100644
--- a/packages/svelte/tests/snapshot/samples/hmr/_expected/client/index.svelte.js
+++ b/packages/svelte/tests/snapshot/samples/hmr/_expected/client/index.svelte.js
@@ -1,5 +1,5 @@
 import "svelte/internal/disclose-version";
-import "svelte/internal/legacy-component";
+import "svelte/internal/flags/legacy";
 import * as $ from "svelte/internal/client";
 
 var root = $.template(`<h1>hello world</h1>`);
diff --git a/packages/svelte/tests/snapshot/samples/imports-in-modules/_expected/client/index.svelte.js b/packages/svelte/tests/snapshot/samples/imports-in-modules/_expected/client/index.svelte.js
index 6a0a38a702cc..9c7d2f3f23b0 100644
--- a/packages/svelte/tests/snapshot/samples/imports-in-modules/_expected/client/index.svelte.js
+++ b/packages/svelte/tests/snapshot/samples/imports-in-modules/_expected/client/index.svelte.js
@@ -1,5 +1,5 @@
 import "svelte/internal/disclose-version";
-import "svelte/internal/legacy-component";
+import "svelte/internal/flags/legacy";
 import * as $ from "svelte/internal/client";
 import { random } from './module.svelte';
 
diff --git a/packages/svelte/tests/snapshot/samples/purity/_expected/client/index.svelte.js b/packages/svelte/tests/snapshot/samples/purity/_expected/client/index.svelte.js
index 09f4e61e4be9..0a627a55ae6c 100644
--- a/packages/svelte/tests/snapshot/samples/purity/_expected/client/index.svelte.js
+++ b/packages/svelte/tests/snapshot/samples/purity/_expected/client/index.svelte.js
@@ -1,5 +1,5 @@
 import "svelte/internal/disclose-version";
-import "svelte/internal/legacy-component";
+import "svelte/internal/flags/legacy";
 import * as $ from "svelte/internal/client";
 
 var root = $.template(`<p></p> <p></p> <!>`, 1);

From 8f63b179f6e8e5f1240a9f1b435ccbe6ed9bf133 Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <dg@domgan.com>
Date: Wed, 6 Nov 2024 14:00:41 +0000
Subject: [PATCH 3/8] feedback and tweaks

---
 .../svelte/scripts/check-treeshakeability.js  | 17 +++++++-
 .../src/internal/client/dom/blocks/each.js    | 39 ++++++++++---------
 2 files changed, 35 insertions(+), 21 deletions(-)

diff --git a/packages/svelte/scripts/check-treeshakeability.js b/packages/svelte/scripts/check-treeshakeability.js
index 9263649956f5..23982adaacea 100644
--- a/packages/svelte/scripts/check-treeshakeability.js
+++ b/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);
 }
 
diff --git a/packages/svelte/src/internal/client/dom/blocks/each.js b/packages/svelte/src/internal/client/dom/blocks/each.js
index 0fef7d343be6..6bc732bb35bd 100644
--- a/packages/svelte/src/internal/client/dom/blocks/each.js
+++ b/packages/svelte/src/internal/client/dom/blocks/each.js
@@ -36,6 +36,7 @@ import { array_from, is_array } from '../../../shared/utils.js';
 import { INERT } from '../../constants.js';
 import { queue_micro_task } from '../task.js';
 import { active_effect, active_reaction } from '../../runtime.js';
+import { legacy_mode_flag } from '../../../flags/index.js';
 
 /**
  * The row of a keyed each block that is currently updating. We track this
@@ -490,27 +491,27 @@ 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 ? (legacy_mode_flag && 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;

From b8955338318c2d502e4231b5725d5f521a39752f Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <dg@domgan.com>
Date: Wed, 6 Nov 2024 14:01:11 +0000
Subject: [PATCH 4/8] feedback and tweaks

---
 packages/svelte/scripts/check-treeshakeability.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/svelte/scripts/check-treeshakeability.js b/packages/svelte/scripts/check-treeshakeability.js
index 23982adaacea..1b50111b8755 100644
--- a/packages/svelte/scripts/check-treeshakeability.js
+++ b/packages/svelte/scripts/check-treeshakeability.js
@@ -136,7 +136,7 @@ if (!bundle.includes('component_context.l')) {
 
 if (failed) {
 	// eslint-disable-next-line no-console
-	// console.error(bundle);
+	console.error(bundle);
 	fs.writeFileSync('scripts/_bundle.js', bundle);
 }
 

From bf0a9348a28dd254c13f2c8d28286b0aedcd8d4d Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <dg@domgan.com>
Date: Wed, 6 Nov 2024 14:11:24 +0000
Subject: [PATCH 5/8] tweaks

---
 packages/svelte/src/internal/client/dom/blocks/each.js    | 3 +--
 packages/svelte/src/internal/client/reactivity/sources.js | 3 ++-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/packages/svelte/src/internal/client/dom/blocks/each.js b/packages/svelte/src/internal/client/dom/blocks/each.js
index 6bc732bb35bd..d62c575391bc 100644
--- a/packages/svelte/src/internal/client/dom/blocks/each.js
+++ b/packages/svelte/src/internal/client/dom/blocks/each.js
@@ -36,7 +36,6 @@ import { array_from, is_array } from '../../../shared/utils.js';
 import { INERT } from '../../constants.js';
 import { queue_micro_task } from '../task.js';
 import { active_effect, active_reaction } from '../../runtime.js';
-import { legacy_mode_flag } from '../../../flags/index.js';
 
 /**
  * The row of a keyed each block that is currently updating. We track this
@@ -494,7 +493,7 @@ function create_item(anchor, state, prev, next, value, key, index, render_fn, fl
 	var reactive = (flags & EACH_ITEM_REACTIVE) !== 0;
 	var mutable = (flags & EACH_ITEM_IMMUTABLE) === 0;
 
-	var v = reactive ? (legacy_mode_flag && mutable ? mutable_source(value) : source(value)) : value;
+	var v = reactive ? (mutable ? mutable_source(value) : source(value)) : value;
 	var i = (flags & EACH_INDEX_REACTIVE) === 0 ? index : source(index);
 
 	/** @type {EachItem} */
diff --git a/packages/svelte/src/internal/client/reactivity/sources.js b/packages/svelte/src/internal/client/reactivity/sources.js
index bb97ea34c882..4bbd470d08c8 100644
--- a/packages/svelte/src/internal/client/reactivity/sources.js
+++ b/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);
 	}
 

From f075248be20d8bf9072674df2f9783f77c111bdd Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <dg@domgan.com>
Date: Wed, 6 Nov 2024 16:04:50 +0000
Subject: [PATCH 6/8] tweaks

---
 packages/svelte/src/index-client.js | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/packages/svelte/src/index-client.js b/packages/svelte/src/index-client.js
index ff477595a191..72811c8f17c9 100644
--- a/packages/svelte/src/index-client.js
+++ b/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(() => {

From e3436e666206f29130f81a4dcd56b19b09d4fbab Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <dg@domgan.com>
Date: Wed, 6 Nov 2024 16:34:03 +0000
Subject: [PATCH 7/8] tweaks

---
 .changeset/warm-snakes-remain.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.changeset/warm-snakes-remain.md b/.changeset/warm-snakes-remain.md
index 5d7ef6e5f5cc..bba83f81d336 100644
--- a/.changeset/warm-snakes-remain.md
+++ b/.changeset/warm-snakes-remain.md
@@ -2,4 +2,4 @@
 'svelte': patch
 ---
 
-feat: adds legacy mode flag
+chore: adds legacy mode flag

From 88992636aa742b7711b03caeef35958feed393ae Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <dg@domgan.com>
Date: Wed, 6 Nov 2024 16:34:56 +0000
Subject: [PATCH 8/8] tweaks

---
 .changeset/warm-snakes-remain.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.changeset/warm-snakes-remain.md b/.changeset/warm-snakes-remain.md
index bba83f81d336..508e8763e5f3 100644
--- a/.changeset/warm-snakes-remain.md
+++ b/.changeset/warm-snakes-remain.md
@@ -2,4 +2,4 @@
 'svelte': patch
 ---
 
-chore: adds legacy mode flag
+chore: adds legacy mode flag reducing bundle size in runes mode only apps