From ea22840e8c0537388e9839ad337d1aa6f0bd90c8 Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <trueadm@users.noreply.github.com>
Date: Wed, 17 Jul 2024 23:11:43 +0100
Subject: [PATCH] breaking: remove deep reactivity from non-bindable props
 (#12484)

* breaking: remove deep reactivity from non-bindable props

* breaking: remove deep reactivity from non-bindable props

* lint
---
 .changeset/heavy-feet-attend.md                           | 5 +++++
 .../src/compiler/phases/3-transform/client/utils.js       | 3 ++-
 .../3-transform/client/visitors/javascript-runes.js       | 6 +++++-
 .../samples/props-default-reactivity/_config.js           | 8 ++++----
 4 files changed, 16 insertions(+), 6 deletions(-)
 create mode 100644 .changeset/heavy-feet-attend.md

diff --git a/.changeset/heavy-feet-attend.md b/.changeset/heavy-feet-attend.md
new file mode 100644
index 0000000000..24b3519c9a
--- /dev/null
+++ b/.changeset/heavy-feet-attend.md
@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+breaking: remove deep reactivity from non-bindable props
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/utils.js b/packages/svelte/src/compiler/phases/3-transform/client/utils.js
index e77ee236ef..ea0d3d55e7 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/utils.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/utils.js
@@ -336,7 +336,8 @@ export function serialize_set_binding(node, context, fallback, prefix, options)
 						left,
 						context.state.analysis.runes &&
 							!options?.skip_proxy_and_freeze &&
-							should_proxy_or_freeze(value, context.state.scope)
+							should_proxy_or_freeze(value, context.state.scope) &&
+							binding.kind === 'bindable_prop'
 							? serialize_proxy_reassignment(value, left_name, state)
 							: value
 					);
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/javascript-runes.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/javascript-runes.js
index 4588f71d47..71524b539a 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/javascript-runes.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/javascript-runes.js
@@ -276,7 +276,11 @@ export const javascript_visitors_runes = {
 								/** @type {import('estree').Expression} */ (visit(binding.initial));
 							// We're adding proxy here on demand and not within the prop runtime function so that
 							// people not using proxied state anywhere in their code don't have to pay the additional bundle size cost
-							if (initial && binding.mutated && should_proxy_or_freeze(initial, state.scope)) {
+							if (
+								initial &&
+								binding.kind === 'bindable_prop' &&
+								should_proxy_or_freeze(initial, state.scope)
+							) {
 								initial = b.call('$.proxy', initial);
 							}
 
diff --git a/packages/svelte/tests/runtime-runes/samples/props-default-reactivity/_config.js b/packages/svelte/tests/runtime-runes/samples/props-default-reactivity/_config.js
index 4ebb27fc9a..5eff145cb7 100644
--- a/packages/svelte/tests/runtime-runes/samples/props-default-reactivity/_config.js
+++ b/packages/svelte/tests/runtime-runes/samples/props-default-reactivity/_config.js
@@ -63,8 +63,8 @@ export default test({
 			`
 				<button>mutate: 3</button>
 				<button>reassign: 3</button>
-				<button>mutate: 1</button>
-				<button>reassign: 1</button>
+				<button>mutate: 0</button>
+				<button>reassign: 0</button>
 			`
 		);
 
@@ -91,8 +91,8 @@ export default test({
 			`
 				<button>mutate: 3</button>
 				<button>reassign: 3</button>
-				<button>mutate: 3</button>
-				<button>reassign: 3</button>
+				<button>mutate: 2</button>
+				<button>reassign: 2</button>
 			`
 		);
 	}