fix: avoid migrating slots in custom elements (#13406)

They'll be preserved as-is in some future Svelte version

.changeset/eight-waves-mate.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid migrating slots in custom elements

packages/svelte/src/compiler/migrate/index.js

@@ -599,6 +599,7 @@ const template = {
 		next();
 	},
 	SlotElement(node, { state, next }) {
+		if (state.analysis.custom_element) return;
 		let name = 'children';
 		let slot_name = 'default';
 		let slot_props = '{ ';

packages/svelte/tests/migrate/samples/slots-custom-element/input.svelte

@@ -0,0 +1,23 @@
+<svelte:options customElement="my-element" />
+
+<script>
+	// to show that it doesn't bail out from the whole migration
+	let count = 0;
+</script>
+
+<button on:click={()=>count++}><slot /></button>
+
+{count}
+
+{#if foo}
+	<slot name="foo" {foo} />
+{/if}
+
+{#if $$slots.bar}
+	{$$slots}
+	<slot name="bar" />
+{/if}
+
+{#if $$slots.default}foo{/if}
+
+<slot name="dashed-name" />

packages/svelte/tests/migrate/samples/slots-custom-element/output.svelte

@@ -0,0 +1,23 @@
+<svelte:options customElement="my-element" />
+
+<script>
+	// to show that it doesn't bail out from the whole migration
+	let count = $state(0);
+</script>
+
+<button onclick={()=>count++}><slot /></button>
+
+{count}
+
+{#if foo}
+	<slot name="foo" {foo} />
+{/if}
+
+{#if bar}
+	{$$slots}
+	<slot name="bar" />
+{/if}
+
+{#if children}foo{/if}
+
+<slot name="dashed-name" />