From 2f4936c0f50b6aa2ff4c86f11ab4e3c53d956422 Mon Sep 17 00:00:00 2001
From: Lars Francke <git@lars-francke.de>
Date: Mon, 28 Apr 2025 15:58:56 +0200
Subject: [PATCH 1/3] feat: The migration warning when slots cannot be migrated
 due to naming conflicts now states the name of the slot causing the issue

---
 .changeset/strong-coins-peel.md                              | 5 +++++
 packages/svelte/src/compiler/migrate/index.js                | 4 ++--
 .../impossible-migrate-$derived-derived-var-3/output.svelte  | 4 ++--
 .../impossible-migrate-slot-change-name/output.svelte        | 4 ++--
 .../impossible-migrate-slot-non-identifier/output.svelte     | 4 ++--
 5 files changed, 13 insertions(+), 8 deletions(-)
 create mode 100644 .changeset/strong-coins-peel.md

diff --git a/.changeset/strong-coins-peel.md b/.changeset/strong-coins-peel.md
new file mode 100644
index 000000000000..d54b0c56e6b1
--- /dev/null
+++ b/.changeset/strong-coins-peel.md
@@ -0,0 +1,5 @@
+---
+'svelte': minor
+---
+
+The migration warning when slots cannot be migrated due to naming conflicts now states the name of the slot causing the issue
diff --git a/packages/svelte/src/compiler/migrate/index.js b/packages/svelte/src/compiler/migrate/index.js
index 75a9a6490539..2d5a4dcd9e5d 100644
--- a/packages/svelte/src/compiler/migrate/index.js
+++ b/packages/svelte/src/compiler/migrate/index.js
@@ -1307,7 +1307,7 @@ const template = {
 			name = state.scope.generate(slot_name);
 			if (name !== slot_name) {
 				throw new MigrationError(
-					'This migration would change the name of a slot making the component unusable'
+					`This migration would change the name of a slot (${slot_name} to ${name}) making the component unusable`
 				);
 			}
 		}
@@ -1880,7 +1880,7 @@ function handle_identifier(node, state, path) {
 				let new_name = state.scope.generate(name);
 				if (new_name !== name) {
 					throw new MigrationError(
-						'This migration would change the name of a slot making the component unusable'
+						`This migration would change the name of a slot (${name} to ${new_name}) making the component unusable`
 					);
 				}
 			}
diff --git a/packages/svelte/tests/migrate/samples/impossible-migrate-$derived-derived-var-3/output.svelte b/packages/svelte/tests/migrate/samples/impossible-migrate-$derived-derived-var-3/output.svelte
index 9e4f086aedd3..26012e11151d 100644
--- a/packages/svelte/tests/migrate/samples/impossible-migrate-$derived-derived-var-3/output.svelte
+++ b/packages/svelte/tests/migrate/samples/impossible-migrate-$derived-derived-var-3/output.svelte
@@ -1,7 +1,7 @@
-<!-- @migration-task Error while migrating Svelte code: This migration would change the name of a slot making the component unusable -->
+<!-- @migration-task Error while migrating Svelte code: This migration would change the name of a slot (derived to derived_1) making the component unusable -->
 <script>
 	let derived;
 </script>
 <Component>
 	<slot name="derived" slot="derived" />
-</Component>
\ No newline at end of file
+</Component>
diff --git a/packages/svelte/tests/migrate/samples/impossible-migrate-slot-change-name/output.svelte b/packages/svelte/tests/migrate/samples/impossible-migrate-slot-change-name/output.svelte
index 2b6838a1d681..328966b63b6a 100644
--- a/packages/svelte/tests/migrate/samples/impossible-migrate-slot-change-name/output.svelte
+++ b/packages/svelte/tests/migrate/samples/impossible-migrate-slot-change-name/output.svelte
@@ -1,6 +1,6 @@
-<!-- @migration-task Error while migrating Svelte code: This migration would change the name of a slot making the component unusable -->
+<!-- @migration-task Error while migrating Svelte code: This migration would change the name of a slot (body to body_1) making the component unusable -->
 <script>
 	let body;
 </script>
 
-<slot name="body"></slot>
\ No newline at end of file
+<slot name="body"></slot>
diff --git a/packages/svelte/tests/migrate/samples/impossible-migrate-slot-non-identifier/output.svelte b/packages/svelte/tests/migrate/samples/impossible-migrate-slot-non-identifier/output.svelte
index 6e5ab103109b..1e763577df34 100644
--- a/packages/svelte/tests/migrate/samples/impossible-migrate-slot-non-identifier/output.svelte
+++ b/packages/svelte/tests/migrate/samples/impossible-migrate-slot-non-identifier/output.svelte
@@ -1,2 +1,2 @@
-<!-- @migration-task Error while migrating Svelte code: This migration would change the name of a slot making the component unusable -->
-<slot name="dashed-name"></slot>
\ No newline at end of file
+<!-- @migration-task Error while migrating Svelte code: This migration would change the name of a slot (dashed-name to dashed_name) making the component unusable -->
+<slot name="dashed-name"></slot>

From 8f8f417705bb1c6347b065564cd7c3fc5120e89e Mon Sep 17 00:00:00 2001
From: Paolo Ricciuti <ricciutipaolo@gmail.com>
Date: Wed, 30 Apr 2025 08:49:16 +0200
Subject: [PATCH 2/3] chore: update changeset

---
 .changeset/strong-coins-peel.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.changeset/strong-coins-peel.md b/.changeset/strong-coins-peel.md
index d54b0c56e6b1..63cf22123e47 100644
--- a/.changeset/strong-coins-peel.md
+++ b/.changeset/strong-coins-peel.md
@@ -1,5 +1,5 @@
 ---
-'svelte': minor
+'svelte': patch
 ---
 
 The migration warning when slots cannot be migrated due to naming conflicts now states the name of the slot causing the issue

From 895b9a0add99c5b6d91a6710ed3283fc056d99d0 Mon Sep 17 00:00:00 2001
From: Paolo Ricciuti <ricciutipaolo@gmail.com>
Date: Wed, 30 Apr 2025 08:50:43 +0200
Subject: [PATCH 3/3] chore: update changeset

---
 .changeset/strong-coins-peel.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.changeset/strong-coins-peel.md b/.changeset/strong-coins-peel.md
index 63cf22123e47..013e8e44a107 100644
--- a/.changeset/strong-coins-peel.md
+++ b/.changeset/strong-coins-peel.md
@@ -2,4 +2,4 @@
 'svelte': patch
 ---
 
-The migration warning when slots cannot be migrated due to naming conflicts now states the name of the slot causing the issue
+fix: improve error message for migration errors when slot would be renamed