fix: improve error message for migration errors when slot would be renamed (#15841)

Co-authored-by: Paolo Ricciuti <ricciutipaolo@gmail.com>

.changeset/strong-coins-peel.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: improve error message for migration errors when slot would be renamed

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`
 					);
 				}
 			}

packages/svelte/tests/migrate/samples/impossible-migrate-$derived-derived-var-3/output.svelte

@@ -1,4 +1,4 @@
-<!-- @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>

packages/svelte/tests/migrate/samples/impossible-migrate-slot-change-name/output.svelte

@@ -1,4 +1,4 @@
-<!-- @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>

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 -->
+<!-- @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>