fix: don't print errors on migration errors (#13754)

makes it look like the migration script failed when in reality you need to update some of your files by hand

.changeset/large-carrots-behave.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't print errors on migration errors

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

@@ -25,6 +25,15 @@ const style_placeholder = '/*$$__STYLE_CONTENT__$$*/';
 
 let has_migration_task = false;
 
+class MigrationError extends Error {
+	/**
+	 * @param {string} msg
+	 */
+	constructor(msg) {
+		super(msg);
+	}
+}
+
 /**
  * Does a best-effort migration of Svelte code towards using runes, event attributes and render tags.
  * May throw an error if the code is too complex to migrate automatically.
@@ -310,8 +319,10 @@ export function migrate(source, { filename } = {}) {
 		}
 		return { code: str.toString() };
 	} catch (e) {
-		// eslint-disable-next-line no-console
+		if (!(e instanceof MigrationError)) {
-		console.error('Error while migrating Svelte code', e);
+			// eslint-disable-next-line no-console
+			console.error('Error while migrating Svelte code', e);
+		}
 		has_migration_task = true;
 		return {
 			code: `<!-- @migration-task Error while migrating Svelte code: ${/** @type {any} */ (e).message} -->\n${og_source}`
@@ -398,7 +409,7 @@ const instance_script = {
 				state.str.remove(/** @type {number} */ (node.start), /** @type {number} */ (node.end));
 			}
 			if (illegal_specifiers.length > 0) {
-				throw new Error(
+				throw new MigrationError(
 					`Can't migrate code with ${illegal_specifiers.join(' and ')}. Please migrate by hand.`
 				);
 			}
@@ -462,7 +473,7 @@ const instance_script = {
 
 				if (declarator.id.type !== 'Identifier') {
 					// TODO invest time in this?
-					throw new Error(
+					throw new MigrationError(
 						'Encountered an export declaration pattern that is not supported for automigration.'
 					);
 					// Turn export let into props. It's really really weird because export let { x: foo, z: [bar]} = ..
@@ -493,7 +504,7 @@ const instance_script = {
 				const binding = /** @type {Binding} */ (state.scope.get(name));
 
 				if (state.analysis.uses_props && (declarator.init || binding.updated)) {
-					throw new Error(
+					throw new MigrationError(
 						'$$props is used together with named props in a way that cannot be automatically migrated.'
 					);
 				}
@@ -1065,7 +1076,7 @@ const template = {
 		} else if (slot_name !== 'default') {
 			name = state.scope.generate(slot_name);
 			if (name !== slot_name) {
-				throw new Error(
+				throw new MigrationError(
 					'This migration would change the name of a slot making the component unusable'
 				);
 			}
@@ -1520,7 +1531,7 @@ function handle_identifier(node, state, path) {
 			} else if (name !== 'default') {
 				let new_name = state.scope.generate(name);
 				if (new_name !== name) {
-					throw new Error(
+					throw new MigrationError(
 						'This migration would change the name of a slot making the component unusable'
 					);
 				}

packages/svelte/tests/migrate/samples/impossible-migrate-beforeUpdate-afterUpdate/_config.js

@@ -3,5 +3,6 @@ import { test } from '../../test';
 export default test({
 	logs: [
 		'One or more `@migration-task` comments were added to `output.svelte`, please check them and complete the migration manually.'
-	]
+	],
+	errors: []
 });

packages/svelte/tests/migrate/samples/impossible-migrate-prop-and-$$props/_config.js

@@ -3,5 +3,6 @@ import { test } from '../../test';
 export default test({
 	logs: [
 		'One or more `@migration-task` comments were added to `output.svelte`, please check them and complete the migration manually.'
-	]
+	],
+	errors: []
 });

packages/svelte/tests/migrate/samples/impossible-migrate-prop-non-identifier/_config.js

@@ -3,5 +3,6 @@ import { test } from '../../test';
 export default test({
 	logs: [
 		'One or more `@migration-task` comments were added to `output.svelte`, please check them and complete the migration manually.'
-	]
+	],
+	errors: []
 });

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

@@ -3,5 +3,6 @@ import { test } from '../../test';
 export default test({
 	logs: [
 		'One or more `@migration-task` comments were added to `output.svelte`, please check them and complete the migration manually.'
-	]
+	],
+	errors: []
 });

packages/svelte/tests/migrate/samples/impossible-migrate-slot-non-identifier/_config.js

@@ -3,5 +3,6 @@ import { test } from '../../test';
 export default test({
 	logs: [
 		'One or more `@migration-task` comments were added to `output.svelte`, please check them and complete the migration manually.'
-	]
+	],
+	errors: []
 });

packages/svelte/tests/migrate/samples/impossible-migrate-with-errors/_config.js

@@ -0,0 +1,31 @@
+import { test } from '../../test';
+
+export default test({
+	logs: [
+		'One or more `@migration-task` comments were added to `output.svelte`, please check them and complete the migration manually.'
+	],
+	errors: [
+		'Error while migrating Svelte code',
+		{
+			code: 'unexpected_eof',
+			end: {
+				character: 30,
+				column: 21,
+				line: 3
+			},
+			filename: 'output.svelte',
+			frame: `1: <script
+2: 
+3: unterminated template
+                        ^`,
+			message: 'Unexpected end of input',
+			name: 'CompileError',
+			position: [30, 30],
+			start: {
+				character: 30,
+				column: 21,
+				line: 3
+			}
+		}
+	]
+});

packages/svelte/tests/migrate/samples/impossible-migrate-with-errors/input.svelte

@@ -0,0 +1,3 @@
+<script
+
+unterminated template

packages/svelte/tests/migrate/samples/impossible-migrate-with-errors/output.svelte

@@ -0,0 +1,4 @@
+<!-- @migration-task Error while migrating Svelte code: Unexpected end of input -->
+<script
+
+unterminated template

packages/svelte/tests/migrate/samples/svelte-self-skip-filename/_config.js

@@ -4,5 +4,6 @@ export default test({
 	skip_filename: true,
 	logs: [
 		"One or more `@migration-task` comments were added to a file (unfortunately we don't know the name), please check them and complete the migration manually."
-	]
+	],
+	errors: []
 });

packages/svelte/tests/migrate/test.ts

@@ -6,7 +6,8 @@ import { suite, type BaseTest } from '../suite.js';
 
 interface ParserTest extends BaseTest {
 	skip_filename?: boolean;
-	logs?: string[];
+	logs?: any[];
+	errors?: any[];
 }
 
 const { test, run } = suite<ParserTest>(async (config, cwd) => {
@@ -16,6 +17,7 @@ const { test, run } = suite<ParserTest>(async (config, cwd) => {
 		.replace(/\r/g, '');
 
 	const logs: any[] = [];
+	const errors: any[] = [];
 
 	if (config.logs) {
 		console.log = (...args) => {
@@ -23,6 +25,12 @@ const { test, run } = suite<ParserTest>(async (config, cwd) => {
 		};
 	}
 
+	if (config.errors) {
+		console.error = (...args) => {
+			errors.push(...args);
+		};
+	}
+
 	const actual = migrate(input, {
 		filename: config.skip_filename ? undefined : `output.svelte`
 	}).code;
@@ -31,6 +39,10 @@ const { test, run } = suite<ParserTest>(async (config, cwd) => {
 		assert.deepEqual(logs, config.logs);
 	}
 
+	if (config.errors) {
+		assert.deepEqual(errors, config.errors);
+	}
+
 	// run `UPDATE_SNAPSHOTS=true pnpm test migrate` to update parser tests
 	if (process.env.UPDATE_SNAPSHOTS || !fs.existsSync(`${cwd}/output.svelte`)) {
 		fs.writeFileSync(`${cwd}/output.svelte`, actual);