[fix] prevent unused types warning (#6293)

* [fix] prevent unused types warning

Fixes #6290

* ts-nocheck

Co-authored-by: Rich Harris <hello@rich-harris.dev>

Author: dummdidumm

.changeset/neat-cameras-swim.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+[fix] prevent unused types warning

packages/kit/src/core/sync/write_types/index.js

@@ -559,7 +559,13 @@ export function tweak_types(content, names) {
 										arg.name.end,
 										`: Parameters<${type}>[0]` + (add_parens ? ')' : '')
 									);
+								} else {
+									// prevent "type X is imported but not used" (isn't silenced by @ts-nocheck) when svelte-check runs
+									code.append(`;${type};`);
 								}
+							} else {
+								// prevent "type X is imported but not used" (isn't silenced by @ts-nocheck) when svelte-check runs
+								code.append(`;${type};`);
 							}
 
 							modified = true;
@@ -569,6 +575,16 @@ export function tweak_types(content, names) {
 			}
 		});
 
+		if (modified) {
+			// Ignore all type errors so they don't show up twice when svelte-check runs
+			// Account for possible @ts-check which would overwrite @ts-nocheck
+			if (code.original.startsWith('// @ts-check')) {
+				code.prependLeft('// @ts-check'.length, '\n// @ts-nocheck\n');
+			} else {
+				code.prepend('// @ts-nocheck\n');
+			}
+		}
+
 		return {
 			modified,
 			code: code.toString(),

packages/kit/src/core/sync/write_types/index.spec.js

@@ -1,3 +1,4 @@
+// @ts-nocheck
 import path from 'path';
 import fs from 'fs';
 import { format } from 'prettier';
@@ -101,7 +102,8 @@ test('Rewrites types for a TypeScript module', () => {
 	assert.equal(rewritten?.exports, ['GET']);
 	assert.equal(
 		rewritten?.code,
-		`
+		`// @ts-nocheck
+
 		export const GET = ({ params }: Parameters<Get>[0]) => {
 			return {
 				a: 1
@@ -125,13 +127,14 @@ test('Rewrites types for a TypeScript module without param', () => {
 	assert.equal(rewritten?.exports, ['GET']);
 	assert.equal(
 		rewritten?.code,
-		`
+		`// @ts-nocheck
+
 		export const GET = () => {
 			return {
 				a: 1
 			};
 		};
-	`
+	;Get;`
 	);
 });
 
@@ -150,7 +153,8 @@ test('Rewrites types for a JavaScript module with `function`', () => {
 	assert.equal(rewritten?.exports, ['GET']);
 	assert.equal(
 		rewritten?.code,
-		`
+		`// @ts-nocheck
+
 		/** @param {Parameters<import('./$types').Get>[0]} event */
 		export function GET({ params }) {
 			return {
@@ -176,7 +180,36 @@ test('Rewrites types for a JavaScript module with `const`', () => {
 	assert.equal(rewritten?.exports, ['GET']);
 	assert.equal(
 		rewritten?.code,
-		`
+		`// @ts-nocheck
+
+		/** @param {Parameters<import('./$types').Get>[0]} event */
+		export const GET = ({ params }) => {
+			return {
+				a: 1
+			};
+		};
+	`
+	);
+});
+
+test('Appends @ts-nocheck after @ts-check', () => {
+	const source = `// @ts-check
+		/** @type {import('./$types').Get} */
+		export const GET = ({ params }) => {
+			return {
+				a: 1
+			};
+		};
+	`;
+
+	const rewritten = tweak_types(source, new Set(['GET']));
+
+	assert.equal(rewritten?.exports, ['GET']);
+	assert.equal(
+		rewritten?.code,
+		`// @ts-check
+// @ts-nocheck
+
 		/** @param {Parameters<import('./$types').Get>[0]} event */
 		export const GET = ({ params }) => {
 			return {