feat(typescript): error when no tsconfig and no rootDir (#794)

* feat(typescript): error when no tsconfig and no rootDir

* fix: only emit when declaration: true

packages/typescript/src/preflight.ts

@@ -17,6 +17,8 @@ ${pluginName}: Rollup requires that TypeScript produces ES Modules. Unfortunatel
  "module" other than "esnext". Unless you know what you're doing, please change "module" to "esnext"
  in the target tsconfig.json file or plugin options.`.replace(/\n/g, '');
 
+const rootDirErrorMessage = `${pluginName}: The "rootDir" or "rootDirs" option is required when the "tsconfig" option is not specified and "declaration" is "true".`;
+
 const tsLibErrorMessage = `${pluginName}: Could not find module 'tslib', which is required by this plugin. Is it installed?`;
 
 let undef;
@@ -28,6 +30,11 @@ export const preflight = ({ config, context, rollupOptions, tslib }: PreflightOp
     context.warn(moduleErrorMessage);
   }
 
+  const { options } = config;
+  if (options.declaration && !options.configFilePath && !options.rootDir && !options.rootDirs) {
+    context.error(rootDirErrorMessage);
+  }
+
   if (!rollupOptions.preserveModules && tslib === null) {
     context.error(tsLibErrorMessage);
   }

packages/typescript/test/snapshots/tsconfig.ts.md

@@ -0,0 +1,25 @@
+# Snapshot report for `test/tsconfig.ts`
+
+The actual snapshot is saved in `tsconfig.ts.snap`.
+
+Generated by [AVA](https://avajs.dev).
+
+## inline config without tsconfig + rootDir
+
+> Snapshot 1
+
+    [
+      'main.js',
+      'types/main.d.ts',
+    ]
+
+## inline config without tsconfig without rootDir fails
+
+> Snapshot 1
+
+    Error {
+      code: 'PLUGIN_ERROR',
+      hook: 'buildStart',
+      plugin: 'typescript',
+      message: '@rollup/plugin-typescript: The "rootDir" or "rootDirs" option is required when the "tsconfig" option is not specified and "declaration" is "true".',
+    }

packages/typescript/test/tsconfig.ts

@@ -0,0 +1,50 @@
+import test from 'ava';
+import { rollup } from 'rollup';
+
+import typescript from '..';
+
+import { getCode, onwarn } from '../../../util/test';
+
+test.beforeEach(() => process.chdir(__dirname));
+
+test.serial('inline config without tsconfig + rootDir', async (t) => {
+  const bundle = await rollup({
+    input: 'fixtures/basic/main.ts',
+    plugins: [
+      typescript({
+        declaration: true,
+        declarationDir: 'fixtures/basic/dist/types',
+        exclude: 'fixtures/basic/dist/types',
+        include: 'fixtures/basic/*.ts',
+        tsconfig: false,
+        rootDir: 'fixtures/basic'
+      })
+    ],
+    onwarn
+  });
+  const files = await getCode(bundle, { format: 'esm', dir: 'fixtures/basic/dist' }, true);
+  const [, { source }] = files;
+
+  t.snapshot(files.map(({ fileName }) => fileName));
+  t.true((source as string)?.includes('declare const answer = 42;'));
+});
+
+test.serial('inline config without tsconfig without rootDir fails', async (t) => {
+  const fail = () =>
+    rollup({
+      input: 'fixtures/basic/main.ts',
+      plugins: [
+        typescript({
+          declaration: true,
+          declarationDir: 'fixtures/basic/dist/types',
+          exclude: 'fixtures/basic/dist/types',
+          include: 'fixtures/basic/*.ts',
+          tsconfig: false
+        })
+      ],
+      onwarn
+    });
+
+  const error = await t.throwsAsync(fail);
+  t.snapshot(error);
+});