fix(transformer): output empty file for TS definition files (#3500)

As discussed in
https://github.com/oxc-project/oxc/pull/3489#issuecomment-2143482458,
transformer should output an empty file for TS definition files.

crates/oxc_transformer/src/typescript/mod.rs

@@ -69,7 +69,14 @@ impl<'a> TypeScript<'a> {
 // Transforms
 impl<'a> TypeScript<'a> {
     pub fn transform_program(&self, program: &mut Program<'a>, ctx: &mut TraverseCtx) {
-        self.transform_program_for_namespace(program, ctx);
+        if self.ctx.source_type.is_typescript_definition() {
+            // Output empty file for TS definitions
+            program.directives.clear();
+            program.hashbang = None;
+            program.body.clear();
+        } else {
+            self.transform_program_for_namespace(program, ctx);
+        }
     }
 
     pub fn transform_program_on_exit(&self, program: &mut Program<'a>) {

tasks/transform_conformance/oxc.snap.md

@@ -1,6 +1,6 @@
 commit: 4bd1b2c2
 
-Passed: 2/2
+Passed: 3/3
 
 # All Passed:
 * babel-plugin-transform-typescript

tasks/transform_conformance/src/test_case.rs

@@ -37,9 +37,12 @@ impl TestCaseKind {
             return Some(Self::Exec(ExecTestCase::new(cwd, path)));
         }
 
-        // named `input.[ext]``
-        if path.file_stem().is_some_and(|name| name == "input")
-            && path.extension().is_some_and(|ext| VALID_EXTENSIONS.contains(&ext.to_str().unwrap()))
+        // named `input.[ext]` or `input.d.ts`
+        if (path.file_stem().is_some_and(|name| name == "input")
+            && path
+                .extension()
+                .is_some_and(|ext| VALID_EXTENSIONS.contains(&ext.to_str().unwrap())))
+            || path.file_name().is_some_and(|name| name == "input.d.ts")
         {
             return Some(Self::Transform(ConformanceTestCase::new(cwd, path)));
         }
@@ -156,11 +159,15 @@ pub trait TestCase {
         let allocator = Allocator::default();
         let source_text = fs::read_to_string(path).unwrap();
 
-        let source_type = SourceType::from_path(path).unwrap().with_typescript(
-            // Some babel test cases have a js extension, but contain typescript code.
-            // Therefore, if the typescript plugin exists, enable the typescript.
-            self.options().get_plugin("transform-typescript").is_some(),
-        );
+        // Some babel test cases have a js extension, but contain typescript code.
+        // Therefore, if the typescript plugin exists, enable typescript.
+        let mut source_type = SourceType::from_path(path).unwrap();
+        if !source_type.is_typescript()
+            && (self.options().get_plugin("transform-typescript").is_some()
+                || self.options().get_plugin("syntax-typescript").is_some())
+        {
+            source_type = source_type.with_typescript(true);
+        }
 
         let ret = Parser::new(&allocator, &source_text, source_type).parse();
         let mut program = ret.program;
@@ -224,18 +231,20 @@ impl TestCase for ConformanceTestCase {
             .as_ref()
             .is_some_and(|path| path.extension().and_then(std::ffi::OsStr::to_str) == Some("js"));
 
-        let source_type = SourceType::from_path(&self.path)
+        let mut source_type = SourceType::from_path(&self.path)
             .unwrap()
             .with_script(if self.options.source_type.is_some() {
                 !self.options.is_module()
             } else {
                 input_is_js && output_is_js
             })
-            .with_typescript(
-                self.options.get_plugin("transform-typescript").is_some()
-                    || self.options.get_plugin("syntax-typescript").is_some(),
-            )
             .with_jsx(self.options.get_plugin("syntax-jsx").is_some());
+        if !source_type.is_typescript()
+            && (self.options.get_plugin("transform-typescript").is_some()
+                || self.options.get_plugin("syntax-typescript").is_some())
+        {
+            source_type = source_type.with_typescript(true);
+        }
 
         if filtered {
             println!("input_path: {:?}", &self.path);

tasks/transform_conformance/tests/babel-plugin-transform-typescript/test/fixtures/ts-declaration-empty-output/input.d.ts

@@ -0,0 +1,14 @@
+export interface Things<P, T> {
+    p: P;
+    t: T;
+}
+
+export interface Props {
+}
+
+export default class MyComponent {
+    props: Props;
+}
+export namespace Something {
+    export const foo = 123;
+}