feat(codegen): allow attaching comment to the top of the file. (#13048)

closes #12982

Author: Boshen

crates/oxc_codegen/src/gen.rs

@@ -46,6 +46,8 @@ impl Gen for Program<'_> {
     fn r#gen(&self, p: &mut Codegen, ctx: Context) {
         p.is_jsx = self.source_type.is_jsx();
 
+        // Allow for inserting comments to the top of the file.
+        p.print_comments_at(0);
         if let Some(hashbang) = &self.hashbang {
             hashbang.print(p, ctx);
         }

crates/oxc_codegen/tests/integration/comments.rs

@@ -1,5 +1,22 @@
 use crate::tester::{test, test_same};
 
+#[test]
+fn test_comment_at_top_of_file() {
+    use oxc_allocator::Allocator;
+    use oxc_ast::CommentPosition;
+    use oxc_codegen::Codegen;
+    use oxc_parser::Parser;
+    use oxc_span::SourceType;
+    let source_type = SourceType::mjs();
+    let allocator = Allocator::default();
+    let mut ret = Parser::new(&allocator, "export{} /** comment */", source_type).parse();
+    // Move comment to top of the file.
+    ret.program.comments[0].attached_to = 0;
+    ret.program.comments[0].position = CommentPosition::Leading;
+    let code = Codegen::new().build(&ret.program).code;
+    assert_eq!(code, "/** comment */ export {};\n");
+}
+
 #[test]
 fn unit() {
     test_same("<div>{/* Hello */}</div>;\n");