feat: empty module should have ExportsKind::None (#317)

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### Description

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### Test Plan

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---

crates/rolldown/src/bundler/visitors/scanner.rs

@@ -40,8 +40,10 @@ pub struct Scanner<'a> {
   current_stmt_info: StmtInfo,
   result: ScanResult,
   esm_export_keyword: Option<Span>,
-  cjs_export_keyword: Option<Span>,
+  esm_import_keyword: Option<Span>,
   pub namespace_symbol: SymbolRef,
+  used_exports_ref: bool,
+  used_module_ref: bool,
 }
 
 impl<'ast> Scanner<'ast> {
@@ -68,45 +70,49 @@ impl<'ast> Scanner<'ast> {
       current_stmt_info: StmtInfo::default(),
       result,
       esm_export_keyword: None,
-      cjs_export_keyword: None,
+      esm_import_keyword: None,
       module_type,
       namespace_symbol: namespace_ref,
+      used_exports_ref: false,
+      used_module_ref: false,
     }
   }
 
   pub fn scan(mut self, program: &Program<'ast>) -> ScanResult {
     self.visit_program(program);
-    self.result
-  }
+    let mut exports_kind = ExportsKind::None;
 
-  fn set_esm_export_keyword(&mut self, span: Span) {
-    if self.esm_export_keyword.is_none() {
-      self.esm_export_keyword = Some(span);
+    if self.esm_export_keyword.is_some() {
+      exports_kind = ExportsKind::Esm;
+    } else if self.used_exports_ref || self.used_module_ref {
+      exports_kind = ExportsKind::CommonJs;
+    } else {
+      // TODO(hyf0): need review this. Why `ModuleType` doesn't have higher priority?
+      match self.module_type {
+        ModuleType::CJS | ModuleType::CjsPackageJson => {
+          exports_kind = ExportsKind::CommonJs;
+        }
+        ModuleType::EsmMjs | ModuleType::EsmPackageJson => {
+          exports_kind = ExportsKind::Esm;
+        }
+        ModuleType::Unknown => {
+          if self.esm_import_keyword.is_some() {
+            exports_kind = ExportsKind::Esm;
+          }
+        }
+      }
     }
+
+    self.result.exports_kind = exports_kind;
+    self.result
   }
 
   fn is_unresolved_reference(&self, ident_ref: &IdentifierReference) -> bool {
     self.scope.is_unresolved(ident_ref.reference_id.get().unwrap())
   }
 
-  fn set_cjs_export_keyword(&mut self, span: Span) {
-    if self.cjs_export_keyword.is_none() {
-      self.cjs_export_keyword = Some(span);
-    }
-  }
-
-  fn set_exports_kind(&mut self) {
-    if self.esm_export_keyword.is_some() {
-      self.result.exports_kind = ExportsKind::Esm;
-    } else if self.cjs_export_keyword.is_some() {
-      self.result.exports_kind = ExportsKind::CommonJs;
-    } else if self.module_type.is_esm() {
-      self.result.exports_kind = ExportsKind::Esm;
-    } else if self.module_type.is_commonjs() {
-      self.result.exports_kind = ExportsKind::CommonJs;
-    } else {
-      self.result.exports_kind = ExportsKind::Esm;
-    }
+  fn set_esm_export_keyword(&mut self, span: Span) {
+    self.esm_export_keyword.get_or_insert(span);
   }
 
   fn add_declared_id(&mut self, id: SymbolId) {
@@ -276,6 +282,8 @@ impl<'ast> Scanner<'ast> {
   fn scan_module_decl(&mut self, decl: &ModuleDeclaration) {
     match decl {
       oxc::ast::ast::ModuleDeclaration::ImportDeclaration(decl) => {
+        // TODO: this should be the span of `import` keyword, while now it is the span of the whole import declaration.
+        self.esm_import_keyword.get_or_insert(decl.span);
         self.scan_import_decl(decl);
       }
       oxc::ast::ast::ModuleDeclaration::ExportAllDeclaration(decl) => {
@@ -311,7 +319,6 @@ impl<'ast> Visit<'ast> for Scanner<'ast> {
       self.visit_statement(stmt);
       self.result.stmt_infos.add_stmt_info(std::mem::take(&mut self.current_stmt_info));
     }
-    self.set_exports_kind();
   }
 
   fn visit_binding_identifier(&mut self, ident: &oxc::ast::ast::BindingIdentifier) {
@@ -327,14 +334,15 @@ impl<'ast> Visit<'ast> for Scanner<'ast> {
       Some(symbol_id) if self.is_top_level(symbol_id) => {
         self.add_referenced_symbol(symbol_id);
       }
-      _ => {}
-    }
-    if ident.name == "module" || ident.name == "exports" {
-      if let Some(refs) = self.scope.root_unresolved_references().get(&ident.name) {
-        if refs.iter().any(|r| (*r).eq(&ident.reference_id.get().unwrap())) {
-          self.set_cjs_export_keyword(ident.span);
+      None => {
+        if ident.name == "module" {
+          self.used_module_ref = true;
+        }
+        if ident.name == "exports" {
+          self.used_exports_ref = true;
         }
       }
+      _ => {}
     }
   }
 

crates/rolldown/tests/esbuild/default/nested_common_js/artifacts.snap

@@ -8,7 +8,7 @@ input_file: crates/rolldown/tests/esbuild/default/nested_common_js
 ## entry_js.mjs
 
 ```js
-import { __commonJS, __toESM } from "./_rolldown_runtime.mjs";
+import { __commonJS } from "./_rolldown_runtime.mjs";
 
 // foo.js
 var require_foo = __commonJS({

crates/rolldown/tests/esbuild/default/new_expression_common_js/artifacts.snap

@@ -8,7 +8,7 @@ input_file: crates/rolldown/tests/esbuild/default/new_expression_common_js
 ## entry_js.mjs
 
 ```js
-import { __commonJS, __toESM } from "./_rolldown_runtime.mjs";
+import { __commonJS } from "./_rolldown_runtime.mjs";
 
 // foo.js
 var require_foo = __commonJS({

crates/rolldown/tests/esbuild/default/require_child_dir_common_js/artifacts.snap

@@ -8,7 +8,7 @@ input_file: crates/rolldown/tests/esbuild/default/require_child_dir_common_js
 ## entry_js.mjs
 
 ```js
-import { __commonJS, __toESM } from "./_rolldown_runtime.mjs";
+import { __commonJS } from "./_rolldown_runtime.mjs";
 
 // dir/index.js
 var require_dir_index = __commonJS({

crates/rolldown/tests/esbuild/default/require_parent_dir_common_js/artifacts.snap

@@ -8,7 +8,7 @@ input_file: crates/rolldown/tests/esbuild/default/require_parent_dir_common_js
 ## dir_entry_js.mjs
 
 ```js
-import { __commonJS, __toESM } from "./_rolldown_runtime.mjs";
+import { __commonJS } from "./_rolldown_runtime.mjs";
 
 // index.js
 var require_require_parent_dir_common_js_index = __commonJS({

crates/rolldown/tests/esbuild/default/simple_common_js/artifacts.snap

@@ -8,7 +8,7 @@ input_file: crates/rolldown/tests/esbuild/default/simple_common_js
 ## entry_js.mjs
 
 ```js
-import { __commonJS, __toESM } from "./_rolldown_runtime.mjs";
+import { __commonJS } from "./_rolldown_runtime.mjs";
 
 // foo.js
 var require_foo = __commonJS({

crates/rolldown/tests/esbuild/default/use_strict_directive_bundle_issue1837/artifacts.snap

@@ -8,7 +8,7 @@ input_file: crates/rolldown/tests/esbuild/default/use_strict_directive_bundle_is
 ## entry_js.mjs
 
 ```js
-import { __commonJS, __toESM } from "./_rolldown_runtime.mjs";
+import { __commonJS } from "./_rolldown_runtime.mjs";
 
 // cjs.js
 var require_cjs = __commonJS({

crates/rolldown/tests/esbuild/splitting/shared-commonjs-into-es6/artifacts.snap

@@ -21,7 +21,6 @@ export { require_shared };
 ## a.mjs
 
 ```js
-import { __toESM } from "./_rolldown_runtime.mjs";
 import { require_shared } from "./2.mjs";
 
 // a.js
@@ -31,7 +30,6 @@ console.log(foo)
 ## b.mjs
 
 ```js
-import { __toESM } from "./_rolldown_runtime.mjs";
 import { require_shared } from "./2.mjs";
 
 // b.js

crates/rolldown/tests/fixtures/mix-cjs-esm/artifacts.snap

@@ -8,15 +8,12 @@ input_file: crates/rolldown/tests/fixtures/mix-cjs-esm
 ## main.mjs
 
 ```js
-import { __commonJS, __esm, __toCommonJS, __toESM } from "./_rolldown_runtime.mjs";
+import { __commonJS, __esm, __toESM } from "./_rolldown_runtime.mjs";
 
 // esm-export-cjs-export.js
 module.exports = 1;
 const value$1 = 1;
 // foo.js
-var foo_ns = {
-
-};
 var init_foo = __esm({
 	'foo.js'() {
 

crates/rolldown/tests/fixtures/require/require_cjs/artifacts.snap

@@ -8,7 +8,7 @@ input_file: crates/rolldown/tests/fixtures/require/require_cjs
 ## main.mjs
 
 ```js
-import { __commonJS, __toESM } from "./_rolldown_runtime.mjs";
+import { __commonJS } from "./_rolldown_runtime.mjs";
 
 // cjs.js
 var require_cjs = __commonJS({