feat(tree_shaking): shake unused import variables (#771)

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### Description

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crates/rolldown/src/ast_scanner/side_effect_detector.rs

@@ -207,8 +207,12 @@ impl<'a> SideEffectDetector<'a> {
       Statement::Declaration(decl) => self.detect_side_effect_of_decl(decl),
       Statement::ExpressionStatement(expr) => self.detect_side_effect_of_expr(&expr.expression),
       Statement::ModuleDeclaration(module_decl) => match &**module_decl {
-        oxc::ast::ast::ModuleDeclaration::ImportDeclaration(_)
-        | oxc::ast::ast::ModuleDeclaration::ExportAllDeclaration(_) => true,
+        oxc::ast::ast::ModuleDeclaration::ExportAllDeclaration(_) => true,
+        oxc::ast::ast::ModuleDeclaration::ImportDeclaration(_) => {
+          // We consider `import ...` has no side effect. However, `import ...` might be rewritten to other statements by the bundler.
+          // In that case, we will mark the statement as having side effect in link stage.
+          false
+        }
         oxc::ast::ast::ModuleDeclaration::ExportDefaultDeclaration(default_decl) => {
           match &default_decl.declaration {
             oxc::ast::ast::ExportDefaultDeclarationKind::Expression(expr) => {

crates/rolldown/src/stages/link_stage/mod.rs

@@ -215,65 +215,82 @@ impl<'a> LinkStage<'a> {
       stmt_infos.iter_mut().for_each(|stmt_info| {
         stmt_info.import_records.iter().for_each(|rec_id| {
           let rec = &importer.import_records[*rec_id];
-          let ModuleId::Normal(importee_id) = rec.resolved_module else {
-            return;
-          };
-          let importee_linking_info = &self.metas[importee_id];
-          match rec.kind {
-            ImportKind::Import => {
-              let is_reexport_all = importer.star_exports.contains(rec_id);
-              match importee_linking_info.wrap_kind {
-                WrapKind::None => {}
-                WrapKind::Cjs => {
-                  if is_reexport_all {
-                    // something like `__reExport(foo_exports, __toESM(require_bar()))`
-                    // Reference to `require_bar`
-                    stmt_info.referenced_symbols.push(importee_linking_info.wrapper_ref.unwrap());
-                    stmt_info.referenced_symbols.push(self.runtime.resolve_symbol("__toESM"));
-                    stmt_info.referenced_symbols.push(self.runtime.resolve_symbol("__reExport"));
-                    stmt_info.referenced_symbols.push(importer.namespace_symbol);
-                  } else {
-                    // something like `var import_foo = __toESM(require_foo())`
+          match rec.resolved_module {
+            ModuleId::External(_) => {
+              // Make sure symbols from external modules are included and de_conflicted
+              stmt_info.side_effect = true;
+            }
+            ModuleId::Normal(importee_id) => {
+              let importee_linking_info = &self.metas[importee_id];
+              match rec.kind {
+                ImportKind::Import => {
+                  let is_reexport_all = importer.star_exports.contains(rec_id);
+                  match importee_linking_info.wrap_kind {
+                    WrapKind::None => {}
+                    WrapKind::Cjs => {
+                      stmt_info.side_effect = true;
+                      if is_reexport_all {
+                        // Turn `export * from 'bar_cjs'` into `__reExport(foo_exports, __toESM(require_bar_cjs()))`
+                        // Reference to `require_bar_cjs`
+                        stmt_info
+                          .referenced_symbols
+                          .push(importee_linking_info.wrapper_ref.unwrap());
+                        stmt_info.referenced_symbols.push(self.runtime.resolve_symbol("__toESM"));
+                        stmt_info
+                          .referenced_symbols
+                          .push(self.runtime.resolve_symbol("__reExport"));
+                        stmt_info.referenced_symbols.push(importer.namespace_symbol);
+                      } else {
+                        // Turn `import * as bar from 'bar_cjs'` into `var import_bar_cjs = __toESM(require_bar_cjs())`
+                        // Turn `import { prop } from 'bar_cjs'; prop;` into `var import_bar_cjs = __toESM(require_bar_cjs()); import_bar_cjs.prop;`
+                        // Reference to `require_bar_cjs`
+                        stmt_info
+                          .referenced_symbols
+                          .push(importee_linking_info.wrapper_ref.unwrap());
+                        stmt_info.referenced_symbols.push(self.runtime.resolve_symbol("__toESM"));
+                        stmt_info.declared_symbols.push(rec.namespace_ref);
+                        let importee = &self.module_table.normal_modules[importee_id];
+                        symbols.lock().unwrap().get_mut(rec.namespace_ref).name =
+                          format!("import_{}", &importee.repr_name).into();
+                      }
+                    }
+                    WrapKind::Esm => {
+                      stmt_info.side_effect = true;
+                      // Turn `import ... from 'bar_esm'` into `init_bar_esm()`
+                      // Reference to `init_foo`
+                      stmt_info.referenced_symbols.push(importee_linking_info.wrapper_ref.unwrap());
+                      if is_reexport_all && importee_linking_info.has_dynamic_exports {
+                        // Turn `export * from 'bar_esm'` into `init_bar_esm();__reExport(foo_exports, bar_esm_exports);`
+                        // something like `__reExport(foo_exports, other_exports)`
+                        stmt_info
+                          .referenced_symbols
+                          .push(self.runtime.resolve_symbol("__reExport"));
+                        stmt_info.referenced_symbols.push(importer.namespace_symbol);
+                        let importee = &self.module_table.normal_modules[importee_id];
+                        stmt_info.referenced_symbols.push(importee.namespace_symbol);
+                      }
+                    }
+                  }
+                }
+                ImportKind::Require => match importee_linking_info.wrap_kind {
+                  WrapKind::None => {}
+                  WrapKind::Cjs => {
+                    // something like `require_foo()`
                     // Reference to `require_foo`
                     stmt_info.referenced_symbols.push(importee_linking_info.wrapper_ref.unwrap());
-                    stmt_info.referenced_symbols.push(self.runtime.resolve_symbol("__toESM"));
-                    stmt_info.declared_symbols.push(rec.namespace_ref);
-                    let importee = &self.module_table.normal_modules[importee_id];
-                    symbols.lock().unwrap().get_mut(rec.namespace_ref).name =
-                      format!("import_{}", &importee.repr_name).into();
                   }
-                }
-                WrapKind::Esm => {
-                  // something like `init_foo()`
-                  // Reference to `init_foo`
-                  stmt_info.referenced_symbols.push(importee_linking_info.wrapper_ref.unwrap());
-                  if is_reexport_all && importee_linking_info.has_dynamic_exports {
-                    // something like `__reExport(foo_exports, other_exports)`
-                    stmt_info.referenced_symbols.push(self.runtime.resolve_symbol("__reExport"));
-                    stmt_info.referenced_symbols.push(importer.namespace_symbol);
+                  WrapKind::Esm => {
+                    // something like `(init_foo(), toCommonJS(foo_exports))`
+                    // Reference to `init_foo`
+                    stmt_info.referenced_symbols.push(importee_linking_info.wrapper_ref.unwrap());
+                    stmt_info.referenced_symbols.push(self.runtime.resolve_symbol("__toCommonJS"));
                     let importee = &self.module_table.normal_modules[importee_id];
                     stmt_info.referenced_symbols.push(importee.namespace_symbol);
                   }
-                }
+                },
+                ImportKind::DynamicImport => {}
               }
             }
-            ImportKind::Require => match importee_linking_info.wrap_kind {
-              WrapKind::None => {}
-              WrapKind::Cjs => {
-                // something like `require_foo()`
-                // Reference to `require_foo`
-                stmt_info.referenced_symbols.push(importee_linking_info.wrapper_ref.unwrap());
-              }
-              WrapKind::Esm => {
-                // something like `(init_foo(), toCommonJS(foo_exports))`
-                // Reference to `init_foo`
-                stmt_info.referenced_symbols.push(importee_linking_info.wrapper_ref.unwrap());
-                stmt_info.referenced_symbols.push(self.runtime.resolve_symbol("__toCommonJS"));
-                let importee = &self.module_table.normal_modules[importee_id];
-                stmt_info.referenced_symbols.push(importee.namespace_symbol);
-              }
-            },
-            ImportKind::DynamicImport => {}
           }
         });
       });

crates/rolldown/src/stages/link_stage/tree_shaking.rs

@@ -78,6 +78,8 @@ fn include_statement(ctx: &mut Context, module: &NormalModule, stmt_info_id: Stm
   // include statements that are referenced by this statement
   stmt_info.declared_symbols.iter().chain(stmt_info.referenced_symbols.iter()).for_each(
     |symbol_ref| {
+      // Notice we also include `declared_symbols`. This for case that import statements declare new symbols, but they are not
+      // really declared by the module itself. We need to include them where they are really declared.
       include_symbol(ctx, *symbol_ref);
     },
   );

crates/rolldown/tests/esbuild/import_star/import_star_export_import_star_unused/artifacts.snap

@@ -8,15 +8,6 @@ input_file: crates/rolldown/tests/esbuild/import_star/import_star_export_import_
 ## entry_js.mjs
 
 ```js
-import { __export } from "./$runtime$.mjs";
-
-// foo.js
-var foo_ns = {};
-__export(foo_ns, {
-	foo:() => foo$1
-});
-const foo$1 = 123;
-
 // entry.js
 let foo = 234;
 console.log(foo);

crates/rolldown/tests/esbuild/import_star/import_star_export_star_unused/artifacts.snap

@@ -8,17 +8,6 @@ input_file: crates/rolldown/tests/esbuild/import_star/import_star_export_star_un
 ## entry_js.mjs
 
 ```js
-import { __export } from "./$runtime$.mjs";
-
-// foo.js
-const foo$1 = 123;
-
-// bar.js
-var bar_ns = {};
-__export(bar_ns, {
-	foo:() => foo$1
-});
-
 // entry.js
 let foo = 234;
 console.log(foo);

crates/rolldown/tests/esbuild/import_star/import_star_unused/artifacts.snap

@@ -8,15 +8,6 @@ input_file: crates/rolldown/tests/esbuild/import_star/import_star_unused
 ## entry_js.mjs
 
 ```js
-import { __export } from "./$runtime$.mjs";
-
-// foo.js
-var foo_ns = {};
-__export(foo_ns, {
-	foo:() => foo$1
-});
-const foo$1 = 123;
-
 // entry.js
 let foo = 234;
 console.log(foo);

crates/rolldown/tests/esbuild/import_star/other_file_import_export_self_as_namespace_unused_es6/artifacts.snap

@@ -8,14 +8,7 @@ input_file: crates/rolldown/tests/esbuild/import_star/other_file_import_export_s
 ## entry_js.mjs
 
 ```js
-import { __export } from "./$runtime$.mjs";
-
 // foo.js
-var foo_ns = {};
-__export(foo_ns, {
-	foo:() => foo,
-	ns:() => foo_ns
-});
 const foo = 123;
 
 export { foo };

crates/rolldown/tests/fixtures/basic/artifacts.snap

@@ -9,11 +9,11 @@ input_file: crates/rolldown/tests/fixtures/basic
 
 ```js
 // shared.js
-const a = 'shared.js';
+const a$1 = 'shared.js';
 
 // main.js
-const a$1 = 'index.js';
-console.log(a$1, a);
+const a = 'index.js';
+console.log(a, a$1);
 
 //# sourceMappingURL=main.mjs.map
 ```
@@ -22,14 +22,14 @@ console.log(a$1, a);
 
 - ../shared.js
 (0:0-0:6) "const " --> (2:0-2:6) "\nconst"
-(0:6-0:10) "a = " --> (2:6-2:10) " a ="
-(0:10-2:13) "'shared.js'\n\nexport { a }" --> (2:10-5:0) " 'shared.js';\n\n// main.js"
+(0:6-0:10) "a = " --> (2:6-2:12) " a$1 ="
+(0:10-2:13) "'shared.js'\n\nexport { a }" --> (2:12-5:0) " 'shared.js';\n\n// main.js"
 - ../main.js
 (1:0-1:6) "\nconst" --> (5:0-5:6) "\nconst"
-(1:6-1:10) " a =" --> (5:6-5:12) " a$1 ="
-(1:10-2:0) " 'index.js'" --> (5:12-6:0) " 'index.js';"
+(1:6-1:10) " a =" --> (5:6-5:10) " a ="
+(1:10-2:0) " 'index.js'" --> (5:10-6:0) " 'index.js';"
 (2:0-2:8) "\nconsole" --> (6:0-6:8) "\nconsole"
 (2:8-2:12) ".log" --> (6:8-6:12) ".log"
-(2:12-2:15) "(a," --> (6:12-6:17) "(a$1,"
-(2:15-2:18) " a2" --> (6:17-6:19) " a"
+(2:12-2:15) "(a," --> (6:12-6:15) "(a,"
+(2:15-2:18) " a2" --> (6:15-6:19) " a$1"
 (2:18-3:1) ")\n" --> (6:19-8:34) ");\n\n//# sourceMappingURL=main.mjs.map"

crates/rolldown/tests/fixtures/basic_re_export/artifacts.snap

@@ -9,9 +9,9 @@ input_file: crates/rolldown/tests/fixtures/basic_re_export
 
 ```js
 // a.js
-const a = 'a.js';
+const a$1 = 'a.js';
 
 // main.js
-const a$1 = 'index.js';
-console.log(a$1, a);
+const a = 'index.js';
+console.log(a, a$1);
 ```

crates/rolldown/tests/fixtures/deconflict/basic/artifacts.snap

@@ -11,10 +11,10 @@ input_file: crates/rolldown/tests/fixtures/deconflict/basic
 import { default as assert } from "assert";
 
 // a.js
-const a = 'a.js';
+const a$1 = 'a.js';
 
 // main.js
-const a$1 = 'main.js';
-assert.equal(a$1, 'main.js');
-assert.equal(a, 'a.js');
+const a = 'main.js';
+assert.equal(a, 'main.js');
+assert.equal(a$1, 'a.js');
 ```

crates/rolldown/tests/fixtures/deconflict/basic_scoped/artifacts.snap

@@ -11,12 +11,12 @@ input_file: crates/rolldown/tests/fixtures/deconflict/basic_scoped
 import { default as assert } from "assert";
 
 // a.js
-const a = 'a.js';
+const a$1 = 'a.js';
 
 // main.js
-const a$1 = 'main.js';
+const a = 'main.js';
 function foo(a$1$1) {
-	return [a$1$1, a$1, a];
+	return [a$1$1, a, a$1];
 }
 assert.deepEqual(foo('foo'), ['foo', 'main.js', 'a.js']);
 
@@ -27,21 +27,21 @@ assert.deepEqual(foo('foo'), ['foo', 'main.js', 'a.js']);
 
 - ../a.js
 (0:0-0:6) "const " --> (3:0-3:6) "\nconst"
-(0:6-0:10) "a = " --> (3:6-3:10) " a ="
-(0:10-3:1) "'a.js'\n\nexport { a }\n" --> (3:10-6:0) " 'a.js';\n\n// main.js"
+(0:6-0:10) "a = " --> (3:6-3:12) " a$1 ="
+(0:10-3:1) "'a.js'\n\nexport { a }\n" --> (3:12-6:0) " 'a.js';\n\n// main.js"
 - ../main.js
 (2:0-2:6) "\nconst" --> (6:0-6:6) "\nconst"
-(2:6-2:10) " a =" --> (6:6-6:12) " a$1 ="
-(2:10-5:0) " 'main.js'\n\n" --> (6:12-7:0) " 'main.js';"
+(2:6-2:10) " a =" --> (6:6-6:10) " a ="
+(2:10-5:0) " 'main.js'\n\n" --> (6:10-7:0) " 'main.js';"
 (5:0-5:9) "\nfunction" --> (7:0-7:9) "\nfunction"
 (5:9-5:13) " foo" --> (7:9-7:13) " foo"
 (5:13-5:18) "(a$1)" --> (7:13-7:20) "(a$1$1)"
 (5:18-6:2) " {\n " --> (7:20-8:0) " {"
 (6:2-6:9) " return" --> (8:0-8:8) "\n\treturn"
 (6:9-6:10) " " --> (8:8-8:9) " "
 (6:10-6:15) "[a$1," --> (8:9-8:16) "[a$1$1,"
-(6:15-6:18) " a," --> (8:16-8:21) " a$1,"
-(6:18-6:22) " aJs" --> (8:21-8:23) " a"
+(6:15-6:18) " a," --> (8:16-8:19) " a,"
+(6:18-6:22) " aJs" --> (8:19-8:23) " a$1"
 (6:22-7:1) "]\n" --> (8:23-9:0) "];"
 (7:1-9:0) "}\n" --> (9:0-10:0) "\n}"
 (9:0-9:7) "\nassert" --> (10:0-10:7) "\nassert"