feat(transformer): enable ArrowFunctionConverter in async-to-generator and async-generator-functions plugins (#7113)

Part of https://github.com/oxc-project/oxc/pull/7074

In async-to-generator and async-generator-functions plugins, we may need to transform the async arrow function to a regular generator function, now we can reuse the ability of the ArrowFunction plugin by enabling `ArrowFunctionConverter`.

I will fix semantic errors in the follow-up PR

crates/oxc_transformer/src/common/arrow_function_converter.rs

@@ -97,6 +97,8 @@ use oxc_syntax::{
 };
 use oxc_traverse::{Ancestor, BoundIdentifier, Traverse, TraverseCtx};
 
+use crate::TransformOptions;
+
 /// Mode for arrow function conversion
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum ArrowFunctionConverterMode {
@@ -107,23 +109,27 @@ pub enum ArrowFunctionConverterMode {
     Enabled,
 
     /// Only convert async arrow functions
-    #[expect(unused)]
     AsyncOnly,
 }
 
-pub struct ArrowFunctionConverterOptions {
-    pub mode: ArrowFunctionConverterMode,
-}
-
 pub struct ArrowFunctionConverter<'a> {
     mode: ArrowFunctionConverterMode,
     this_var_stack: SparseStack<BoundIdentifier<'a>>,
 }
 
 impl<'a> ArrowFunctionConverter<'a> {
-    pub fn new(options: &ArrowFunctionConverterOptions) -> Self {
+    pub fn new(options: &TransformOptions) -> Self {
+        let mode = if options.env.es2015.arrow_function.is_some() {
+            ArrowFunctionConverterMode::Enabled
+        } else if options.env.es2017.async_to_generator
+            || options.env.es2018.async_generator_functions
+        {
+            ArrowFunctionConverterMode::AsyncOnly
+        } else {
+            ArrowFunctionConverterMode::Disabled
+        };
         // `SparseStack` is created with 1 empty entry, for `Program`
-        Self { mode: options.mode, this_var_stack: SparseStack::new() }
+        Self { mode, this_var_stack: SparseStack::new() }
     }
 }
 
@@ -254,7 +260,11 @@ impl<'a> Traverse<'a> for ArrowFunctionConverter<'a> {
             return;
         }
 
-        if let Expression::ArrowFunctionExpression(_) = expr {
+        if let Expression::ArrowFunctionExpression(arrow_function_expr) = expr {
+            if self.is_async_only() && !arrow_function_expr.r#async {
+                return;
+            }
+
             let Expression::ArrowFunctionExpression(arrow_function_expr) =
                 ctx.ast.move_expression(expr)
             else {
@@ -272,13 +282,18 @@ impl<'a> ArrowFunctionConverter<'a> {
         self.mode == ArrowFunctionConverterMode::Disabled
     }
 
+    /// Check if arrow function conversion has enabled for transform async arrow functions
+    fn is_async_only(&self) -> bool {
+        self.mode == ArrowFunctionConverterMode::AsyncOnly
+    }
+
     fn get_this_identifier(
         &mut self,
         span: Span,
         ctx: &mut TraverseCtx<'a>,
     ) -> Option<ArenaBox<'a, IdentifierReference<'a>>> {
         // Find arrow function we are currently in (if we are)
-        let arrow_scope_id = Self::get_arrow_function_scope(ctx)?;
+        let arrow_scope_id = self.get_arrow_function_scope(ctx)?;
 
         // TODO(improve-on-babel): We create a new UID for every scope. This is pointless, as only one
         // `this` can be in scope at a time. We could create a single `_this` UID and reuse it in each
@@ -304,7 +319,7 @@ impl<'a> ArrowFunctionConverter<'a> {
 
     /// Find arrow function we are currently in, if it's between current node, and where `this` is bound.
     /// Return its `ScopeId`.
-    fn get_arrow_function_scope(ctx: &mut TraverseCtx<'a>) -> Option<ScopeId> {
+    fn get_arrow_function_scope(&self, ctx: &mut TraverseCtx<'a>) -> Option<ScopeId> {
         // `this` inside a class resolves to `this` *outside* the class in:
         // * `extends` clause
         // * Computed method key
@@ -346,13 +361,13 @@ impl<'a> ArrowFunctionConverter<'a> {
         // ```
         //
         // So in this loop, we only exit when we encounter one of the above.
-        for ancestor in ctx.ancestors() {
+        let mut ancestors = ctx.ancestors();
+        while let Some(ancestor) = ancestors.next() {
             match ancestor {
                 // Top level
                 Ancestor::ProgramBody(_)
                 // Function (includes class method body)
                 | Ancestor::FunctionParams(_)
-                | Ancestor::FunctionBody(_)
                 // Class property body
                 | Ancestor::PropertyDefinitionValue(_)
                 // Class accessor property body
@@ -361,10 +376,29 @@ impl<'a> ArrowFunctionConverter<'a> {
                 | Ancestor::StaticBlockBody(_) => return None,
                 // Arrow function
                 Ancestor::ArrowFunctionExpressionParams(func) => {
-                    return Some(func.scope_id().get().unwrap())
+                    return if self.is_async_only() && !*func.r#async() {
+                        None
+                    } else {
+                        Some(func.scope_id().get().unwrap())
+                    }
                 }
                 Ancestor::ArrowFunctionExpressionBody(func) => {
-                    return Some(func.scope_id().get().unwrap())
+                    return if self.is_async_only() && !*func.r#async() {
+                        None
+                    } else {
+                        Some(func.scope_id().get().unwrap())
+                    }
+                }
+                Ancestor::FunctionBody(func) => {
+                    return if self.is_async_only() && *func.r#async()
+                    && matches!(
+                        ancestors.next().unwrap(),
+                        Ancestor::MethodDefinitionValue(_) | Ancestor::ObjectPropertyValue(_)
+                    ) {
+                        Some(func.scope_id().get().unwrap())
+                    } else {
+                        None
+                    }
                 }
                 _ => {}
             }

crates/oxc_transformer/src/common/mod.rs

@@ -1,8 +1,6 @@
 //! Utility transforms which are in common between other transforms.
 
-use arrow_function_converter::{
+use arrow_function_converter::ArrowFunctionConverter;
-    ArrowFunctionConverter, ArrowFunctionConverterMode, ArrowFunctionConverterOptions,
-};
 use oxc_allocator::Vec as ArenaVec;
 use oxc_ast::ast::*;
 use oxc_traverse::{Traverse, TraverseCtx};
@@ -31,23 +29,12 @@ pub struct Common<'a, 'ctx> {
 
 impl<'a, 'ctx> Common<'a, 'ctx> {
     pub fn new(options: &TransformOptions, ctx: &'ctx TransformCtx<'a>) -> Self {
-        let arrow_function_converter_options = {
-            let mode = if options.env.es2015.arrow_function.is_some() {
-                ArrowFunctionConverterMode::Enabled
-            } else {
-                ArrowFunctionConverterMode::Disabled
-            };
-            ArrowFunctionConverterOptions { mode }
-        };
-
         Self {
             module_imports: ModuleImports::new(ctx),
             var_declarations: VarDeclarations::new(ctx),
             statement_injector: StatementInjector::new(ctx),
             top_level_statements: TopLevelStatements::new(ctx),
-            arrow_function_converter: ArrowFunctionConverter::new(
+            arrow_function_converter: ArrowFunctionConverter::new(options),
-                &arrow_function_converter_options,
-            ),
         }
     }
 }

crates/oxc_transformer/src/es2017/async_to_generator.rs

@@ -131,7 +131,14 @@ impl<'a, 'ctx> Traverse<'a> for AsyncToGenerator<'a, 'ctx> {
     }
 
     fn exit_function(&mut self, func: &mut Function<'a>, ctx: &mut TraverseCtx<'a>) {
-        if func.r#async && matches!(ctx.parent(), Ancestor::MethodDefinitionValue(_)) {
+        if func.r#async
+            && !func.is_typescript_syntax()
+            && matches!(
+                ctx.parent(),
+                // `class A { async foo() {} }` | `({ async foo() {} })`
+                Ancestor::MethodDefinitionValue(_) | Ancestor::PropertyDefinitionValue(_)
+            )
+        {
             self.executor.transform_function_for_method_definition(func, ctx);
         }
     }

crates/oxc_transformer/src/es2018/async_generator_functions/mod.rs

@@ -172,7 +172,11 @@ impl<'a, 'ctx> Traverse<'a> for AsyncGeneratorFunctions<'a, 'ctx> {
         if func.r#async
             && func.generator
             && !func.is_typescript_syntax()
-            && matches!(ctx.parent(), Ancestor::MethodDefinitionValue(_))
+            && matches!(
+                ctx.parent(),
+                // `class A { async foo() {} }` | `({ async foo() {} })`
+                Ancestor::MethodDefinitionValue(_) | Ancestor::ObjectPropertyValue(_)
+            )
         {
             self.executor.transform_function_for_method_definition(func, ctx);
         }

tasks/transform_conformance/snapshots/babel.snap.md

@@ -1452,13 +1452,37 @@ x Output mismatch
 
 # babel-plugin-transform-async-generator-functions (15/19)
 * async-generators/class-method/input.js
-x Output mismatch
+Bindings mismatch:
+after transform: ScopeId(0): ["C", "_this"]
+rebuilt        : ScopeId(0): ["C"]
+Bindings mismatch:
+after transform: ScopeId(3): []
+rebuilt        : ScopeId(2): ["_this"]
+Symbol scope ID mismatch for "_this":
+after transform: SymbolId(1): ScopeId(0)
+rebuilt        : SymbolId(1): ScopeId(2)
 
 * async-generators/object-method/input.js
-x Output mismatch
+Bindings mismatch:
+after transform: ScopeId(0): ["_this"]
+rebuilt        : ScopeId(0): []
+Bindings mismatch:
+after transform: ScopeId(2): []
+rebuilt        : ScopeId(1): ["_this"]
+Symbol scope ID mismatch for "_this":
+after transform: SymbolId(0): ScopeId(0)
+rebuilt        : SymbolId(0): ScopeId(1)
 
 * async-generators/static-method/input.js
-x Output mismatch
+Bindings mismatch:
+after transform: ScopeId(0): ["C", "_this"]
+rebuilt        : ScopeId(0): ["C"]
+Bindings mismatch:
+after transform: ScopeId(3): []
+rebuilt        : ScopeId(2): ["_this"]
+Symbol scope ID mismatch for "_this":
+after transform: SymbolId(1): ScopeId(0)
+rebuilt        : SymbolId(1): ScopeId(2)
 
 * nested/arrows-in-declaration/input.js
 x Output mismatch

tasks/transform_conformance/snapshots/babel_exec.snap.md

@@ -234,33 +234,3 @@ AssertionError: expected false to be true // Object.is equality
 
 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯[14/14]⎯
 
-
-⎯⎯⎯⎯ Unhandled Rejection ⎯⎯⎯⎯⎯
-AssertionError: expected [Function Object] to be [Function Bar] // Object.is equality
-
-- Expected
-+ Received
-
-- [Function Bar]
-+ [Function Object]
-
- ❯ fixtures/babel-preset-env-test-fixtures-plugins-integration-regression-7064-exec.test.js:13:30
-     11|    }).call(this);
-     12|    _asyncToGenerator(function* () {
-     13|     expect(this.constructor).toBe(Bar);
-       |                              ^
-     14|    })();
-     15|    _asyncToGenerator(function* () {
- ❯ asyncGeneratorStep ../../node_modules/.pnpm/@babel+runtime@7.26.0/node_modules/@babel/runtime/helpers/asyncToGenerator.js:3:17
- ❯ _next ../../node_modules/.pnpm/@babel+runtime@7.26.0/node_modules/@babel/runtime/helpers/asyncToGenerator.js:17:9
- ❯ ../../node_modules/.pnpm/@babel+runtime@7.26.0/node_modules/@babel/runtime/helpers/asyncToGenerator.js:22:7
- ❯ ../../node_modules/.pnpm/@babel+runtime@7.26.0/node_modules/@babel/runtime/helpers/asyncToGenerator.js:14:12
- ❯ Bar.test fixtures/babel-preset-env-test-fixtures-plugins-integration-regression-7064-exec.test.js:14:6
- ❯ fixtures/babel-preset-env-test-fixtures-plugins-integration-regression-7064-exec.test.js:20:12
- ❯ ../../node_modules/.pnpm/@vitest+runner@2.1.2/node_modules/@vitest/runner/dist/index.js:146:14
-
-This error originated in "fixtures/babel-preset-env-test-fixtures-plugins-integration-regression-7064-exec.test.js" test file. It doesn't mean the error was thrown inside the file itself, but while it was running.
-The latest test that might've caused the error is "fixtures/babel-preset-env-test-fixtures-plugins-integration-regression-7064-exec.test.js". It might mean one of the following:
-- The error was thrown, while Vitest was running this test.
-- If the error occurred after the test had been completed, this was the last documented test before it was thrown.
-