Implement async functions using generators (#2821)

This should hopefully fix more async/futures issues related to resuming execution in the future, since we can leverage generator logic to handle this for us.

It changes the following:

- Refactors `GeneratorContext` to handle context preparation.
- Reuses the functionality of `GeneratorContext` in `Await`.
- Removes `EarlyReturnType` in favour of a single `r#await` bool flag in `CallFrame`.

boa_cli/src/debug/function.rs

@@ -159,10 +159,26 @@ fn trace(_: &JsValue, args: &[JsValue], context: &mut Context<'_>) -> JsResult<J
     result
 }
 
+fn traceable(_: &JsValue, args: &[JsValue], _: &mut Context<'_>) -> JsResult<JsValue> {
+    let value = args.get_or_undefined(0);
+    let traceable = args.get_or_undefined(1).to_boolean();
+
+    let Some(callable) = value.as_callable() else {
+        return Err(JsNativeError::typ()
+        .with_message("expected callable object")
+        .into());
+    };
+
+    set_trace_flag_in_function_object(callable, traceable)?;
+
+    Ok(value.clone())
+}
+
 pub(super) fn create_object(context: &mut Context<'_>) -> JsObject {
     ObjectInitializer::new(context)
         .function(NativeFunction::from_fn_ptr(flowgraph), "flowgraph", 1)
         .function(NativeFunction::from_fn_ptr(bytecode), "bytecode", 1)
         .function(NativeFunction::from_fn_ptr(trace), "trace", 1)
+        .function(NativeFunction::from_fn_ptr(traceable), "traceable", 2)
         .build()
 }

boa_engine/src/builtins/async_function/mod.rs

@@ -70,7 +70,7 @@ impl BuiltInConstructor for AsyncFunction {
             context
                 .intrinsics()
                 .constructors()
-                .generator_function()
+                .async_function()
                 .constructor()
         });
         BuiltInFunctionObject::create_dynamic_function(

boa_engine/src/builtins/async_generator/mod.rs

@@ -490,49 +490,22 @@ impl AsyncGenerator {
             .expect("already checked before")
             .state = AsyncGeneratorState::Executing;
 
+        let (value, resume_kind) = match completion {
+            (Ok(value), r#return) => (
+                value,
+                if r#return {
+                    GeneratorResumeKind::Return
+                } else {
+                    GeneratorResumeKind::Normal
+                },
+            ),
+            (Err(value), _) => (value.to_opaque(context), GeneratorResumeKind::Throw),
+        };
         // 6. Push genContext onto the execution context stack; genContext is now the running execution context.
-        std::mem::swap(
-            &mut context.vm.environments,
-            &mut generator_context.environments,
-        );
-        std::mem::swap(&mut context.vm.stack, &mut generator_context.stack);
-        std::mem::swap(
-            &mut context.vm.active_function,
-            &mut generator_context.active_function,
-        );
-        let old_realm = context.enter_realm(generator_context.realm.clone());
 
-        context.vm.push_frame(generator_context.call_frame.clone());
+        let result = generator_context.resume(Some(value), resume_kind, context);
 
         // 7. Resume the suspended evaluation of genContext using completion as the result of the operation that suspended it. Let result be the Completion Record returned by the resumed computation.
-        match completion {
-            (Ok(value), r#return) => {
-                context.vm.push(value);
-                if r#return {
-                    context.vm.frame_mut().generator_resume_kind = GeneratorResumeKind::Return;
-                } else {
-                    context.vm.frame_mut().generator_resume_kind = GeneratorResumeKind::Normal;
-                }
-            }
-            (Err(value), _) => {
-                let value = value.to_opaque(context);
-                context.vm.push(value);
-                context.vm.frame_mut().generator_resume_kind = GeneratorResumeKind::Throw;
-            }
-        }
-        let result = context.run();
-
-        std::mem::swap(
-            &mut context.vm.environments,
-            &mut generator_context.environments,
-        );
-        std::mem::swap(&mut context.vm.stack, &mut generator_context.stack);
-        generator_context.call_frame = context.vm.pop_frame().expect("generator frame must exist");
-        std::mem::swap(
-            &mut context.vm.active_function,
-            &mut generator_context.active_function,
-        );
-        context.enter_realm(old_realm);
 
         generator
             .borrow_mut()
@@ -543,7 +516,8 @@ impl AsyncGenerator {
         // 8. Assert: result is never an abrupt completion.
         assert!(!result.is_throw_completion());
 
-        // 9. Assert: When we return here, genContext has already been removed from the execution context stack and callerContext is the currently running execution context.
+        // 9. Assert: When we return here, genContext has already been removed from the execution context stack and
+        // callerContext is the currently running execution context.
         // 10. Return unused.
     }
 

boa_engine/src/builtins/function/mod.rs

@@ -416,18 +416,6 @@ impl Function {
         }
     }
 
-    /// Returns the promise capability if the function is an async function.
-    pub(crate) const fn get_promise_capability(&self) -> Option<&PromiseCapability> {
-        if let FunctionKind::Async {
-            promise_capability, ..
-        } = &self.kind
-        {
-            Some(promise_capability)
-        } else {
-            None
-        }
-    }
-
     ///  Sets the class object.
     pub(crate) fn set_class_object(&mut self, object: JsObject) {
         match &mut self.kind {

boa_engine/src/builtins/generator/mod.rs

@@ -44,12 +44,72 @@ pub(crate) enum GeneratorState {
 #[derive(Debug, Clone, Finalize, Trace)]
 pub(crate) struct GeneratorContext {
     pub(crate) environments: DeclarativeEnvironmentStack,
-    pub(crate) call_frame: CallFrame,
     pub(crate) stack: Vec<JsValue>,
     pub(crate) active_function: Option<JsObject>,
+    pub(crate) call_frame: Option<CallFrame>,
     pub(crate) realm: Realm,
 }
 
+impl GeneratorContext {
+    /// Creates a new `GeneratorContext` from the raw `Context` state components.
+    pub(crate) fn new(
+        environments: DeclarativeEnvironmentStack,
+        stack: Vec<JsValue>,
+        active_function: Option<JsObject>,
+        call_frame: CallFrame,
+        realm: Realm,
+    ) -> Self {
+        Self {
+            environments,
+            stack,
+            active_function,
+            call_frame: Some(call_frame),
+            realm,
+        }
+    }
+
+    /// Creates a new `GeneratorContext` from the current `Context` state.
+    pub(crate) fn from_current(context: &mut Context<'_>) -> Self {
+        Self {
+            environments: context.vm.environments.clone(),
+            call_frame: Some(context.vm.frame().clone()),
+            stack: context.vm.stack.clone(),
+            active_function: context.vm.active_function.clone(),
+            realm: context.realm().clone(),
+        }
+    }
+
+    /// Resumes execution with `GeneratorContext` as the current execution context.
+    pub(crate) fn resume(
+        &mut self,
+        value: Option<JsValue>,
+        resume_kind: GeneratorResumeKind,
+        context: &mut Context<'_>,
+    ) -> CompletionRecord {
+        std::mem::swap(&mut context.vm.environments, &mut self.environments);
+        std::mem::swap(&mut context.vm.stack, &mut self.stack);
+        std::mem::swap(&mut context.vm.active_function, &mut self.active_function);
+        context.swap_realm(&mut self.realm);
+        context
+            .vm
+            .push_frame(self.call_frame.take().expect("should have a call frame"));
+        context.vm.frame_mut().generator_resume_kind = resume_kind;
+        if let Some(value) = value {
+            context.vm.push(value);
+        }
+
+        let result = context.run();
+
+        std::mem::swap(&mut context.vm.environments, &mut self.environments);
+        std::mem::swap(&mut context.vm.stack, &mut self.stack);
+        std::mem::swap(&mut context.vm.active_function, &mut self.active_function);
+        context.swap_realm(&mut self.realm);
+        self.call_frame = context.vm.pop_frame();
+        assert!(self.call_frame.is_some());
+        result
+    }
+}
+
 /// The internal representation of a `Generator` object.
 #[derive(Debug, Finalize, Trace)]
 pub struct Generator {
@@ -117,15 +177,19 @@ impl Generator {
         args: &[JsValue],
         context: &mut Context<'_>,
     ) -> JsResult<JsValue> {
+        // Extracted `GeneratorValidate` since we don't need to worry about validation
+        // of async functions.
+        let generator = Self::generator_validate(this)?;
+
         // 1. Return ? GeneratorResume(this value, value, empty).
-        this.as_object().map_or_else(
-            || {
-                Err(JsNativeError::typ()
-                    .with_message("Generator.prototype.next called on non generator")
-                    .into())
-            },
-            |obj| Self::generator_resume(obj, args.get_or_undefined(0), context),
-        )
+        let completion =
+            Self::generator_resume(generator, args.get_or_undefined(0).clone(), context);
+
+        match completion {
+            CompletionRecord::Return(value) => Ok(create_iter_result_object(value, false, context)),
+            CompletionRecord::Normal(value) => Ok(create_iter_result_object(value, true, context)),
+            CompletionRecord::Throw(err) => Err(err),
+        }
     }
 
     /// `Generator.prototype.return ( value )`
@@ -144,9 +208,17 @@ impl Generator {
         context: &mut Context<'_>,
     ) -> JsResult<JsValue> {
         // 1. Let g be the this value.
+        let generator = Self::generator_validate(this)?;
         // 2. Let C be Completion { [[Type]]: return, [[Value]]: value, [[Target]]: empty }.
         // 3. Return ? GeneratorResumeAbrupt(g, C, empty).
-        Self::generator_resume_abrupt(this, Ok(args.get_or_undefined(0).clone()), context)
+        let completion =
+            Self::generator_resume_abrupt(generator, Ok(args.get_or_undefined(0).clone()), context);
+
+        match completion {
+            CompletionRecord::Return(value) => Ok(create_iter_result_object(value, false, context)),
+            CompletionRecord::Normal(value) => Ok(create_iter_result_object(value, true, context)),
+            CompletionRecord::Throw(err) => Err(err),
+        }
     }
 
     /// `Generator.prototype.throw ( exception )`
@@ -166,46 +238,67 @@ impl Generator {
         context: &mut Context<'_>,
     ) -> JsResult<JsValue> {
         // 1. Let g be the this value.
+        let generator = Self::generator_validate(this)?;
         // 2. Let C be ThrowCompletion(exception).
         // 3. Return ? GeneratorResumeAbrupt(g, C, empty).
-        Self::generator_resume_abrupt(
-            this,
+        let completion = Self::generator_resume_abrupt(
+            generator,
             Err(JsError::from_opaque(args.get_or_undefined(0).clone())),
             context,
-        )
+        );
+
+        match completion {
+            CompletionRecord::Return(value) => Ok(create_iter_result_object(value, false, context)),
+            CompletionRecord::Normal(value) => Ok(create_iter_result_object(value, true, context)),
+            CompletionRecord::Throw(err) => Err(err),
+        }
     }
 
-    /// `27.5.3.3 GeneratorResume ( generator, value, generatorBrand )`
+    /// `27.5.3.3 GeneratorValidate ( generator, generatorBrand )`
     ///
     /// More information:
     ///  - [ECMAScript reference][spec]
     ///
-    /// [spec]: https://tc39.es/ecma262/#sec-generatorresume
-    pub(crate) fn generator_resume(
-        generator_obj: &JsObject,
-        value: &JsValue,
-        context: &mut Context<'_>,
-    ) -> JsResult<JsValue> {
-        // 1. Let state be ? GeneratorValidate(generator, generatorBrand).
+    /// [spec]:https://tc39.es/ecma262/#sec-generatorvalidate
+    fn generator_validate(gen: &JsValue) -> JsResult<&JsObject> {
+        let generator_obj = gen.as_object().ok_or_else(|| {
+            JsNativeError::typ().with_message("Generator method called on non generator")
+        })?;
         let mut generator_obj_mut = generator_obj.borrow_mut();
         let generator = generator_obj_mut.as_generator_mut().ok_or_else(|| {
             JsNativeError::typ().with_message("generator resumed on non generator object")
         })?;
-        let state = generator.state;
 
-        if state == GeneratorState::Executing {
-            return Err(JsNativeError::typ()
+        if generator.state == GeneratorState::Executing {
+            Err(JsNativeError::typ()
                 .with_message("Generator should not be executing")
-                .into());
+                .into())
+        } else {
+            Ok(generator_obj)
+        }
     }
 
+    /// `27.5.3.3 GeneratorResume ( generator, value, generatorBrand )`
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-generatorresume
+    pub(crate) fn generator_resume(
+        gen: &JsObject,
+        value: JsValue,
+        context: &mut Context<'_>,
+    ) -> CompletionRecord {
+        // 1. Let state be ? GeneratorValidate(generator, generatorBrand).
+        let mut generator_obj_mut = gen.borrow_mut();
+        let generator = generator_obj_mut
+            .as_generator_mut()
+            .expect("already validated that this is a generator");
+        let state = generator.state;
+
         // 2. If state is completed, return CreateIterResultObject(undefined, true).
         if state == GeneratorState::Completed {
-            return Ok(create_iter_result_object(
-                JsValue::undefined(),
-                true,
-                context,
-            ));
+            return CompletionRecord::Normal(JsValue::undefined());
         }
 
         // 3. Assert: state is either suspendedStart or suspendedYield.
@@ -227,64 +320,30 @@ impl Generator {
             .expect("generator context cannot be empty here");
         drop(generator_obj_mut);
 
-        std::mem::swap(
-            &mut context.vm.environments,
-            &mut generator_context.environments,
-        );
-        std::mem::swap(&mut context.vm.stack, &mut generator_context.stack);
-        std::mem::swap(
-            &mut context.vm.active_function,
-            &mut generator_context.active_function,
-        );
-        let old_realm = context.enter_realm(generator_context.realm.clone());
-        context.vm.push_frame(generator_context.call_frame.clone());
-        if !first_execution {
-            context.vm.push(value.clone());
-        }
-
-        context.vm.frame_mut().generator_resume_kind = GeneratorResumeKind::Normal;
-
-        let record = context.run();
-
-        generator_context.call_frame = context
-            .vm
-            .pop_frame()
-            .expect("generator call frame must exist");
-        std::mem::swap(
-            &mut context.vm.environments,
-            &mut generator_context.environments,
-        );
-        std::mem::swap(&mut context.vm.stack, &mut generator_context.stack);
-        std::mem::swap(
-            &mut context.vm.active_function,
-            &mut generator_context.active_function,
+        let record = generator_context.resume(
+            (!first_execution).then_some(value),
+            GeneratorResumeKind::Normal,
+            context,
         );
-        context.enter_realm(old_realm);
 
-        let mut generator_obj_mut = generator_obj.borrow_mut();
+        let mut generator_obj_mut = gen.borrow_mut();
         let generator = generator_obj_mut
             .as_generator_mut()
             .expect("already checked this object type");
 
-        match record {
-            CompletionRecord::Return(value) => {
-                generator.state = GeneratorState::SuspendedYield;
+        generator.state = match record {
+            CompletionRecord::Return(_) => {
                 generator.context = Some(generator_context);
-                Ok(create_iter_result_object(value, false, context))
-            }
-            CompletionRecord::Normal(value) => {
-                generator.state = GeneratorState::Completed;
-                Ok(create_iter_result_object(value, true, context))
-            }
-            CompletionRecord::Throw(err) => {
-                generator.state = GeneratorState::Completed;
-                Err(err)
-            }
+                GeneratorState::SuspendedYield
             }
+            CompletionRecord::Normal(_) | CompletionRecord::Throw(_) => GeneratorState::Completed,
+        };
+
         // 8. Push genContext onto the execution context stack; genContext is now the running execution context.
         // 9. Resume the suspended evaluation of genContext using NormalCompletion(value) as the result of the operation that suspended it. Let result be the value returned by the resumed computation.
         // 10. Assert: When we return here, genContext has already been removed from the execution context stack and methodContext is the currently running execution context.
         // 11. Return Completion(result).
+        record
     }
 
     /// `27.5.3.4 GeneratorResumeAbrupt ( generator, abruptCompletion, generatorBrand )`
@@ -294,26 +353,17 @@ impl Generator {
     ///
     /// [spec]: https://tc39.es/ecma262/#sec-generatorresumeabrupt
     pub(crate) fn generator_resume_abrupt(
-        this: &JsValue,
+        gen: &JsObject,
         abrupt_completion: JsResult<JsValue>,
         context: &mut Context<'_>,
-    ) -> JsResult<JsValue> {
+    ) -> CompletionRecord {
         // 1. Let state be ? GeneratorValidate(generator, generatorBrand).
-        let generator_obj = this.as_object().ok_or_else(|| {
-            JsNativeError::typ().with_message("generator resumed on non generator object")
-        })?;
-        let mut generator_obj_mut = generator_obj.borrow_mut();
-        let generator = generator_obj_mut.as_generator_mut().ok_or_else(|| {
-            JsNativeError::typ().with_message("generator resumed on non generator object")
-        })?;
+        let mut generator_obj_mut = gen.borrow_mut();
+        let generator = generator_obj_mut
+            .as_generator_mut()
+            .expect("already validated the generator object");
         let mut state = generator.state;
 
-        if state == GeneratorState::Executing {
-            return Err(JsNativeError::typ()
-                .with_message("Generator should not be executing")
-                .into());
-        }
-
         // 2. If state is suspendedStart, then
         if state == GeneratorState::SuspendedStart {
             // a. Set generator.[[GeneratorState]] to completed.
@@ -327,12 +377,10 @@ impl Generator {
         // 3. If state is completed, then
         if state == GeneratorState::Completed {
             // a. If abruptCompletion.[[Type]] is return, then
-            if let Ok(value) = abrupt_completion {
             // i. Return CreateIterResultObject(abruptCompletion.[[Value]], true).
-                return Ok(create_iter_result_object(value, true, context));
-            }
             // b. Return Completion(abruptCompletion).
-            return abrupt_completion;
+            return abrupt_completion
+                .map_or_else(CompletionRecord::Throw, CompletionRecord::Normal);
         }
 
         // 4. Assert: state is suspendedYield.
@@ -352,65 +400,26 @@ impl Generator {
         generator.state = GeneratorState::Executing;
         drop(generator_obj_mut);
 
-        std::mem::swap(
-            &mut context.vm.environments,
-            &mut generator_context.environments,
-        );
-        std::mem::swap(&mut context.vm.stack, &mut generator_context.stack);
-        std::mem::swap(
-            &mut context.vm.active_function,
-            &mut generator_context.active_function,
-        );
-        let old_realm = context.enter_realm(generator_context.realm.clone());
-        context.vm.push_frame(generator_context.call_frame.clone());
-
-        let completion_record = match abrupt_completion {
-            Ok(value) => {
-                context.vm.push(value);
-                context.vm.frame_mut().generator_resume_kind = GeneratorResumeKind::Return;
-                context.run()
-            }
-            Err(value) => {
-                let value = value.to_opaque(context);
-                context.vm.push(value);
-                context.vm.frame_mut().generator_resume_kind = GeneratorResumeKind::Throw;
-                context.run()
-            }
+        let (value, resume_kind) = match abrupt_completion {
+            Ok(value) => (value, GeneratorResumeKind::Return),
+            Err(err) => (err.to_opaque(context), GeneratorResumeKind::Throw),
         };
-        generator_context.call_frame = context
-            .vm
-            .pop_frame()
-            .expect("generator call frame must exist");
-        std::mem::swap(
-            &mut context.vm.environments,
-            &mut generator_context.environments,
-        );
-        std::mem::swap(&mut context.vm.stack, &mut generator_context.stack);
-        std::mem::swap(
-            &mut context.vm.active_function,
-            &mut generator_context.active_function,
-        );
-        context.enter_realm(old_realm);
 
-        let mut generator_obj_mut = generator_obj.borrow_mut();
+        let record = generator_context.resume(Some(value), resume_kind, context);
+
+        let mut generator_obj_mut = gen.borrow_mut();
         let generator = generator_obj_mut
             .as_generator_mut()
             .expect("already checked this object type");
 
-        match completion_record {
-            CompletionRecord::Return(value) => {
-                generator.state = GeneratorState::SuspendedYield;
+        generator.state = match record {
+            CompletionRecord::Return(_) => {
                 generator.context = Some(generator_context);
-                Ok(create_iter_result_object(value, false, context))
-            }
-            CompletionRecord::Normal(value) => {
-                generator.state = GeneratorState::Completed;
-                Ok(create_iter_result_object(value, true, context))
-            }
-            CompletionRecord::Throw(err) => {
-                generator.state = GeneratorState::Completed;
-                Err(err)
-            }
+                GeneratorState::SuspendedYield
             }
+            CompletionRecord::Normal(_) | CompletionRecord::Throw(_) => GeneratorState::Completed,
+        };
+
+        record
     }
 }

boa_engine/src/context/mod.rs

@@ -536,6 +536,13 @@ impl<'host> Context<'host> {
 
 // ==== Private API ====
 
+impl Context<'_> {
+    /// Swaps the currently active realm with `realm`.
+    pub(crate) fn swap_realm(&mut self, realm: &mut Realm) {
+        std::mem::swap(&mut self.realm, realm);
+    }
+}
+
 #[cfg(feature = "intl")]
 impl<'host> Context<'host> {
     /// Get the ICU related utilities

boa_engine/src/environments/runtime.rs

@@ -552,26 +552,6 @@ impl DeclarativeEnvironmentStack {
             .expect("environment stack is cannot be empty")
     }
 
-    /// Get the most outer function environment slots.
-    ///
-    /// # Panics
-    ///
-    /// Panics if no environment exists on the stack.
-    pub(crate) fn current_function_slots(&self) -> &EnvironmentSlots {
-        for env in self
-            .stack
-            .iter()
-            .filter_map(Environment::as_declarative)
-            .rev()
-        {
-            if let Some(slots) = &env.slots {
-                return slots;
-            }
-        }
-
-        panic!("global environment must exist")
-    }
-
     /// Get the most outer environment.
     ///
     /// # Panics

boa_engine/src/vm/call_frame/mod.rs

@@ -5,7 +5,7 @@
 mod abrupt_record;
 mod env_stack;
 
-use crate::{object::JsObject, vm::CodeBlock};
+use crate::{builtins::promise::PromiseCapability, object::JsObject, vm::CodeBlock};
 use boa_gc::{Finalize, Gc, Trace};
 use thin_vec::ThinVec;
 
@@ -21,7 +21,7 @@ pub struct CallFrame {
     #[unsafe_ignore_trace]
     pub(crate) abrupt_completion: Option<AbruptCompletionRecord>,
     #[unsafe_ignore_trace]
-    pub(crate) early_return: Option<EarlyReturnType>,
+    pub(crate) r#yield: bool,
     pub(crate) pop_on_return: usize,
     // Tracks the number of environments in environment entry.
     // On abrupt returns this is used to decide how many environments need to be pop'ed.
@@ -31,6 +31,7 @@ pub struct CallFrame {
     pub(crate) arg_count: usize,
     #[unsafe_ignore_trace]
     pub(crate) generator_resume_kind: GeneratorResumeKind,
+    pub(crate) promise_capability: Option<PromiseCapability>,
 
     // When an async generator is resumed, the generator object is needed
     // to fulfill the steps 4.e-j in [AsyncGeneratorStart](https://tc39.es/ecma262/#sec-asyncgeneratorstart).
@@ -52,10 +53,11 @@ impl CallFrame {
             pop_on_return: 0,
             env_stack: Vec::from([EnvStackEntry::new(0, max_length)]),
             abrupt_completion: None,
-            early_return: None,
+            r#yield: false,
             param_count: 0,
             arg_count: 0,
             generator_resume_kind: GeneratorResumeKind::Normal,
+            promise_capability: None,
             async_generator: None,
             iterators: ThinVec::new(),
         }
@@ -107,10 +109,3 @@ pub(crate) enum GeneratorResumeKind {
     Throw,
     Return,
 }
-
-// An enum to mark whether a return is early due to Async or Yield
-#[derive(Copy, Clone, Debug, PartialEq)]
-pub(crate) enum EarlyReturnType {
-    Await,
-    Yield,
-}

boa_engine/src/vm/code_block.rs

@@ -824,7 +824,7 @@ impl JsObject {
 
         let context = &mut ContextCleanupGuard::new(context, realm, active_function);
 
-        let (code, mut environments, class_object, promise, async_, gen) =
+        let (code, mut environments, class_object, promise_cap, async_, gen) =
             match function_object.kind() {
                 FunctionKind::Native {
                     function,
@@ -874,7 +874,7 @@ impl JsObject {
                     code.clone(),
                     environments.clone(),
                     class_object.clone(),
-                    Some(promise_capability.promise().clone()),
+                    Some(promise_capability.clone()),
                     true,
                     false,
                 ),
@@ -1009,11 +1009,12 @@ impl JsObject {
 
         let param_count = code.params.as_ref().len();
 
-        context.vm.push_frame(
-            CallFrame::new(code)
+        let mut frame = CallFrame::new(code)
             .with_param_count(param_count)
-                .with_arg_count(arg_count),
-        );
+            .with_arg_count(arg_count);
+        frame.promise_capability = promise_cap.clone();
+
+        context.vm.push_frame(frame);
 
         let result = context
             .run()
@@ -1024,13 +1025,10 @@ impl JsObject {
         std::mem::swap(&mut environments, &mut context.vm.environments);
         std::mem::swap(&mut context.vm.stack, &mut stack);
 
-        if let Some(promise) = promise {
-            return Ok(promise.into());
-        }
-
-        let value = result?;
-
-        let value = if gen {
+        if let Some(promise_cap) = promise_cap {
+            Ok(promise_cap.promise().clone().into())
+        } else if gen {
+            result?;
             let proto = this_function_object
                 .get(PROTOTYPE, context)
                 .expect("generator must have a prototype property")
@@ -1045,34 +1043,32 @@ impl JsObject {
                     },
                     Clone::clone,
                 );
-
-            let generator = Self::from_proto_and_data(
-                proto,
-                if async_ {
+            let data = if async_ {
                 ObjectData::async_generator(AsyncGenerator {
                     state: AsyncGeneratorState::SuspendedStart,
-                        context: Some(GeneratorContext {
+                    context: Some(GeneratorContext::new(
                         environments,
-                            call_frame,
                         stack,
-                            active_function: context.vm.active_function.clone(),
-                            realm: context.realm().clone(),
-                        }),
+                        context.vm.active_function.clone(),
+                        call_frame,
+                        context.realm().clone(),
+                    )),
                     queue: VecDeque::new(),
                 })
             } else {
                 ObjectData::generator(Generator {
                     state: GeneratorState::SuspendedStart,
-                        context: Some(GeneratorContext {
+                    context: Some(GeneratorContext::new(
                         environments,
-                            call_frame,
                         stack,
-                            active_function: context.vm.active_function.clone(),
-                            realm: context.realm().clone(),
-                        }),
+                        context.vm.active_function.clone(),
+                        call_frame,
+                        context.realm().clone(),
+                    )),
                 })
-                },
-            );
+            };
+
+            let generator = Self::from_proto_and_data(proto, data);
 
             if async_ {
                 let gen_clone = generator.clone();
@@ -1081,15 +1077,18 @@ impl JsObject {
                     .as_async_generator_mut()
                     .expect("must be object here");
                 let gen_context = gen.context.as_mut().expect("must exist");
-                gen_context.call_frame.async_generator = Some(gen_clone);
+                // TODO: try to move this to the context itself.
+                gen_context
+                    .call_frame
+                    .as_mut()
+                    .expect("should have a call frame initialized")
+                    .async_generator = Some(gen_clone);
             }
 
-            generator.into()
+            Ok(generator.into())
         } else {
-            value
-        };
-
-        Ok(value)
+            result
+        }
     }
 
     pub(crate) fn construct_internal(

boa_engine/src/vm/mod.rs

@@ -7,7 +7,7 @@
 use crate::{
     builtins::async_generator::{AsyncGenerator, AsyncGeneratorState},
     environments::{DeclarativeEnvironment, DeclarativeEnvironmentStack},
-    vm::{call_frame::EarlyReturnType, code_block::Readable},
+    vm::code_block::Readable,
     Context, JsError, JsObject, JsResult, JsValue,
 };
 #[cfg(feature = "fuzz")]
@@ -98,6 +98,7 @@ impl Vm {
     /// # Panics
     ///
     /// If there is no frame, then this will panic.
+    #[track_caller]
     pub(crate) fn frame(&self) -> &CallFrame {
         self.frames.last().expect("no frame found")
     }
@@ -107,6 +108,7 @@ impl Vm {
     /// # Panics
     ///
     /// If there is no frame, then this will panic.
+    #[track_caller]
     pub(crate) fn frame_mut(&mut self) -> &mut CallFrame {
         self.frames.last_mut().expect("no frame found")
     }
@@ -189,19 +191,7 @@ impl Context<'_> {
 
         // If the current executing function is an async function we have to resolve/reject it's promise at the end.
         // The relevant spec section is 3. in [AsyncBlockStart](https://tc39.es/ecma262/#sec-asyncblockstart).
-        let promise_capability = self
-            .vm
-            .environments
-            .current_function_slots()
-            .as_function_slots()
-            .and_then(|slots| {
-                let slots_borrow = slots.borrow();
-                let function_object = slots_borrow.function_object();
-                let function = function_object.borrow();
-                function
-                    .as_function()
-                    .and_then(|f| f.get_promise_capability().cloned())
-            });
+        let promise_capability = self.vm.frame().promise_capability.clone();
 
         let execution_completion = loop {
             // 1. Exit the execution loop if program counter ever is equal to or exceeds the amount of instructions
@@ -300,21 +290,11 @@ impl Context<'_> {
         };
 
         // Early return immediately after loop.
-        if let Some(early_return) = self.vm.frame().early_return {
-            match early_return {
-                EarlyReturnType::Await => {
-                    let result = self.vm.pop();
-                    self.vm.stack.truncate(self.vm.frame().fp);
-                    self.vm.frame_mut().early_return = None;
-                    return CompletionRecord::Normal(result);
-                }
-                EarlyReturnType::Yield => {
+        if self.vm.frame().r#yield {
             let result = self.vm.pop();
-                    self.vm.frame_mut().early_return = None;
+            self.vm.frame_mut().r#yield = false;
             return CompletionRecord::Return(result);
         }
-            }
-        }
 
         #[cfg(feature = "trace")]
         if self.vm.trace {
@@ -359,7 +339,6 @@ impl Context<'_> {
         };
 
         if let Some(promise) = promise_capability {
-            // Step 3.e-g in [AsyncGeneratorStart](https://tc39.es/ecma262/#sec-asyncgeneratorstart)
             match execution_completion {
                 CompletionType::Normal => {
                     promise
@@ -383,6 +362,7 @@ impl Context<'_> {
                 }
             }
         } else if let Some(generator_object) = self.vm.frame().async_generator.clone() {
+            // Step 3.e-g in [AsyncGeneratorStart](https://tc39.es/ecma262/#sec-asyncgeneratorstart)
             let mut generator_object_mut = generator_object.borrow_mut();
             let generator = generator_object_mut
                 .as_async_generator_mut()

boa_engine/src/vm/opcode/await_stm/mod.rs

@@ -1,14 +1,10 @@
-use boa_gc::GcRefCell;
+use boa_gc::{Gc, GcRefCell};
 
 use crate::{
     builtins::{generator::GeneratorContext, Promise},
     native_function::NativeFunction,
     object::FunctionObjectBuilder,
-    vm::{
-        call_frame::{EarlyReturnType, GeneratorResumeKind},
-        opcode::Operation,
-        CompletionType,
-    },
+    vm::{opcode::Operation, CompletionType, GeneratorResumeKind},
     Context, JsArgs, JsResult, JsValue,
 };
 
@@ -33,13 +29,9 @@ impl Operation for Await {
             context,
         )?;
 
-        let generator_context = GeneratorContext {
-            environments: context.vm.environments.clone(),
-            call_frame: context.vm.frame().clone(),
-            stack: context.vm.stack.clone(),
-            active_function: context.vm.active_function.clone(),
-            realm: context.realm().clone(),
-        };
+        let gen = GeneratorContext::from_current(context);
+
+        let captures = Gc::new(GcRefCell::new(Some(gen)));
 
         // 3. Let fulfilledClosure be a new Abstract Closure with parameters (value) that captures asyncContext and performs the following steps when called:
         // 4. Let onFulfilled be CreateBuiltinFunction(fulfilledClosure, 1, "", « »).
@@ -47,49 +39,22 @@ impl Operation for Await {
             context,
             NativeFunction::from_copy_closure_with_captures(
                 |_this, args, captures, context| {
-                    let mut generator_context = std::mem::take(&mut *captures.borrow_mut())
-                        .expect("function should only be called once");
                     // a. Let prevContext be the running execution context.
                     // b. Suspend prevContext.
                     // c. Push asyncContext onto the execution context stack; asyncContext is now the running execution context.
                     // d. Resume the suspended evaluation of asyncContext using NormalCompletion(value) as the result of the operation that suspended it.
-                    // e. Assert: When we reach this step, asyncContext has already been removed from the execution context stack and prevContext is the currently running execution context.
-                    // f. Return undefined.
+                    let mut gen = captures.borrow_mut().take().expect("should only run once");
 
-                    std::mem::swap(
-                        &mut context.vm.environments,
-                        &mut generator_context.environments,
-                    );
-                    std::mem::swap(&mut context.vm.stack, &mut generator_context.stack);
-                    std::mem::swap(
-                        &mut context.vm.active_function,
-                        &mut generator_context.active_function,
-                    );
-                    let old_realm = context.enter_realm(generator_context.realm.clone());
-                    context.vm.push_frame(generator_context.call_frame.clone());
-
-                    context.vm.frame_mut().generator_resume_kind = GeneratorResumeKind::Normal;
-                    context.vm.push(args.get_or_undefined(0).clone());
-                    context.run();
-
-                    context
-                        .vm
-                        .pop_frame()
-                        .expect("generator call frame must exist");
-                    std::mem::swap(
-                        &mut context.vm.environments,
-                        &mut generator_context.environments,
-                    );
-                    std::mem::swap(&mut context.vm.stack, &mut generator_context.stack);
-                    std::mem::swap(
-                        &mut context.vm.active_function,
-                        &mut generator_context.active_function,
+                    gen.resume(
+                        Some(args.get_or_undefined(0).clone()),
+                        GeneratorResumeKind::Normal,
+                        context,
                     );
-                    context.enter_realm(old_realm);
-
+                    // e. Assert: When we reach this step, asyncContext has already been removed from the execution context stack and prevContext is the currently running execution context.
+                    // f. Return undefined.
                     Ok(JsValue::undefined())
                 },
-                GcRefCell::new(Some(generator_context.clone())),
+                captures.clone(),
             ),
         )
         .name("")
@@ -102,8 +67,6 @@ impl Operation for Await {
             context,
             NativeFunction::from_copy_closure_with_captures(
                 |_this, args, captures, context| {
-                    let mut generator_context = std::mem::take(&mut *captures.borrow_mut())
-                        .expect("function should only be called once");
                     // a. Let prevContext be the running execution context.
                     // b. Suspend prevContext.
                     // c. Push asyncContext onto the execution context stack; asyncContext is now the running execution context.
@@ -111,40 +74,17 @@ impl Operation for Await {
                     // e. Assert: When we reach this step, asyncContext has already been removed from the execution context stack and prevContext is the currently running execution context.
                     // f. Return undefined.
 
-                    std::mem::swap(
-                        &mut context.vm.environments,
-                        &mut generator_context.environments,
-                    );
-                    std::mem::swap(&mut context.vm.stack, &mut generator_context.stack);
-                    std::mem::swap(
-                        &mut context.vm.active_function,
-                        &mut generator_context.active_function,
-                    );
-                    let old_realm = context.enter_realm(generator_context.realm.clone());
-                    context.vm.push_frame(generator_context.call_frame.clone());
-
-                    context.vm.frame_mut().generator_resume_kind = GeneratorResumeKind::Throw;
-                    context.vm.push(args.get_or_undefined(0).clone());
-                    context.run();
+                    let mut gen = captures.borrow_mut().take().expect("should only run once");
 
-                    context
-                        .vm
-                        .pop_frame()
-                        .expect("generator call frame must exist");
-                    std::mem::swap(
-                        &mut context.vm.environments,
-                        &mut generator_context.environments,
-                    );
-                    std::mem::swap(&mut context.vm.stack, &mut generator_context.stack);
-                    std::mem::swap(
-                        &mut context.vm.active_function,
-                        &mut generator_context.active_function,
+                    gen.resume(
+                        Some(args.get_or_undefined(0).clone()),
+                        GeneratorResumeKind::Throw,
+                        context,
                     );
-                    context.enter_realm(old_realm);
 
                     Ok(JsValue::undefined())
                 },
-                GcRefCell::new(Some(generator_context)),
+                captures,
             ),
         )
         .name("")
@@ -161,7 +101,7 @@ impl Operation for Await {
         );
 
         context.vm.push(JsValue::undefined());
-        context.vm.frame_mut().early_return = Some(EarlyReturnType::Await);
+        context.vm.frame_mut().r#yield = true;
         Ok(CompletionType::Return)
     }
 }

boa_engine/src/vm/opcode/generator/mod.rs

@@ -6,7 +6,7 @@ use crate::{
     error::JsNativeError,
     string::utf16,
     vm::{
-        call_frame::{AbruptCompletionRecord, EarlyReturnType, GeneratorResumeKind},
+        call_frame::{AbruptCompletionRecord, GeneratorResumeKind},
         opcode::Operation,
         CompletionType,
     },
@@ -155,7 +155,7 @@ impl Operation for GeneratorNextDelegate {
                 context.vm.push(iterator.clone());
                 context.vm.push(next_method.clone());
                 context.vm.push(value);
-                context.vm.frame_mut().early_return = Some(EarlyReturnType::Yield);
+                context.vm.frame_mut().r#yield = true;
                 Ok(CompletionType::Return)
             }
             GeneratorResumeKind::Throw => {
@@ -177,7 +177,7 @@ impl Operation for GeneratorNextDelegate {
                     context.vm.push(iterator.clone());
                     context.vm.push(next_method.clone());
                     context.vm.push(value);
-                    context.vm.frame_mut().early_return = Some(EarlyReturnType::Yield);
+                    context.vm.frame_mut().r#yield = true;
                     return Ok(CompletionType::Return);
                 }
                 context.vm.frame_mut().pc = done_address as usize;
@@ -207,7 +207,7 @@ impl Operation for GeneratorNextDelegate {
                     context.vm.push(iterator.clone());
                     context.vm.push(next_method.clone());
                     context.vm.push(value);
-                    context.vm.frame_mut().early_return = Some(EarlyReturnType::Yield);
+                    context.vm.frame_mut().r#yield = true;
                     return Ok(CompletionType::Return);
                 }
                 context.vm.frame_mut().pc = done_address as usize;

boa_engine/src/vm/opcode/generator/yield_stm.rs

@@ -1,5 +1,5 @@
 use crate::{
-    vm::{call_frame::EarlyReturnType, opcode::Operation, CompletionType},
+    vm::{opcode::Operation, CompletionType},
     Context, JsResult,
 };
 
@@ -15,7 +15,7 @@ impl Operation for Yield {
     const INSTRUCTION: &'static str = "INST - Yield";
 
     fn execute(context: &mut Context<'_>) -> JsResult<CompletionType> {
-        context.vm.frame_mut().early_return = Some(EarlyReturnType::Yield);
+        context.vm.frame_mut().r#yield = true;
         Ok(CompletionType::Return)
     }
 }

docs/boa_object.md

@@ -76,6 +76,45 @@ their instructions aren't traced.
 
 The `this` value can be changed as well as the arguments that are passed to the function.
 
+### Function `$boa.function.traceable(func, mode)`
+
+Marks a single function as traceable on all future executions of the function. Both useful to mark
+several functions as traceable and to trace functions that suspend their execution (async functions,
+generators, async generators).
+
+#### Input
+
+```Javascript
+function* g() {
+    yield 1;
+    yield 2;
+    yield 3;
+}
+$boa.function.traceable(g, true);
+var iter = g();
+iter.next();
+iter.next();
+iter.next();
+```
+
+#### Output
+
+```bash
+1μs           RestParameterPop                                      <empty>
+1μs           PushUndefined                                         undefined
+2μs           Yield                                                 undefined
+4μs           GetName                    0000: 'a'                  1
+0μs           Yield                                                 1
+1μs           GeneratorNext                                         undefined
+1μs           Pop                                                   <empty>
+15μs          GetName                    0001: 'b'                  2
+1μs           Yield                                                 2
+1μs           GeneratorNext                                         undefined
+1μs           Pop                                                   <empty>
+4μs           GetName                    0002: 'c'                  3
+1μs           Yield                                                 3
+```
+
 ## Function `$boa.function.flowgraph(func, options)`
 
 It can be used to get the instruction flowgraph, like the command-line flag.