[async-await] (simpler) fix for Return in try/finally in async functions

src/ast/scopes.h

Index: src/ast/scopes.h
diff --git a/src/ast/scopes.h b/src/ast/scopes.h
index ea37890c5e4875406996b0a5916141fc2459fc82..162c9003019934c5c90060afc38f4ff4528566c7 100644
--- a/src/ast/scopes.h
+++ b/src/ast/scopes.h
@@ -680,6 +680,11 @@ class DeclarationScope : public Scope {
   // calls sloppy eval.
   Variable* DeclareFunctionVar(const AstRawString* name);
 
+  // Declare some special internal variables which must be accessible to
+  // Ignition without ScopeInfo.
+  Variable* DeclareGeneratorObjectVar(const AstRawString* name);
+  Variable* DeclarePromiseVar(const AstRawString* name);
+
   // Declare a parameter in this scope.  When there are duplicated
   // parameters the rightmost one 'wins'.  However, the implementation
   // expects all parameters to be declared and from left to right.
@@ -718,6 +723,17 @@ class DeclarationScope : public Scope {
     return function_;
   }
 
+  Variable* generator_object_var() const {
+    DCHECK(is_function_scope() || is_module_scope());
+    return GetRareVariable(RareVariable::kGeneratorObject);
+  }
+
+  Variable* promise_var() const {
+    DCHECK(is_function_scope());
+    DCHECK(IsAsyncFunction(function_kind_));
+    return GetRareVariable(RareVariable::kPromise);
+  }
+
   // Parameters. The left-most parameter has index 0.
   // Only valid for function and module scopes.
   Variable* parameter(int index) const {
@@ -758,12 +774,14 @@ class DeclarationScope : public Scope {
   }
 
   Variable* this_function_var() const {
+    Variable* this_function = GetRareVariable(RareVariable::kThisFunction);
+
     // This is only used in derived constructors atm.
-    DCHECK(this_function_ == nullptr ||
+    DCHECK(this_function == nullptr ||
            (is_function_scope() && (IsClassConstructor(function_kind()) ||
                                     IsConciseMethod(function_kind()) ||
                                     IsAccessorFunction(function_kind()))));
-    return this_function_;
+    return this_function;
   }
 
   // Adds a local variable in this scope's locals list. This is for adjusting
@@ -872,8 +890,50 @@ class DeclarationScope : public Scope {
   Variable* new_target_;
   // Convenience variable; function scopes only.
   Variable* arguments_;
-  // Convenience variable; Subclass constructor only
-  Variable* this_function_;
+
+  struct RareData : public ZoneObject {
+    void* operator new(size_t size, Zone* zone) { return zone->New(size); }
+
+    // Convenience variable; Subclass constructor only
+    Variable* this_function = nullptr;
+
+    // Generator object, if any; generator function scopes and module scopes
+    // only.
+    Variable* generator_object = nullptr;
+    // Promise, if any; async function scopes only.
+    Variable* promise = nullptr;
+  };
+
+  enum class RareVariable {
+    kThisFunction = offsetof(RareData, this_function),
+    kGeneratorObject = offsetof(RareData, generator_object),
+    kPromise = offsetof(RareData, promise)
+  };
+
+  V8_INLINE RareData* EnsureRareData() {
+    if (rare_data_ == nullptr) {
+      rare_data_ = new (zone_) RareData;
+    }
+    return rare_data_;
+  }
+
+  V8_INLINE Variable* GetRareVariable(RareVariable id) const {
+    if (rare_data_ == nullptr) return nullptr;
+    return *reinterpret_cast<Variable**>(
+        reinterpret_cast<uint8_t*>(rare_data_) + static_cast<ptrdiff_t>(id));
+  }
+
+  // Set `var` to null if it's non-null and Predicate (Variable*) -> bool
+  // returns true.
+  template <typename Predicate>
+  V8_INLINE void NullifyRareVariableIf(RareVariable id, Predicate predicate) {
+    if (V8_LIKELY(rare_data_ == nullptr)) return;
+    Variable** var = reinterpret_cast<Variable**>(
+        reinterpret_cast<uint8_t*>(rare_data_) + static_cast<ptrdiff_t>(id));
+    if (*var && predicate(*var)) *var = nullptr;
+  }
+
+  RareData* rare_data_ = nullptr;
 };
 
 class ModuleScope final : public DeclarationScope {