LibJS: Make Reference aware of DeclarativeEnvironment indices

VM::resolve_binding() can now return a Reference that knows the exact
binding index if it's pointing into a DeclarativeEnvironment.

Reading/writing through the Reference will now use direct environment
access when possible.

Userland/Libraries/LibJS/Runtime/Reference.cpp

@@ -5,6 +5,7 @@
  */
 
 #include <LibJS/AST.h>
+#include <LibJS/Runtime/DeclarativeEnvironment.h>
 #include <LibJS/Runtime/Error.h>
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/Reference.h>
@@ -48,6 +49,9 @@ void Reference::put_value(GlobalObject& global_object, Value value)
 
     VERIFY(m_base_type == BaseType::Environment);
     VERIFY(m_base_environment);
+    if (m_index_in_declarative_environment.has_value())
+        static_cast<DeclarativeEnvironment*>(m_base_environment)->set_mutable_binding_direct(global_object, m_index_in_declarative_environment.value(), value, m_strict);
+    else
         m_base_environment->set_mutable_binding(global_object, m_name.as_string(), value, m_strict);
 }
 
@@ -77,6 +81,8 @@ Value Reference::get_value(GlobalObject& global_object) const
 
     VERIFY(m_base_type == BaseType::Environment);
     VERIFY(m_base_environment);
+    if (m_index_in_declarative_environment.has_value())
+        return static_cast<DeclarativeEnvironment*>(m_base_environment)->get_binding_value_direct(global_object, m_index_in_declarative_environment.value(), m_strict);
     return m_base_environment->get_binding_value(global_object, m_name.as_string(), m_strict);
 }
 

Userland/Libraries/LibJS/Runtime/Reference.h

@@ -44,11 +44,12 @@ public:
         }
     }
 
-    Reference(Environment& base, FlyString referenced_name, bool strict = false)
+    Reference(Environment& base, FlyString referenced_name, bool strict = false, Optional<size_t> index_in_declarative_environment = {})
         : m_base_type(BaseType::Environment)
         , m_base_environment(&base)
         , m_name(move(referenced_name))
         , m_strict(strict)
+        , m_index_in_declarative_environment(move(index_in_declarative_environment))
     {
     }
 
@@ -129,6 +130,7 @@ private:
     PropertyName m_name;
     Value m_this_value;
     bool m_strict { false };
+    Optional<size_t> m_index_in_declarative_environment;
 };
 
 }

Userland/Libraries/LibJS/Runtime/VM.cpp

@@ -442,12 +442,14 @@ Reference VM::get_identifier_reference(Environment* environment, FlyString name,
         // a. Return the Reference Record { [[Base]]: unresolvable, [[ReferencedName]]: name, [[Strict]]: strict, [[ThisValue]]: empty }.
         return Reference { Reference::BaseType::Unresolvable, move(name), strict };
     }
-    auto exists = environment->has_binding(name);
+
+    Optional<size_t> index;
+    auto exists = environment->has_binding(name, &index);
     if (exception())
         return {};
 
     if (exists)
-        return Reference { *environment, move(name), strict };
+        return Reference { *environment, move(name), strict, index };
     else
         return get_identifier_reference(environment->outer_environment(), move(name), strict);
 }