LibJS: Taint variable environment chain after non-strict direct eval()

Since non-strict direct eval() can insert new bindings into a
surrounding var scope, we cannot safely cache some assumptions about
environment chain layout after eval() has taken place.

Since eval() is rare, let's do what other engines do and simply
deoptimize in its presence. This patch adds a new "permanently screwed"
flag to JS::Environment that will be set on the entire variable
environment chain upon non-strict direct eval().

Userland/Libraries/LibJS/Runtime/AbstractOperations.cpp

@@ -444,6 +444,13 @@ ThrowCompletionOr<Value> perform_eval(Value x, GlobalObject& caller_realm, Calle
     if (strict_eval)
         variable_environment = lexical_environment;
 
+    if (direct == EvalMode::Direct && !strict_eval) {
+        // NOTE: Non-strict direct eval() forces us to deoptimize variable accesses.
+        //       Mark the variable environment chain as screwed since we will not be able
+        //       to rely on cached environment coordinates from this point on.
+        variable_environment->set_permanently_screwed_by_eval();
+    }
+
     // 18. If runningContext is not already suspended, suspend runningContext.
     // FIXME: We don't have this concept yet.
 

Userland/Libraries/LibJS/Runtime/DeclarativeEnvironment.cpp

@@ -40,7 +40,7 @@ bool DeclarativeEnvironment::has_binding(FlyString const& name, Optional<size_t>
     auto it = m_names.find(name);
     if (it == m_names.end())
         return false;
-    if (out_index)
+    if (!is_permanently_screwed_by_eval() && out_index)
         *out_index = it->value;
     return true;
 }

Userland/Libraries/LibJS/Runtime/Environment.cpp

@@ -28,4 +28,13 @@ void Environment::visit_edges(Visitor& visitor)
     visitor.visit(m_outer_environment);
 }
 
+void Environment::set_permanently_screwed_by_eval()
+{
+    if (m_permanently_screwed_by_eval)
+        return;
+    m_permanently_screwed_by_eval = true;
+    if (outer_environment())
+        outer_environment()->set_permanently_screwed_by_eval();
+}
+
 }

Userland/Libraries/LibJS/Runtime/Environment.h

@@ -53,6 +53,11 @@ public:
 
     virtual char const* class_name() const override { return "Environment"; }
 
+    // This flag is set on the entire variable environment chain when direct eval() is performed.
+    // It is used to disable non-local variable access caching.
+    bool is_permanently_screwed_by_eval() const { return m_permanently_screwed_by_eval; }
+    void set_permanently_screwed_by_eval();
+
 protected:
     explicit Environment(Environment* parent);
 
@@ -63,6 +68,8 @@ private:
 
     GlobalObject* m_global_object { nullptr };
     Environment* m_outer_environment { nullptr };
+
+    bool m_permanently_screwed_by_eval { false };
 };
 
 }