LibJS: Add basic support for "with" statements

with statements evaluate an expression and put the result of it at the
"front" of the scope chain. This is implemented by creating a WithScope
object and placing it in front of the VM's current call frame's scope.

Libraries/LibJS/AST.cpp

@@ -29,6 +29,7 @@
 #include <AK/HashTable.h>
 #include <AK/ScopeGuard.h>
 #include <AK/StringBuilder.h>
+#include <AK/TemporaryChange.h>
 #include <LibCrypto/BigInt/SignedBigInteger.h>
 #include <LibJS/AST.h>
 #include <LibJS/Interpreter.h>
@@ -46,6 +47,7 @@
 #include <LibJS/Runtime/ScriptFunction.h>
 #include <LibJS/Runtime/Shape.h>
 #include <LibJS/Runtime/StringObject.h>
+#include <LibJS/Runtime/WithScope.h>
 #include <stdio.h>
 
 namespace JS {
@@ -255,9 +257,22 @@ Value IfStatement::execute(Interpreter& interpreter, GlobalObject& global_object
     return js_undefined();
 }
 
-Value WithStatement::execute(Interpreter&, GlobalObject&) const
+Value WithStatement::execute(Interpreter& interpreter, GlobalObject& global_object) const
 {
-    ASSERT_NOT_REACHED();
+    auto object_value = m_object->execute(interpreter, global_object);
+    if (interpreter.exception())
+        return {};
+
+    auto* object = object_value.to_object(global_object);
+    if (interpreter.exception())
+        return {};
+
+    ASSERT(object);
+
+    auto* with_scope = interpreter.heap().allocate<WithScope>(global_object, *object, interpreter.vm().call_frame().scope);
+    TemporaryChange<ScopeObject*> scope_change(interpreter.vm().call_frame().scope, with_scope);
+    interpreter.execute_statement(global_object, m_body);
+    return {};
 }
 
 Value WhileStatement::execute(Interpreter& interpreter, GlobalObject& global_object) const

Libraries/LibJS/CMakeLists.txt

@@ -76,6 +76,7 @@ set(SOURCES
     Runtime/Uint8ClampedArray.cpp
     Runtime/VM.cpp
     Runtime/Value.cpp
+    Runtime/WithScope.cpp
     Token.cpp
 )
 

Libraries/LibJS/Runtime/WithScope.cpp

@@ -0,0 +1,67 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <LibJS/AST.h>
+#include <LibJS/Runtime/WithScope.h>
+
+namespace JS {
+
+WithScope::WithScope(Object& object, ScopeObject* parent_scope)
+    : ScopeObject(parent_scope)
+    , m_object(object)
+{
+}
+
+void WithScope::visit_edges(Cell::Visitor& visitor)
+{
+    Base::visit_edges(visitor);
+    visitor.visit(&m_object);
+}
+
+Optional<Variable> WithScope::get_from_scope(const FlyString& name) const
+{
+    auto value = m_object.get(name);
+    if (value.is_empty())
+        return {};
+    return Variable { value, DeclarationKind::Var };
+}
+
+void WithScope::put_to_scope(const FlyString& name, Variable variable)
+{
+    m_object.put(name, variable.value);
+}
+
+bool WithScope::has_this_binding() const
+{
+    return parent()->has_this_binding();
+}
+
+Value WithScope::get_this_binding(GlobalObject& global_object) const
+{
+    return parent()->get_this_binding(global_object);
+}
+
+}

Libraries/LibJS/Runtime/WithScope.h

@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/ScopeObject.h>
+
+namespace JS {
+
+class WithScope : public ScopeObject {
+    JS_OBJECT(WithScope, ScopeObject);
+
+public:
+    WithScope(Object&, ScopeObject* parent_scope);
+
+    virtual Optional<Variable> get_from_scope(const FlyString&) const override;
+    virtual void put_to_scope(const FlyString&, Variable) override;
+    virtual bool has_this_binding()  const override;
+    virtual Value get_this_binding(GlobalObject&) const override;
+
+private:
+    virtual void visit_edges(Visitor&) override;
+
+    Object& m_object;
+};
+
+}