Fix panic when a self mutating function is constructing an object (#710)

boa/src/builtins/function/tests.rs

@@ -2,7 +2,7 @@ use crate::{forward, forward_val, Context};
 
 #[allow(clippy::float_cmp)]
 #[test]
-fn check_arguments_object() {
+fn arguments_object() {
     let mut engine = Context::new();
 
     let init = r#"
@@ -25,7 +25,7 @@ fn check_arguments_object() {
 }
 
 #[test]
-fn check_self_mutating_func() {
+fn self_mutating_function_when_calling() {
     let mut engine = Context::new();
     let func = r#"
         function x() {
@@ -42,3 +42,22 @@ fn check_self_mutating_func() {
         3
     );
 }
+
+#[test]
+fn self_mutating_function_when_constructing() {
+    let mut engine = Context::new();
+    let func = r#"
+        function x() {
+            x.y = 3;
+        }
+        new x();
+        "#;
+    eprintln!("{}", forward(&mut engine, func));
+    let y = forward_val(&mut engine, "x.y").expect("value expected");
+    assert_eq!(y.is_integer(), true);
+    assert_eq!(
+        y.to_i32(&mut engine)
+            .expect("Could not convert value to i32"),
+        3
+    );
+}

boa/src/object/gcobject.rs

@@ -190,16 +190,14 @@ impl GcObject {
     // <https://tc39.es/ecma262/#sec-ecmascript-function-objects-construct-argumentslist-newtarget>
     #[track_caller]
     pub fn construct(&self, args: &[Value], ctx: &mut Context) -> Result<Value> {
-        let this = Object::create(self.borrow().get(&PROTOTYPE.into())).into();
+        let this: Value = Object::create(self.borrow().get(&PROTOTYPE.into())).into();
 
         let this_function_object = self.clone();
-        let object = self.borrow();
-        if let Some(function) = object.as_function() {
+        let body = if let Some(function) = self.borrow().as_function() {
             if function.is_constructable() {
                 match function {
                     Function::BuiltIn(BuiltInFunction(function), _) => {
-                        function(&this, args, ctx)?;
-                        Ok(this)
+                        FunctionBody::BuiltIn(*function)
                     }
                     Function::Ordinary {
                         body,
@@ -211,7 +209,7 @@ impl GcObject {
                         // <https://tc39.es/ecma262/#sec-prepareforordinarycall>
                         let local_env = new_function_environment(
                             this_function_object,
-                            Some(this),
+                            Some(this.clone()),
                             Some(environment.clone()),
                             // Arrow functions do not have a this binding https://tc39.es/ecma262/#sec-function-environment-records
                             if flags.is_lexical_this_mode() {
@@ -244,20 +242,29 @@ impl GcObject {
 
                         ctx.realm_mut().environment.push(local_env);
 
-                        // Call body should be set before reaching here
-                        let _ = body.run(ctx);
-
-                        // local_env gets dropped here, its no longer needed
-                        let binding = ctx.realm_mut().environment.get_this_binding();
-                        Ok(binding)
+                        FunctionBody::Ordinary(body.clone())
                     }
                 }
             } else {
                 let name = this.get_field("name").display().to_string();
-                ctx.throw_type_error(format!("{} is not a constructor", name))
+                return ctx.throw_type_error(format!("{} is not a constructor", name));
             }
         } else {
-            ctx.throw_type_error("not a function")
+            return ctx.throw_type_error("not a function");
+        };
+
+        match body {
+            FunctionBody::BuiltIn(function) => {
+                function(&this, args, ctx)?;
+                Ok(this)
+            }
+            FunctionBody::Ordinary(body) => {
+                let _ = body.run(ctx);
+
+                // local_env gets dropped here, its no longer needed
+                let binding = ctx.realm_mut().environment.get_this_binding();
+                Ok(binding)
+            }
         }
     }
 }