Implement the WeakRef builtin (#2438)

52/60 tests passing. The remaining tests are either features not implemented ([FinalizationRegistry](https://tc39.es/ecma262/multipage/managing-memory.html#sec-finalization-registry-objects)) or features still in development ([symbols-as-weakmap-keys](https://github.com/tc39/proposal-symbols-as-weakmap-keys)).

boa_engine/src/builtins/array/mod.rs

@@ -420,10 +420,7 @@ impl Array {
             JsValue::Object(o) if o.is_callable() => Some(o),
             _ => {
                 return Err(JsNativeError::typ()
-                    .with_message(format!(
-                        "{} is not a function",
-                        mapfn.type_of().to_std_string_escaped()
-                    ))
+                    .with_message(format!("`{}` is not callable", mapfn.type_of()))
                     .into())
             }
         };

boa_engine/src/builtins/mod.rs

@@ -33,6 +33,7 @@ pub mod symbol;
 pub mod typed_array;
 pub mod undefined;
 pub mod uri;
+pub mod weak;
 
 #[cfg(feature = "console")]
 pub mod console;
@@ -82,44 +83,42 @@ use crate::{
     builtins::{
         array_buffer::ArrayBuffer, async_generator::AsyncGenerator,
         async_generator_function::AsyncGeneratorFunction, generator::Generator,
-        generator_function::GeneratorFunction, typed_array::TypedArray, uri::Uri,
+        generator_function::GeneratorFunction, typed_array::TypedArray, uri::Uri, weak::WeakRef,
     },
     property::{Attribute, PropertyDescriptor},
     Context, JsValue,
 };
 
-/// Trait representing a global built-in object such as `Math`, `Object` or
-/// `String`.
+/// Trait representing a global built-in object such as `Math`, `Object` or `String`.
 ///
-/// This trait must be implemented for any global built-in accessible from
-/// Javascript.
+/// This trait must be implemented for any global built-in accessible from JavaScript.
 pub(crate) trait BuiltIn {
     /// Binding name of the built-in inside the global object.
     ///
-    /// E.g. If you want access the properties of a `Complex` built-in
-    /// with the name `Cplx` you must assign `"Cplx"` to this constant,
-    /// making any property inside it accessible from Javascript as `Cplx.prop`
+    /// E.g. If you want access the properties of a `Complex` built-in with the name `Cplx` you must
+    /// assign `"Cplx"` to this constant, making any property inside it accessible from Javascript
+    /// as `Cplx.prop`
     const NAME: &'static str;
 
-    /// Property attribute flags of the built-in.
-    /// Check [Attribute] for more information.
+    /// Property attribute flags of the built-in. Check [`Attribute`] for more information.
     const ATTRIBUTE: Attribute = Attribute::WRITABLE
         .union(Attribute::NON_ENUMERABLE)
         .union(Attribute::CONFIGURABLE);
 
     /// Initialization code for the built-in.
-    /// This is where the methods, properties, static methods and the constructor
-    /// of a built-in must be initialized to be accessible from Javascript.
+    ///
+    /// This is where the methods, properties, static methods and the constructor of a built-in must
+    /// be initialized to be accessible from Javascript.
     ///
     /// # Note
     ///
-    /// A return value of `None` indicates that the value must not be added as
-    /// a global attribute for the global object.
+    /// A return value of `None` indicates that the value must not be added as a global attribute
+    /// for the global object.
     fn init(context: &mut Context) -> Option<JsValue>;
 }
 
-/// Utility function that checks if a type implements `BuiltIn` before
-/// initializing it as a global built-in.
+/// Utility function that checks if a type implements `BuiltIn` before initializing it as a global
+/// built-in.
 #[inline]
 fn init_builtin<B: BuiltIn>(context: &mut Context) {
     if let Some(value) = B::init(context) {
@@ -195,7 +194,8 @@ pub fn init(context: &mut Context) {
         AsyncFunction,
         AsyncGenerator,
         AsyncGeneratorFunction,
-        Uri
+        Uri,
+        WeakRef
     };
 
     #[cfg(feature = "intl")]
@@ -206,17 +206,15 @@ pub fn init(context: &mut Context) {
 }
 
 pub trait JsArgs {
-    /// Utility function to `get` a parameter from
-    /// a `[JsValue]` or default to `JsValue::Undefined`
+    /// Utility function to `get` a parameter from a `[JsValue]` or default to `JsValue::Undefined`
     /// if `get` returns `None`.
     ///
     /// Call this if you are thinking of calling something similar to
     /// `args.get(n).cloned().unwrap_or_default()` or
     /// `args.get(n).unwrap_or(&undefined)`.
     ///
-    /// This returns a reference for efficiency, in case
-    /// you only need to call methods of `JsValue`, so
-    /// try to minimize calling `clone`.
+    /// This returns a reference for efficiency, in case you only need to call methods of `JsValue`,
+    /// so try to minimize calling `clone`.
     fn get_or_undefined(&self, index: usize) -> &JsValue;
 }
 

boa_engine/src/builtins/object/mod.rs

@@ -628,8 +628,8 @@ impl Object {
             val => {
                 return Err(JsNativeError::typ()
                     .with_message(format!(
-                        "expected an object or null, got {}",
-                        val.type_of().to_std_string_escaped()
+                        "expected an object or null, got `{}`",
+                        val.type_of()
                     ))
                     .into())
             }

boa_engine/src/builtins/weak/mod.rs

@@ -0,0 +1,3 @@
+mod weak_ref;
+
+pub(crate) use weak_ref::WeakRef;

boa_engine/src/builtins/weak/weak_ref.rs

@@ -0,0 +1,166 @@
+use boa_gc::{Finalize, Trace, WeakGc};
+use boa_profiler::Profiler;
+use tap::{Conv, Pipe};
+
+use crate::{
+    builtins::{BuiltIn, JsArgs},
+    context::intrinsics::StandardConstructors,
+    object::{
+        internal_methods::get_prototype_from_constructor, ConstructorBuilder, JsObject, ObjectData,
+    },
+    property::Attribute,
+    symbol::WellKnownSymbols,
+    Context, JsNativeError, JsResult, JsValue,
+};
+
+/// The [`WeakRef`][wr] builtin object.
+///
+/// [wr]: https://tc39.es/ecma262/#sec-weak-ref-objects
+#[derive(Debug, Clone, Trace, Finalize)]
+pub(crate) struct WeakRef;
+
+impl BuiltIn for WeakRef {
+    const NAME: &'static str = "WeakRef";
+
+    const ATTRIBUTE: Attribute = Attribute::WRITABLE.union(Attribute::CONFIGURABLE);
+
+    fn init(context: &mut Context) -> Option<JsValue> {
+        let _timer = Profiler::global().start_event(Self::NAME, "init");
+        ConstructorBuilder::with_standard_constructor(
+            context,
+            WeakRef::constructor,
+            context.intrinsics().constructors().weak_ref().clone(),
+        )
+        .name(Self::NAME)
+        .length(Self::LENGTH)
+        .property(
+            WellKnownSymbols::to_string_tag(),
+            "WeakRef",
+            Attribute::CONFIGURABLE,
+        )
+        .method(WeakRef::deref, "deref", 0)
+        .build()
+        .conv::<JsValue>()
+        .pipe(Some)
+    }
+}
+
+impl WeakRef {
+    /// The amount of arguments the `WeakRef` constructor takes.
+    pub(crate) const LENGTH: usize = 1;
+
+    /// Constructor [`WeakRef ( target )`][cons]
+    ///
+    /// [cons]: https://tc39.es/ecma262/#sec-weak-ref-target
+    pub(crate) fn constructor(
+        new_target: &JsValue,
+        args: &[JsValue],
+        context: &mut Context,
+    ) -> JsResult<JsValue> {
+        // If NewTarget is undefined, throw a TypeError exception.
+        if new_target.is_undefined() {
+            return Err(JsNativeError::typ()
+                .with_message("WeakRef: cannot call constructor without `new`")
+                .into());
+        }
+
+        // 2. If target is not an Object, throw a TypeError exception.
+        let target = args.get(0).and_then(JsValue::as_object).ok_or_else(|| {
+            JsNativeError::typ().with_message(format!(
+                "WeakRef: expected target argument of type `object`, got target of type `{}`",
+                args.get_or_undefined(0).type_of()
+            ))
+        })?;
+
+        // 3. Let weakRef be ? OrdinaryCreateFromConstructor(NewTarget, "%WeakRef.prototype%", « [[WeakRefTarget]] »).
+        // 5. Set weakRef.[[WeakRefTarget]] to target.
+        let weak_ref = JsObject::from_proto_and_data(
+            get_prototype_from_constructor(new_target, StandardConstructors::weak_ref, context)?,
+            ObjectData::weak_ref(WeakGc::new(target.inner())),
+        );
+
+        // 4. Perform AddToKeptObjects(target).
+        context.kept_alive.push(target.clone());
+
+        // 6. Return weakRef.
+        Ok(weak_ref.into())
+    }
+
+    /// Method [`WeakRef.prototype.deref ( )`][spec].
+    ///
+    /// If the referenced object hasn't been collected, this method promotes a `WeakRef` into a
+    /// proper [`JsObject`], or returns `undefined` otherwise.
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-weak-ref.prototype.deref
+    pub(crate) fn deref(this: &JsValue, _: &[JsValue], context: &mut Context) -> JsResult<JsValue> {
+        // 1. Let weakRef be the this value.
+        // 2. Perform ? RequireInternalSlot(weakRef, [[WeakRefTarget]]).
+        let weak_ref = this.as_object().map(JsObject::borrow).ok_or_else(|| {
+            JsNativeError::typ().with_message(format!(
+                "WeakRef.prototype.deref: expected `this` to be an `object`, got value of type `{}`",
+                this.type_of()
+            ))
+        })?;
+
+        let weak_ref = weak_ref.as_weak_ref().ok_or_else(|| {
+            JsNativeError::typ()
+                .with_message("WeakRef.prototype.deref: expected `this` to be a `WeakRef` object")
+        })?;
+
+        // 3. Return WeakRefDeref(weakRef).
+
+        // `WeakRefDeref`
+        // https://tc39.es/ecma262/multipage/managing-memory.html#sec-weakrefderef
+        // 1. Let target be weakRef.[[WeakRefTarget]].
+        // 2. If target is not empty, then
+        if let Some(object) = weak_ref.upgrade() {
+            let object = JsObject::from(object);
+
+            // a. Perform AddToKeptObjects(target).
+            context.kept_alive.push(object.clone());
+
+            // b. Return target.
+            Ok(object.into())
+        } else {
+            // 3. Return undefined.
+            Ok(JsValue::undefined())
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{Context, JsValue};
+
+    #[test]
+    fn weak_ref_collected() {
+        let context = &mut Context::default();
+
+        assert!(context
+            .eval(
+                r#"
+            var ptr;
+            {
+                let obj = {a: 5, b: 6};
+                ptr = new WeakRef(obj);
+            }
+            ptr.deref()
+        "#
+            )
+            .unwrap()
+            .is_object());
+
+        boa_gc::force_collect();
+
+        assert_eq!(
+            context
+                .eval(
+                    r#"
+            ptr.deref()
+        "#
+                )
+                .unwrap(),
+            JsValue::undefined()
+        )
+    }
+}

boa_engine/src/context/intrinsics.rs

@@ -116,6 +116,7 @@ pub struct StandardConstructors {
     data_view: StandardConstructor,
     date_time_format: StandardConstructor,
     promise: StandardConstructor,
+    weak_ref: StandardConstructor,
 }
 
 impl Default for StandardConstructors {
@@ -175,6 +176,7 @@ impl Default for StandardConstructors {
             data_view: StandardConstructor::default(),
             date_time_format: StandardConstructor::default(),
             promise: StandardConstructor::default(),
+            weak_ref: StandardConstructor::default(),
         };
 
         // The value of `Array.prototype` is the Array prototype object.
@@ -402,6 +404,11 @@ impl StandardConstructors {
     pub fn promise(&self) -> &StandardConstructor {
         &self.promise
     }
+
+    #[inline]
+    pub fn weak_ref(&self) -> &StandardConstructor {
+        &self.weak_ref
+    }
 }
 
 /// Cached intrinsic objects

boa_engine/src/context/mod.rs

@@ -104,6 +104,8 @@ pub struct Context {
     pub(crate) vm: Vm,
 
     pub(crate) promise_job_queue: VecDeque<JobCallback>,
+
+    pub(crate) kept_alive: Vec<JsObject>,
 }
 
 impl Default for Context {
@@ -538,6 +540,7 @@ impl Context {
         self.realm.set_global_binding_number();
         let result = self.run();
         self.vm.pop_frame();
+        self.clear_kept_objects();
         self.run_queued_jobs()?;
         let (result, _) = result?;
         Ok(result)
@@ -547,6 +550,7 @@ impl Context {
     fn run_queued_jobs(&mut self) -> JsResult<()> {
         while let Some(job) = self.promise_job_queue.pop_front() {
             job.call_job_callback(&JsValue::Undefined, &[], self)?;
+            self.clear_kept_objects();
         }
         Ok(())
     }
@@ -580,6 +584,18 @@ impl Context {
         // TODO
         self.promise_job_queue.push_back(job);
     }
+
+    /// Abstract operation [`ClearKeptObjects`][clear].
+    ///
+    /// Clears all objects maintained alive by calls to the [`AddToKeptObjects`][add] abstract
+    /// operation, used within the [`WeakRef`][weak] constructor.
+    ///
+    /// [clear]: https://tc39.es/ecma262/multipage/executable-code-and-execution-contexts.html#sec-clear-kept-objects
+    /// [add]: https://tc39.es/ecma262/multipage/executable-code-and-execution-contexts.html#sec-addtokeptobjects
+    /// [weak]: https://tc39.es/ecma262/multipage/managing-memory.html#sec-weak-ref-objects
+    pub fn clear_kept_objects(&mut self) {
+        self.kept_alive.clear();
+    }
 }
 /// Builder for the [`Context`] type.
 ///
@@ -661,6 +677,7 @@ impl ContextBuilder {
             #[cfg(feature = "fuzz")]
             instructions_remaining: self.instructions_remaining,
             promise_job_queue: VecDeque::new(),
+            kept_alive: Vec::new(),
         };
 
         // Add new builtIns to Context Realm

boa_engine/src/object/jsobject.rs

@@ -736,6 +736,10 @@ Cannot both specify accessors and a value or writable attribute",
             }
         )
     }
+
+    pub(crate) fn inner(&self) -> &Gc<GcCell<Object>> {
+        &self.inner
+    }
 }
 
 impl AsRef<GcCell<Object>> for JsObject {
@@ -745,6 +749,13 @@ impl AsRef<GcCell<Object>> for JsObject {
     }
 }
 
+impl From<Gc<GcCell<Object>>> for JsObject {
+    #[inline]
+    fn from(inner: Gc<GcCell<Object>>) -> Self {
+        JsObject { inner }
+    }
+}
+
 impl PartialEq for JsObject {
     fn eq(&self, other: &Self) -> bool {
         Self::equals(self, other)