Reduce `WeakGc<T>` memory usage (#3492)

* Reduce `WeakGc<T>` memory usage

Makes inner storage of `WeakGc<T>` store `()` in place of `Ephemeron` value,
insteadthe of a clone of the `Gc<T>` which is already stored in the key.

Reducing the size by the width of one pointer.

* Apply review

boa_gc/src/internals/ephemeron_box.rs

@@ -139,21 +139,33 @@ impl<K: Trace, V: Trace> EphemeronBox<K, V> {
         data.as_ref().map(|data| &data.value)
     }
 
-    /// Returns a reference to the ephemeron's key or None.
+    /// Returns the pointer to the ephemeron's key or None.
     ///
     /// # Safety
     ///
     /// The caller must ensure there are no live mutable references to the ephemeron box's data
     /// before calling this method.
-    pub(crate) unsafe fn key(&self) -> Option<&GcBox<K>> {
+    pub(crate) unsafe fn key_ptr(&self) -> Option<NonNull<GcBox<K>>> {
         // SAFETY: the garbage collector ensures the ephemeron doesn't mutate until
         // finalization.
         unsafe {
             let data = &*self.data.get();
-            data.as_ref().map(|data| data.key.as_ref())
+            data.as_ref().map(|data| data.key)
         }
     }
 
+    /// Returns a reference to the ephemeron's key or None.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure there are no live mutable references to the ephemeron box's data
+    /// before calling this method.
+    pub(crate) unsafe fn key(&self) -> Option<&GcBox<K>> {
+        // SAFETY: the garbage collector ensures the ephemeron doesn't mutate until
+        // finalization.
+        unsafe { self.key_ptr().map(|data| data.as_ref()) }
+    }
+
     /// Marks this `EphemeronBox` as live.
     ///
     /// This doesn't mark the inner value of the ephemeron. [`ErasedEphemeronBox::trace`]

boa_gc/src/pointers/ephemeron.rs

@@ -33,6 +33,23 @@ impl<K: Trace, V: Trace + Clone> Ephemeron<K, V> {
         unsafe { self.inner_ptr.as_ref().value().cloned() }
     }
 
+    /// Gets the stored key of this `Ephemeron`, or `None` if the key was already garbage collected.
+    #[inline]
+    #[must_use]
+    pub fn key(&self) -> Option<Gc<K>> {
+        // SAFETY: this is safe because `Ephemeron` is tracked to always point to a valid pointer
+        // `inner_ptr`.
+        let key_ptr = unsafe { self.inner_ptr.as_ref().key_ptr() }?;
+
+        // SAFETY: Returned pointer is valid, so this is safe.
+        unsafe {
+            key_ptr.as_ref().inc_ref_count();
+        }
+
+        // SAFETY: The gc pointer's reference count has been incremented, so this is safe.
+        Some(unsafe { Gc::from_raw(key_ptr) })
+    }
+
     /// Checks if the [`Ephemeron`] has a value.
     #[must_use]
     pub fn has_value(&self) -> bool {

boa_gc/src/pointers/gc.rs

@@ -59,7 +59,7 @@ impl<T: Trace> Gc<T> {
         //   `unsafe` block.
         // - `set_kv`: `weak` is a newly created `EphemeronBox`, meaning it isn't possible to
         //   collect it since `weak` is still live.
-        unsafe { weak.inner().inner_ptr().as_mut().set(&gc, gc.clone()) }
+        unsafe { weak.inner().inner_ptr().as_mut().set(&gc, ()) }
 
         gc
     }

boa_gc/src/pointers/weak.rs

@@ -8,33 +8,36 @@ use std::hash::{Hash, Hasher};
 #[derive(Debug, Trace, Finalize)]
 #[repr(transparent)]
 pub struct WeakGc<T: Trace + 'static> {
-    inner: Ephemeron<T, Gc<T>>,
+    inner: Ephemeron<T, ()>,
 }
 
 impl<T: Trace> WeakGc<T> {
     /// Creates a new weak pointer for a garbage collected value.
+    #[inline]
     #[must_use]
     pub fn new(value: &Gc<T>) -> Self {
         Self {
-            inner: Ephemeron::new(value, value.clone()),
+            inner: Ephemeron::new(value, ()),
         }
     }
 
     /// Upgrade returns a `Gc` pointer for the internal value if the pointer is still live, or `None`
     /// if the value was already garbage collected.
+    #[inline]
     #[must_use]
     pub fn upgrade(&self) -> Option<Gc<T>> {
-        self.inner.value()
+        self.inner.key()
     }
 
     /// Check if the [`WeakGc`] can be upgraded.
+    #[inline]
     #[must_use]
     pub fn is_upgradable(&self) -> bool {
         self.inner.has_value()
     }
 
     #[must_use]
-    pub(crate) const fn inner(&self) -> &Ephemeron<T, Gc<T>> {
+    pub(crate) const fn inner(&self) -> &Ephemeron<T, ()> {
         &self.inner
     }
 }
@@ -47,8 +50,8 @@ impl<T: Trace> Clone for WeakGc<T> {
     }
 }
 
-impl<T: Trace> From<Ephemeron<T, Gc<T>>> for WeakGc<T> {
-    fn from(inner: Ephemeron<T, Gc<T>>) -> Self {
+impl<T: Trace> From<Ephemeron<T, ()>> for WeakGc<T> {
+    fn from(inner: Ephemeron<T, ()>) -> Self {
         Self { inner }
     }
 }