Fix compilation error

boa/src/builtins/object/internal_state.rs

@@ -0,0 +1,64 @@
+//! Implementations for storing normal rust structs inside any object as internal state.
+
+use std::{
+    any::Any,
+    fmt::{self, Debug},
+    ops::{Deref, DerefMut},
+    rc::Rc,
+};
+
+use gc::{unsafe_empty_trace, Finalize, Trace};
+
+/// Wrapper around `Rc` to implement `Trace` and `Finalize`.
+#[derive(Clone)]
+pub struct InternalStateCell {
+    /// The internal state.
+    state: Rc<dyn Any>,
+}
+
+impl Finalize for InternalStateCell {}
+
+unsafe impl Trace for InternalStateCell {
+    unsafe_empty_trace!();
+}
+
+impl Deref for InternalStateCell {
+    type Target = dyn Any;
+    fn deref(&self) -> &Self::Target {
+        Deref::deref(&self.state)
+    }
+}
+
+impl DerefMut for InternalStateCell {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        Rc::get_mut(&mut self.state).expect("failed to get mutable")
+    }
+}
+
+/// The derived version would print 'InternalStateCell { state: ... }', this custom implementation
+/// only prints the actual internal state.
+impl Debug for InternalStateCell {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        Debug::fmt(&self.state, f)
+    }
+}
+
+impl InternalStateCell {
+    /// Create new `InternalStateCell` from a value.
+    pub fn new<T: Any + InternalState>(value: T) -> Self {
+        Self {
+            state: Rc::new(value),
+        }
+    }
+    /// Get a reference to the stored value and cast it to `T`.
+    pub fn downcast_ref<T: Any + InternalState>(&self) -> Option<&T> {
+        self.deref().downcast_ref::<T>()
+    }
+    /// Get a mutable reference to the stored value and cast it to `T`.
+    pub fn downcast_mut<T: Any + InternalState>(&mut self) -> Option<&mut T> {
+        self.deref_mut().downcast_mut::<T>()
+    }
+}
+
+/// This trait must be implemented by all structs used for internal state.
+pub trait InternalState: Debug {}