Remove DerefMut impl for RootedGuard (#572)

* Construct handle_mut without going through a mutable reference

Signed-off-by: Greg Morenz <[email protected]>

* Remove DerefMut impl for RootedGuard

Signed-off-by: Greg Morenz <[email protected]>

* Allow unrooted interior in MutableHandle

Signed-off-by: Greg Morenz <[email protected]>

---------

Signed-off-by: Greg Morenz <[email protected]>

Author: gmorenz

mozjs/src/gc/root.rs

@@ -43,7 +43,20 @@ impl<'a, T: 'a + RootKind> RootedGuard<'a, T> {
     }
 
     pub fn handle_mut(&mut self) -> MutableHandle<T> {
-        unsafe { MutableHandle::from_marked_location(self.deref_mut()) }
+        unsafe { MutableHandle::from_marked_location(self.as_ptr()) }
+    }
+
+    pub fn as_ptr(&self) -> *mut T {
+        // SAFETY: self.root points to an inbounds allocation
+        unsafe { (&raw mut (*self.root).ptr).cast() }
+    }
+
+    /// Safety: GC must not run during the lifetime of the returned reference.
+    pub unsafe fn as_mut<'b>(&'b mut self) -> &'b mut T
+    where
+        'a: 'b,
+    {
+        &mut *(self.as_ptr())
     }
 
     pub fn get(&self) -> T
@@ -64,6 +77,16 @@ impl<'a, T: 'a + RootKind> RootedGuard<'a, T> {
     }
 }
 
+impl<'a, T> RootedGuard<'a, Option<T>>
+where
+    Option<T>: RootKind,
+{
+    pub fn take(&mut self) -> Option<T> {
+        // Safety: No GC occurs during take call
+        unsafe { self.as_mut().take() }
+    }
+}
+
 impl<'a, T: 'a + RootKind> Deref for RootedGuard<'a, T> {
     type Target = T;
     fn deref(&self) -> &T {
@@ -72,13 +95,6 @@ impl<'a, T: 'a + RootKind> Deref for RootedGuard<'a, T> {
     }
 }
 
-impl<'a, T: 'a + RootKind> DerefMut for RootedGuard<'a, T> {
-    fn deref_mut(&mut self) -> &mut T {
-        // SAFETY: The rooted value is initialized as long as we exist
-        unsafe { (*self.root).ptr.assume_init_mut() }
-    }
-}
-
 impl<'a, T: 'a + RootKind> Drop for RootedGuard<'a, T> {
     fn drop(&mut self) {
         // SAFETY: The rooted value is initialized as long as we exist
@@ -112,6 +128,10 @@ impl<T> Clone for Handle<'_, T> {
 
 impl<T> Copy for Handle<'_, T> {}
 
+#[cfg_attr(
+    feature = "crown",
+    crown::unrooted_must_root_lint::allow_unrooted_interior
+)]
 pub struct MutableHandle<'a, T: 'a> {
     pub(crate) ptr: *mut T,
     anchor: PhantomData<&'a mut T>,