Allow dereferencing a rooted Gc during collection

Ordinarily, objects dropped during collections would only contain
unrooted Gc pointers.  But with `#[unsafe_ignore_trace]` it’s possible
to smuggle a rooted Gc pointer into the heap.  When we collect a
rooted Gc pointer, we need to dereference it in order to unroot its
contents.  This should be safe because the associated allocation
cannot have been collected while the Gc is rooted.

Fixes an “assertion failed: finalizer_safe()” panic in the included
test case (refs Manishearth#52).

Signed-off-by: Anders Kaseorg <[email protected]>

gc/src/lib.rs

@@ -141,12 +141,12 @@ impl<T: ?Sized> Gc<T> {
     #[inline]
     fn inner_ptr(&self) -> *mut GcBox<T> {
         // If we are currently in the dropping phase of garbage collection,
-        // it would be undefined behavior to dereference this pointer.
+        // it would be undefined behavior to dereference an unrooted Gc.
         // By opting into `Trace` you agree to not dereference this pointer
         // within your drop method, meaning that it should be safe.
         //
         // This assert exists just in case.
-        assert!(finalizer_safe());
+        assert!(finalizer_safe() || self.rooted());
 
         unsafe { clear_root_bit(self.ptr_root.get()).as_ptr() }
     }

gc/tests/ignore_trace.rs

@@ -0,0 +1,12 @@
+use gc::{force_collect, Finalize, Gc, Trace};
+
+#[derive(Finalize, Trace)]
+struct S(#[unsafe_ignore_trace] Gc<()>);
+
+/// Using `#[unsafe_ignore_trace]` on a `Gc` may inhibit collection of
+/// cycles through that `Gc`, but it should not result in panics.
+#[test]
+fn ignore_trace_gc() {
+    Gc::new(S(Gc::new(())));
+    force_collect();
+}