Mark brand types as repr(transparent) to try to ensure safety.

src/lib.rs

@@ -38,6 +38,7 @@ pub trait BrandState: Copy + fmt::Debug + Eq + Hash + Ord + sealed::Sealed {}
 /// Zero-sized marker type that indicates a graph or graph handle is in the open
 /// state. See [`Graph::open`].
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq, PartialOrd, Ord)]
+#[repr(transparent)]
 pub struct Open<'id>(PhantomData<*mut &'id ()>);
 impl<'id> BrandState for Open<'id> {}
 unsafe impl<'id> Send for Open<'id> {}
@@ -51,6 +52,7 @@ unsafe impl<'id> Sync for Open<'id> {}
 /// around without having to stay within the bounds of a single call to
 /// `Graph::open`. See [`Graph::open`] (and [`Graph::closed`]).
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq, PartialOrd, Ord)]
+#[repr(transparent)]
 pub struct Closed;
 impl BrandState for Closed {}