seal object reference trait.

This change adds the private ObjectReferencePrivate trait which is used
to seal the ObjectReference trait and prevent outside implementations.

Refs: #36

hdf5/src/hl/references.rs

@@ -13,7 +13,15 @@ pub use legacy::ObjectReference1;
 #[cfg(feature = "1.12.1")]
 pub use standard::ObjectReference2;
 
-pub trait ObjectReference: Sized + H5Type {
+mod private {
+    pub trait ObjectReferencePrivate {}
+}
+
+/// The trait for all object references. This provides a common interface
+/// over the legacy and standard reference types.
+///
+/// This trait is sealed and cannot be implemented for types outside `hdf5::hl`.
+pub trait ObjectReference: Sized + H5Type + private::ObjectReferencePrivate {
     const REF_TYPE: H5R_type_t;
     fn ptr(&self) -> *const c_void;
 

hdf5/src/hl/references/legacy.rs

@@ -14,7 +14,8 @@ use hdf5_sys::h5r::H5R_OBJECT as H5R_OBJECT1;
 #[cfg(feature = "1.12.0")]
 use hdf5_sys::h5r::H5R_OBJECT1;
 
-use crate::{Location, ObjectReference};
+use crate::{Location};
+use super::{ObjectReference, private::ObjectReferencePrivate};
 
 #[repr(transparent)]
 #[derive(Debug, Copy, Clone)]
@@ -28,6 +29,8 @@ unsafe impl H5Type for ObjectReference1 {
     }
 }
 
+impl ObjectReferencePrivate for ObjectReference1 {}
+
 impl ObjectReference for ObjectReference1 {
     const REF_TYPE: hdf5_sys::h5r::H5R_type_t = H5R_OBJECT1;
 

hdf5/src/hl/references/standard.rs

@@ -6,7 +6,7 @@ use hdf5_sys::h5o::H5O_type_t;
 use hdf5_sys::h5r::H5R_type_t::H5R_OBJECT2;
 use hdf5_sys::h5r::{H5R_ref_t, H5Rcreate_object, H5Rdestroy, H5Rget_obj_type3, H5Ropen_object};
 
-use super::ObjectReference;
+use super::{ObjectReference, private::ObjectReferencePrivate};
 use crate::internal_prelude::*;
 use crate::Location;
 
@@ -43,6 +43,8 @@ impl Drop for StdReference {
 #[derive(Debug)]
 pub struct ObjectReference2(StdReference);
 
+impl ObjectReferencePrivate for ObjectReference2 {}
+
 impl ObjectReference for ObjectReference2 {
     const REF_TYPE: hdf5_sys::h5r::H5R_type_t = H5R_OBJECT2;