Add AsRef and Borrow for RelativeDname. (#251)

This PR adds impls for AsRef<RelativeDname<[u8]>> and
Borrow<RelativeDname<[u8]>> to RelativeDname<_>.

src/base/name/relative.rs

@@ -13,7 +13,7 @@ use bytes::Bytes;
 use core::cmp::Ordering;
 use core::ops::{Bound, RangeBounds};
 use core::str::FromStr;
-use core::{cmp, fmt, hash};
+use core::{borrow, cmp, fmt, hash};
 use octseq::builder::{
     EmptyBuilder, FreezeBuilder, FromBuilder, IntoBuilder, Truncate,
 };
@@ -741,6 +741,45 @@ impl<Octs: AsRef<[u8]> + ?Sized> fmt::Debug for RelativeDname<Octs> {
     }
 }
 
+//--- AsRef and Borrow
+
+impl<Octs> AsRef<RelativeDname<[u8]>> for RelativeDname<Octs>
+where
+    Octs: AsRef<[u8]> + ?Sized,
+{
+    fn as_ref(&self) -> &RelativeDname<[u8]> {
+        self.for_slice()
+    }
+}
+
+/// Borrow a relative domain name.
+///
+/// Containers holding an owned `RelativeDname<_>` may be queried with name
+/// over a slice. This `Borrow<_>` impl supports user code querying containers
+/// with compatible-but-different types like the following example:
+///
+/// ```
+/// use std::collections::HashMap;
+///
+/// use domain::base::RelativeDname;
+///
+/// fn get_description(
+///     hash: &HashMap<RelativeDname<Vec<u8>>, String>
+/// ) -> Option<&str> {
+///     let lookup_name: &RelativeDname<[u8]> =
+///         RelativeDname::from_slice(b"\x03ftp").unwrap();
+///     hash.get(lookup_name).map(|x| x.as_ref())
+/// }
+/// ```
+impl<Octs> borrow::Borrow<RelativeDname<[u8]>> for RelativeDname<Octs>
+where
+    Octs: AsRef<[u8]>,
+{
+    fn borrow(&self) -> &RelativeDname<[u8]> {
+        self.for_slice()
+    }
+}
+
 //--- Serialize and Deserialize
 
 #[cfg(feature = "serde")]