subject_name: deprecate From<DnsNameRef> for DnsName.

As mentioned in the comment we can't outright `#[deprecate]` this one
because it's a trait impl.

Instead, promote the deprecation note to the rustdoc comment, add a TODO
to remove it outright, and switch our own tests to call `to_owned()`
directly in preparation of an eventual removal.

src/subject_name/dns_name.rs

@@ -55,9 +55,12 @@ impl AsRef<str> for DnsName {
 }
 
 /// Requires the `alloc` feature.
-// Deprecated
+/// Deprecated.
 #[cfg(feature = "alloc")]
 impl From<DnsNameRef<'_>> for DnsName {
+    // TODO(XXX): Remove this trait impl in the next release. We can't mark it as
+    //            hard deprecated as this isn't supported and produces a
+    //            'useless_deprecated' warning.
     fn from(dns_name: DnsNameRef) -> Self {
         dns_name.to_owned()
     }

tests/name_tests.rs

@@ -41,6 +41,7 @@ fn test_dns_name_traits() {
     let a_ref = DnsNameRef::try_from_ascii(b"example.com").unwrap();
 
     // `From<DnsNameRef>`
+    // TODO(XXX): Remove when deprecated From<DnsNameRef> for DnsName trait is removed.
     let a: DnsName = DnsName::from(a_ref);
 
     // `Clone`, `Debug`, `PartialEq`.
@@ -52,16 +53,22 @@ fn test_dns_name_traits() {
     // PartialEq is case-insensitive
     assert_eq!(
         a,
-        DnsName::from(DnsNameRef::try_from_ascii(b"Example.Com").unwrap())
+        DnsNameRef::try_from_ascii(b"Example.Com")
+            .unwrap()
+            .to_owned()
     );
 
     // PartialEq isn't completely wrong.
     assert_ne!(
         a,
-        DnsName::from(DnsNameRef::try_from_ascii(b"fxample.com").unwrap())
+        DnsNameRef::try_from_ascii(b"fxample.com")
+            .unwrap()
+            .to_owned()
     );
     assert_ne!(
         a,
-        DnsName::from(DnsNameRef::try_from_ascii(b"example.co").unwrap())
+        DnsNameRef::try_from_ascii(b"example.co")
+            .unwrap()
+            .to_owned()
     );
 }