der: support Cow<[u8]> in derive(Sequence) (#1850)

Author: dishmaker

der/src/asn1/octet_string.rs

@@ -160,6 +160,7 @@ pub use self::allocating::OctetString;
 mod allocating {
     use super::*;
     use crate::referenced::*;
+    use alloc::borrow::Cow;
     use alloc::vec::Vec;
 
     /// ASN.1 `OCTET STRING` type: owned form..
@@ -282,6 +283,22 @@ mod allocating {
         }
     }
 
+    impl<'a> TryFrom<&'a Cow<'a, [u8]>> for OctetStringRef<'a> {
+        type Error = Error;
+
+        fn try_from(byte_slice: &'a Cow<'a, [u8]>) -> Result<Self, Error> {
+            OctetStringRef::new(byte_slice)
+        }
+    }
+
+    impl<'a> TryFrom<OctetStringRef<'a>> for Cow<'a, [u8]> {
+        type Error = Error;
+
+        fn try_from(octet_string: OctetStringRef<'a>) -> Result<Self, Self::Error> {
+            Ok(Cow::Borrowed(octet_string.as_bytes()))
+        }
+    }
+
     // Implement by hand because the derive would create invalid values.
     // Use the constructor to create a valid value.
     #[cfg(feature = "arbitrary")]