x509-cert: allow certificate to be serialized to pem (#763)

Author: baloo

x509-cert/src/certificate.rs

@@ -9,6 +9,9 @@ use der::asn1::{BitStringRef, UIntRef};
 use der::{Decode, Enumerated, Error, ErrorKind, Sequence};
 use spki::{AlgorithmIdentifier, SubjectPublicKeyInfo};
 
+#[cfg(feature = "pem")]
+use der::pem::PemLabel;
+
 /// Certificate `Version` as defined in [RFC 5280 Section 4.1].
 ///
 /// ```text
@@ -143,6 +146,12 @@ pub struct Certificate<'a> {
     pub signature: BitStringRef<'a>,
 }
 
+#[cfg(feature = "pem")]
+#[cfg_attr(docsrs, doc(cfg(feature = "pem")))]
+impl PemLabel for Certificate<'_> {
+    const PEM_LABEL: &'static str = "CERTIFICATE";
+}
+
 /// `PkiPath` as defined by X.509 and referenced by [RFC 6066].
 ///
 /// This contains a series of certificates in validation order from the