rsa/der: Avoid as conversions when serializing ASN.1 tags.

Work around the lack of `const fn` for `From` implementations.

src/io/der.rs

@@ -44,14 +44,21 @@ pub enum Tag {
 
 impl From<Tag> for usize {
     fn from(tag: Tag) -> Self {
-        tag as Self
+        Self::from(Tag::into(tag))
     }
 }
 
 impl From<Tag> for u8 {
     fn from(tag: Tag) -> Self {
-        tag as Self
-    } // XXX: narrowing conversion.
+        Tag::into(tag)
+    }
+}
+
+// `impl From<Tag> for u8` but as a `const fn`.
+impl Tag {
+    pub const fn into(self) -> u8 {
+        self as u8
+    }
 }
 
 pub fn expect_tag_and_get_value<'a>(

src/rsa/padding/pkcs1.rs

@@ -139,11 +139,11 @@ macro_rules! pkcs1_digestinfo_prefix {
     ( $name:ident, $digest_len:expr, $digest_oid_len:expr,
       [ $( $digest_oid:expr ),* ] ) => {
         static $name: [u8; 2 + 8 + $digest_oid_len] = [
-            der::Tag::Sequence as u8, 8 + $digest_oid_len + $digest_len,
-                der::Tag::Sequence as u8, 2 + $digest_oid_len + 2,
-                    der::Tag::OID as u8, $digest_oid_len, $( $digest_oid ),*,
-                    der::Tag::Null as u8, 0,
-                der::Tag::OctetString as u8, $digest_len,
+            der::Tag::Sequence.into(), 8 + $digest_oid_len + $digest_len,
+                der::Tag::Sequence.into(), 2 + $digest_oid_len + 2,
+                    der::Tag::OID.into(), $digest_oid_len, $( $digest_oid ),*,
+                    der::Tag::Null.into(), 0,
+                der::Tag::OctetString.into(), $digest_len,
         ];
     }
 }