der: add IsConstructed trait, impl'ed on any FixedTag

Author: dishmaker

der/src/asn1/context_specific.rs

@@ -3,6 +3,7 @@
 use crate::{
     Choice, Decode, DecodeValue, DerOrd, Encode, EncodeValue, EncodeValueRef, Error, Header,
     Length, Reader, Tag, TagMode, TagNumber, Tagged, ValueOrd, Writer, asn1::AnyRef,
+    tag::IsConstructed,
 };
 use core::cmp::Ordering;
 
@@ -60,13 +61,13 @@ impl<T> ContextSpecific<T> {
         tag_number: TagNumber,
     ) -> Result<Option<Self>, T::Error>
     where
-        T: DecodeValue<'a> + Tagged,
+        T: DecodeValue<'a> + IsConstructed,
     {
         Self::decode_with::<_, _, T::Error>(reader, tag_number, |reader| {
             let header = Header::decode(reader)?;
             let value = T::decode_value(reader, header)?;
 
-            if header.tag.is_constructed() != value.tag().is_constructed() {
+            if header.tag.is_constructed() != T::CONSTRUCTED {
                 return Err(header.tag.non_canonical_error().into());
             }
 

der/src/tag.rs

@@ -32,6 +32,17 @@ impl<T: FixedTag + ?Sized> Tagged for T {
     }
 }
 
+/// Types which have a constant ASN.1 constructed bit.
+pub trait IsConstructed {
+    /// ASN.1 constructed bit
+    const CONSTRUCTED: bool;
+}
+
+/// Types which are [`FixedTag`] always known if they are constructed (or primitive).
+impl<T: FixedTag + ?Sized> IsConstructed for T {
+    const CONSTRUCTED: bool = T::TAG.is_constructed();
+}
+
 /// ASN.1 tags.
 ///
 /// Tags are the leading identifier octet of the Tag-Length-Value encoding
@@ -229,7 +240,7 @@ impl Tag {
     }
 
     /// Does this tag represent a constructed (as opposed to primitive) field?
-    pub fn is_constructed(self) -> bool {
+    pub const fn is_constructed(self) -> bool {
         match self {
             Tag::Sequence | Tag::Set => true,
             Tag::Application { constructed, .. }