der: add DecodeValue::decode_value_nested

der/src/asn1/any.rs

@@ -275,7 +275,7 @@ mod allocating {
 
         fn decode<R: Reader<'a>>(reader: &mut R) -> Result<Self, Error> {
             let header = Header::decode(reader)?;
-            read_value(reader, header, Self::decode_value)
+            read_value(reader, header, Self::decode_value_nested)
         }
     }
 

der/src/asn1/internal_macros.rs

@@ -122,6 +122,18 @@ macro_rules! impl_custom_class {
                 Ok(Some(Self::decode(reader)?))
             }
 
+
+            /// Decode inner tag-length-value of `EXPLICIT` field.
+            fn decode_explicit_inner<'a, R: Reader<'a>>(
+                reader: &mut R,
+                header: Header
+            ) -> Result<T, T::Error>
+            where
+                T: Decode<'a>
+            {
+                reader.read_nested(header.length(), |r| T::decode(r))
+            }
+
             #[doc = concat!("Attempt to decode an `IMPLICIT` ASN.1 `", $asn1_class_name, "` field with the")]
             /// provided [`TagNumber`].
             ///
@@ -154,7 +166,7 @@ macro_rules! impl_custom_class {
                 }
 
                 // read_value checks if header matches decoded length
-                let value = crate::reader::read_value(reader, header, T::decode_value)?;
+                let value = crate::reader::read_value(reader, header, T::decode_value_nested)?;
 
                 Ok(Some(Self {
                     tag_number,
@@ -191,10 +203,7 @@ macro_rules! impl_custom_class {
                     Tag::$class_enum_name { number, .. } => Ok(Self {
                         tag_number: number,
                         tag_mode: TagMode::default(),
-                        value: crate::reader::read_value(reader, header, |reader, _| {
-                            // Decode inner tag-length-value of EXPLICIT
-                            T::decode(reader)
-                        })?,
+                        value: crate::reader::read_value(reader, header, Self::decode_explicit_inner)?,
                     }),
                     tag => Err(reader.error(tag.unexpected_error(None)).into())
                 }

der/src/decode.rs

@@ -73,7 +73,7 @@ where
     fn decode<R: Reader<'a>>(reader: &mut R) -> Result<T, <T as DecodeValue<'a>>::Error> {
         let header = Header::decode(reader)?;
         header.tag().assert_eq(T::TAG)?;
-        read_value(reader, header, T::decode_value)
+        read_value(reader, header, T::decode_value_nested)
     }
 }
 
@@ -135,6 +135,16 @@ pub trait DecodeValue<'a>: Sized {
 
     /// Attempt to decode this message using the provided [`Reader`].
     fn decode_value<R: Reader<'a>>(reader: &mut R, header: Header) -> Result<Self, Self::Error>;
+
+    /// Attempt to decode this message using the nested [`Reader`], using [`Reader::read_nested`] wrapper.
+    ///
+    /// For primitive types, this function can be overridden to [`DecodeValue::decode_value`] directly.
+    fn decode_value_nested<R: Reader<'a>>(
+        reader: &mut R,
+        header: Header,
+    ) -> Result<Self, Self::Error> {
+        reader.read_nested(header.length(), |r| Self::decode_value(r, header))
+    }
 }
 
 #[cfg(feature = "alloc")]

der/src/reader.rs

@@ -195,7 +195,7 @@ pub trait Reader<'r>: Clone {
     {
         let header = Header::decode(self)?;
         header.tag().assert_eq(Tag::Sequence)?;
-        read_value(self, header, |r, _| f(r))
+        read_value(self, header, |r, h| r.read_nested(h.length(), |r| f(r)))
     }
 
     /// Obtain a slice of bytes containing a complete TLV production suitable for parsing later.
@@ -219,7 +219,7 @@ where
     #[cfg(feature = "ber")]
     let header = header.with_length(header.length().sans_eoc());
 
-    let ret = reader.read_nested(header.length(), |r| f(r, header))?;
+    let ret = f(reader, header)?;
 
     // Consume EOC marker if the length is indefinite.
     #[cfg(feature = "ber")]