Revert "der: remove Reader::read_value (#1887)"

This reverts commit 5277fcf.

Author: tarcieri

der/src/asn1/internal_macros.rs

@@ -151,8 +151,8 @@ macro_rules! impl_custom_class {
                     return Err(reader.error(header.tag.non_canonical_error()).into());
                 }
 
-                // read_nested checks if header matches decoded length
-                let value = reader.read_nested(header.length, |reader| {
+                // read_value checks if header matches decoded length
+                let value = reader.read_value(header, |reader| {
                     // Decode inner IMPLICIT value
                     T::decode_value(reader, header)
                 })?;
@@ -192,7 +192,7 @@ macro_rules! impl_custom_class {
                     Tag::$class_enum_name { number, .. } => Ok(Self {
                         tag_number: number,
                         tag_mode: TagMode::default(),
-                        value: reader.read_nested(header.length, |reader| {
+                        value: reader.read_value(header, |reader| {
                             // Decode inner tag-length-value of EXPLICIT
                             T::decode(reader)
                         })?,

der/src/decode.rs

@@ -68,7 +68,7 @@ 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)?;
-        reader.read_nested(header.length, |r| T::decode_value(r, header))
+        reader.read_value(header, |r| T::decode_value(r, header))
     }
 }
 

der/src/reader.rs

@@ -32,6 +32,18 @@ pub trait Reader<'r>: Clone {
         E: From<Error>,
         F: FnOnce(&mut Self) -> Result<T, E>;
 
+    /// Read a value (i.e. the "V" part of a "TLV" field) using the provided header.
+    ///
+    /// This calls the provided function `f` with a nested reader created using
+    /// [`Reader::read_nested`].
+    fn read_value<T, F, E>(&mut self, header: Header, f: F) -> Result<T, E>
+    where
+        E: From<Error>,
+        F: FnOnce(&mut Self) -> Result<T, E>,
+    {
+        self.read_nested(header.length, f)
+    }
+
     /// Attempt to read data borrowed directly from the input as a slice,
     /// updating the internal cursor position.
     ///
@@ -192,7 +204,7 @@ pub trait Reader<'r>: Clone {
     {
         let header = Header::decode(self)?;
         header.tag.assert_eq(Tag::Sequence)?;
-        self.read_nested(header.length, f)
+        self.read_value(header, f)
     }
 
     /// Obtain a slice of bytes containing a complete TLV production suitable for parsing later.