der: accept the documented maximum nesting depth

der/src/reader/position.rs

@@ -74,7 +74,7 @@ impl Position {
     /// A [`Resumption`] value which can be used to continue parsing the outer message.
     pub(super) fn split_nested(&mut self, len: Length) -> Result<Resumption> {
         match self.depth.checked_add(1) {
-            Some(depth) if depth < Self::MAX_DEPTH => self.depth = depth,
+            Some(depth) if depth <= Self::MAX_DEPTH => self.depth = depth,
             _ => return Err(self.error(ErrorKind::NestingDepth)),
         }
 

der/tests/nesting.rs

@@ -15,11 +15,9 @@ fn walk<'a>(reader: &mut SliceReader<'a>) -> Result<AnyRef<'a>, Error> {
     }
 }
 
-#[test]
+/// Wrap an innermost `NULL` in `depth` nested `SEQUENCE`s.
 #[allow(clippy::cast_possible_truncation, reason = "test")]
-fn returns_nesting_depth_error_when_max_depth_encountered() {
-    let depth = MAX_DEPTH + 1;
-
+fn nested_sequences(depth: usize) -> Vec<u8> {
     let mut buf = vec![0x05, 0x00]; // innermost NULL
     for _ in 0..depth {
         let len = buf.len();
@@ -43,7 +41,19 @@ fn returns_nesting_depth_error_when_max_depth_encountered() {
         next.extend_from_slice(&buf);
         buf = next;
     }
+    buf
+}
+
+#[test]
+fn accepts_documented_max_depth() {
+    let buf = nested_sequences(MAX_DEPTH);
+    let mut reader = SliceReader::new(&buf).unwrap();
+    walk(&mut reader).expect("MAX_DEPTH levels of nesting should decode");
+}
 
+#[test]
+fn returns_nesting_depth_error_when_max_depth_encountered() {
+    let buf = nested_sequences(MAX_DEPTH + 1);
     let mut reader = SliceReader::new(&buf).unwrap();
     let err = walk(&mut reader).expect_err("should return ErrorKind::NestingDepth");
     assert_eq!(err.kind(), ErrorKind::NestingDepth);