chore: add MaxSizeError::ZSTSequenceNotArray

to make `BorshSchema` more congruent with `BorshSerialize, BorshDeserialize`

borsh/src/schema_helpers.rs

@@ -53,6 +53,9 @@ pub enum MaxSizeError {
     /// Some of the declared types were lacking definition making it impossible
     /// to calculate the size.
     MissingDefinition,
+
+    /// sequences of zero sized types of dynamic length are forbidden by definition
+    ZSTSequenceNotArray,
 }
 
 /// Returns the largest possible size of a serialised object based solely on its type.
@@ -187,6 +190,9 @@ fn max_serialized_size_impl<'a>(
             }
         }
         Ok(Definition::Sequence { elements }) => {
+            if is_zero_size(elements, schema) {
+                return Err(MaxSizeError::ZSTSequenceNotArray);
+            }
             // Assume that sequence has MAX_LEN elements since that’s the most
             // it can have.
             let sz = max_serialized_size_impl(MAX_LEN, elements, schema, stack)?;
@@ -286,7 +292,7 @@ mod tests {
         test_ok::<String>(4 + MAX_LEN);
 
         test_err::<Vec<Vec<u8>>>(MaxSizeError::Overflow);
-        test_ok::<Vec<Vec<()>>>(4 + MAX_LEN * 4);
+        test_err::<Vec<Vec<()>>>(MaxSizeError::ZSTSequenceNotArray);
         test_ok::<[[[(); MAX_LEN]; MAX_LEN]; MAX_LEN]>(0);
     }
 

borsh/tests/test_zero_sized_types.rs

@@ -37,6 +37,22 @@ fn test_serialize_vec_of_zst() {
     assert_eq!(res.unwrap_err().to_string(), ERROR_ZST_FORBIDDEN);
 }
 
+#[test]
+fn test_serialize_vec_of_unit_type() {
+    let v = vec![(), (), ()];
+    let res = to_vec(&v);
+    assert_eq!(res.unwrap_err().to_string(), ERROR_ZST_FORBIDDEN);
+}
+
+#[test]
+fn test_serialize_vec_of_vec_of_unit_type() {
+    let v: Vec<Vec<()>> = vec![vec![(), (), ()]];
+    let res = to_vec(&v);
+    assert_eq!(res.unwrap_err().to_string(), ERROR_ZST_FORBIDDEN);
+}
+
+
+
 #[test]
 fn test_deserialize_vec_deque_of_zst() {
     let v = [0u8, 0u8, 0u8, 64u8];