Fix failing vec hashing test

eth2/utils/ssz/src/cached_tree_hash.rs

@@ -195,15 +195,18 @@ impl TreeHashCache {
         Ok(())
     }
 
-    pub fn chunk_equals(&mut self, chunk: usize, other: &[u8]) -> Result<bool, Error> {
+    pub fn get_chunk(&self, chunk: usize) -> Result<&[u8], Error> {
         let start = chunk * BYTES_PER_CHUNK;
         let end = start + BYTES_PER_CHUNK;
 
         Ok(self
             .cache
             .get(start..end)
-            .ok_or_else(|| Error::NoModifiedFieldForChunk(chunk))?
+            .ok_or_else(|| Error::NoModifiedFieldForChunk(chunk))?)
-            == other)
+    }
+
+    pub fn chunk_equals(&mut self, chunk: usize, other: &[u8]) -> Result<bool, Error> {
+        Ok(self.get_chunk(chunk)? == other)
     }
 
     pub fn changed(&self, chunk: usize) -> Result<bool, Error> {
@@ -218,15 +221,11 @@ impl TreeHashCache {
     }
 
     pub fn hash_children(&self, children: (&usize, &usize)) -> Result<Vec<u8>, Error> {
-        let start = children.0 * BYTES_PER_CHUNK;
+        let mut child_bytes = Vec::with_capacity(BYTES_PER_CHUNK * 2);
-        let end = start + BYTES_PER_CHUNK * 2;
+        child_bytes.append(&mut self.get_chunk(*children.0)?.to_vec());
+        child_bytes.append(&mut self.get_chunk(*children.1)?.to_vec());
 
-        let children = &self
+        Ok(hash(&child_bytes))
-            .cache
-            .get(start..end)
-            .ok_or_else(|| Error::NoChildrenForHashing((*children.0, *children.1)))?;
-
-        Ok(hash(children))
     }
 
     pub fn into_merkle_tree(self) -> Vec<u8> {

eth2/utils/ssz/src/cached_tree_hash/impls.rs

@@ -133,14 +133,13 @@ where
                         // The item existed in the previous list and exsits in the current list.
                         (Some(old), Some(new)) => {
                             new.cached_hash_tree_root(old, cache, *start_chunk)?;
-                        },
+                        }
                         // The item didn't exist in the old list and doesn't exist in the new list,
                         // nothing to do.
-                        (None, None) => {},
+                        (None, None) => {}
-                        _ => panic!("variable sized lists not implemented")
+                        _ => panic!("variable sized lists not implemented"),
                     };
                 }
-                // this thing
             }
         }