Address clippy lints in tree-states (#4414)

* Address some clippy lints

* Box errors to fix error size lint

* Add Default impl for Validator

* Address more clippy lints

* Re-implement `check_state_diff`

* Fix misc test compile errors

testing/ef_tests/src/case_result.rs

@@ -2,7 +2,8 @@ use super::*;
 use compare_fields::{CompareFields, Comparison, FieldComparison};
 use std::fmt::Debug;
 use std::path::{Path, PathBuf};
-use types::{beacon_state::BeaconStateDiff, milhouse::diff::Diff, BeaconState};
+use store::hdiff::{HDiff, HDiffBuffer};
+use types::BeaconState;
 
 pub const MAX_VALUE_STRING_LEN: usize = 500;
 
@@ -121,15 +122,28 @@ where
 pub fn check_state_diff<T: EthSpec>(
     pre_state: &BeaconState<T>,
     opt_post_state: &Option<BeaconState<T>>,
+    spec: &ChainSpec,
 ) -> Result<(), Error> {
     if let Some(post_state) = opt_post_state {
-        let diff = BeaconStateDiff::compute_diff(pre_state, post_state)
-            .expect("BeaconStateDiff should compute");
-        let mut diffed_state = pre_state.clone();
-        diff.apply_diff(&mut diffed_state)
-            .expect("BeaconStateDiff should apply");
+        // Produce a diff between the pre- and post-states.
+        let pre_state_buf = HDiffBuffer::from_state(pre_state.clone());
+        let post_state_buf = HDiffBuffer::from_state(post_state.clone());
+        let diff = HDiff::compute(&pre_state_buf, &post_state_buf).expect("HDiff should compute");
 
-        compare_result_detailed::<_, ()>(&Ok(diffed_state), opt_post_state)
+        // Apply the diff to the pre-state, ensuring the same post-state is
+        // regenerated.
+        let mut reconstructed_buf = HDiffBuffer::from_state(pre_state.clone());
+        diff.apply(&mut reconstructed_buf)
+            .expect("HDiff should apply");
+        let diffed_state = reconstructed_buf
+            .into_state(spec)
+            .expect("HDiffDiffer should convert to state");
+
+        // Drop the caches on the post-state to assist with equality checking.
+        let mut post_state_without_caches = post_state.clone();
+        post_state_without_caches.drop_all_caches().unwrap();
+
+        compare_result_detailed::<_, ()>(&Ok(diffed_state), &Some(post_state_without_caches))
     } else {
         Ok(())
     }