shardtree: Add `std::error::Error` bound to `ShardStore::Error`

shardtree/src/caching.rs

@@ -1,7 +1,6 @@
 //! Implementation of an in-memory shard store with persistence.
 
 use std::convert::Infallible;
-use std::fmt;
 
 use incrementalmerkletree::Address;
 
@@ -25,7 +24,6 @@ where
     S: ShardStore,
     S::H: Clone,
     S::CheckpointId: Clone + Ord,
-    S::Error: fmt::Display,
 {
     backend: S,
     cache: MemoryShardStore<S::H, S::CheckpointId>,
@@ -37,7 +35,6 @@ where
     S: ShardStore,
     S::H: Clone,
     S::CheckpointId: Clone + Ord,
-    S::Error: fmt::Display,
 {
     /// Loads a `CachingShardStore` from the given backend.
     pub fn load(mut backend: S) -> Result<Self, S::Error> {
@@ -110,7 +107,6 @@ where
     S: ShardStore,
     S::H: Clone,
     S::CheckpointId: Clone + Ord,
-    S::Error: fmt::Display,
 {
     type H = S::H;
     type CheckpointId = S::CheckpointId;

shardtree/src/error.rs

@@ -40,7 +40,7 @@ impl<S: fmt::Display> fmt::Display for ShardTreeError<S> {
 
 impl<SE> std::error::Error for ShardTreeError<SE>
 where
-    SE: fmt::Debug + fmt::Display + std::error::Error + 'static,
+    SE: std::error::Error + 'static,
 {
     fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
         match &self {

shardtree/src/lib.rs

@@ -94,7 +94,7 @@ impl Checkpoint {
 pub trait ShardStore {
     type H;
     type CheckpointId;
-    type Error;
+    type Error: std::error::Error;
 
     /// Returns the subtree at the given root address, if any such subtree exists.
     fn get_shard(

shardtree/src/testing.rs

@@ -150,8 +150,6 @@ impl<
         const DEPTH: u8,
         const SHARD_HEIGHT: u8,
     > testing::Tree<H, C> for ShardTree<S, DEPTH, SHARD_HEIGHT>
-where
-    S::Error: std::fmt::Debug,
 {
     fn depth(&self) -> u8 {
         DEPTH