feat: ParentNode fallible interface (#133)

* ParentNode fallible interface

Merge

* ChildReturn alias

* Warnings

* Refactor

* Update node.rs

* Clean up

* Remove unnecessary trait restrictions

* Update path_iterator.rs

* Comments and test

* Rename to child error

* ChildError

* Add Key to ChildError

* Simplify

* Simplify

* Update merkle_tree.rs

* Refactor

* StroageNodeError

* NodeIsLeaf error for leaf nodes

* thiserror for StorageNodeError and ChildError

* Minor refactor common use statements

* Remove extern crate core

* Clean up use statements

* Update path_iterator.rs

* Update path_iterator.rs

fuel-merkle/src/common.rs

@@ -16,6 +16,7 @@ pub use position::Position;
 pub use storage_map::StorageMap;
 pub use subtree::Subtree;
 
+pub(crate) use node::{ChildError, ChildResult};
 pub(crate) use position_path::PositionPath;
 pub(crate) use prefix::{Prefix, PrefixError};
 

fuel-merkle/src/common/node.rs

@@ -10,9 +10,35 @@ pub trait Node {
     fn height(&self) -> u32;
     fn leaf_key(&self) -> Self::Key;
     fn is_leaf(&self) -> bool;
+    fn is_node(&self) -> bool;
 }
 
-pub trait ParentNode: Node {
-    fn left_child(&self) -> Self;
-    fn right_child(&self) -> Self;
+pub trait ParentNode: Node
+where
+    Self: Sized,
+{
+    type Error;
+
+    fn left_child(&self) -> ChildResult<Self>;
+    fn right_child(&self) -> ChildResult<Self>;
+}
+
+#[allow(type_alias_bounds)]
+pub type ChildResult<T: ParentNode> = Result<T, ChildError<T::Key, T::Error>>;
+
+#[derive(Debug, Clone)]
+#[cfg_attr(feature = "std", derive(thiserror::Error))]
+pub enum ChildError<Key, E> {
+    #[cfg_attr(feature = "std", error("Child with key {0} was not found in storage"))]
+    ChildNotFound(Key),
+    #[cfg_attr(feature = "std", error("Node is a leaf with no children"))]
+    NodeIsLeaf,
+    #[cfg_attr(feature = "std", error(transparent))]
+    Error(E),
+}
+
+impl<Key, E> From<E> for ChildError<Key, E> {
+    fn from(e: E) -> Self {
+        Self::Error(e)
+    }
 }