zcash_client_backend: Add `proto::service::TreeState::to_chain_state`

zcash_client_backend/CHANGELOG.md

@@ -24,12 +24,14 @@ and this library adheres to Rust's notion of
   - `ORCHARD_SHARD_HEIGHT`
   - `BlockMetadata::orchard_tree_size`
   - `WalletSummary::next_orchard_subtree_index`
+  - `chain::ChainState`
   - `chain::ScanSummary::{spent_orchard_note_count, received_orchard_note_count}`
 - `zcash_client_backend::fees`:
   - `orchard`
   - `ChangeValue::orchard`
 - `zcash_client_backend::proto`:
   - `service::TreeState::orchard_tree`
+  - `service::TreeState::to_chain_state`
   - `impl TryFrom<&CompactOrchardAction> for CompactAction`
   - `CompactOrchardAction::{cmx, nf, ephemeral_key}`
 - `zcash_client_backend::scanning`:

zcash_client_backend/src/proto.rs

@@ -20,7 +20,7 @@ use zcash_primitives::{
 };
 
 use crate::{
-    data_api::InputSource,
+    data_api::{chain::ChainState, InputSource},
     fees::{ChangeValue, TransactionBalance},
     proposal::{Proposal, ProposalError, ShieldedInputs, Step, StepOutput, StepOutputIndex},
     zip321::{TransactionRequest, Zip321Error},
@@ -290,6 +290,20 @@ impl service::TreeState {
             &orchard_tree_bytes[..],
         )
     }
+
+    /// Parses this tree state into a [`ChainState`] for use with [`scan_cached_blocks`].
+    ///
+    /// [`scan_cached_blocks`]: crate::data_api::chain::scan_cached_blocks
+    pub fn to_chain_state(&self) -> io::Result<ChainState> {
+        Ok(ChainState::new(
+            self.height
+                .try_into()
+                .map_err(|_| io::Error::new(io::ErrorKind::InvalidData, "Invalid block height"))?,
+            self.sapling_tree()?.to_frontier(),
+            #[cfg(feature = "orchard")]
+            self.orchard_tree()?.to_frontier(),
+        ))
+    }
 }
 
 /// Constant for the V1 proposal serialization version.