address complex type clippy warning in merk cache

grovedb/src/merk_cache.rs

@@ -15,13 +15,17 @@ use crate::{Error, GroveDb, Transaction};
 
 type TxMerk<'db> = Merk<PrefixedRocksDbTransactionContext<'db>>;
 
+/// We store Merk on heap to preserve its location as well as borrow flag
+/// alongside.
+type CachedMerkEntry<'db> = Box<(Cell<bool>, TxMerk<'db>)>;
+
 /// Structure to keep subtrees open in memory for repeated access.
 pub(crate) struct MerkCache<'db, 'b, B: AsRef<[u8]>> {
     db: &'db GroveDb,
     pub(crate) version: &'db GroveVersion,
     batch: Box<StorageBatch>,
     tx: &'db Transaction<'db>,
-    merks: UnsafeCell<BTreeMap<SubtreePathBuilder<'b, B>, Box<(Cell<bool>, TxMerk<'db>)>>>,
+    merks: UnsafeCell<BTreeMap<SubtreePathBuilder<'b, B>, CachedMerkEntry<'db>>>,
 }
 
 impl<'db, 'b, B: AsRef<[u8]>> MerkCache<'db, 'b, B> {
@@ -171,7 +175,7 @@ pub(crate) struct MerkHandle<'db, 'c> {
     taken_handle: &'c Cell<bool>,
 }
 
-impl<'db, 'c> MerkHandle<'db, 'c> {
+impl<'db> MerkHandle<'db, '_> {
     pub(crate) fn for_merk<T>(&mut self, f: impl FnOnce(&mut TxMerk<'db>) -> T) -> T {
         if self.taken_handle.get() {
             panic!("Attempt to have double &mut borrow on Merk");