avoid extra alloc on map

-   Before this change the code did:

  StorageRootTargets::new(
      prefix_sets.account_prefix_set.iter().map(|n|
  B256::from_slice(&nibbles.pack())),
      prefix_sets.storage_prefix_sets.clone(),
  ).len()

  StorageRootTargets::new(..) builds a brand-new
  B256Map<PrefixSet> by:

  - cloning every entry in storage_prefix_sets (each PrefixSet can
    hold dozens of nibble prefixes), and
  - allocating a fresh B256Map to hold them.

- We only needed the resulting length, so those clones/allocations
  were immediately thrown away.

crates/trie/parallel/src/proof.rs

@@ -182,11 +182,10 @@ where
         // Extend prefix sets with targets
         let prefix_sets = Self::extend_prefix_sets_with_targets(&self.prefix_sets, &targets);
 
-        let storage_root_targets_len = StorageRootTargets::new(
-            prefix_sets.account_prefix_set.iter().map(|nibbles| B256::from_slice(&nibbles.pack())),
-            prefix_sets.storage_prefix_sets.clone(),
-        )
-        .len();
+        let storage_root_targets_len = StorageRootTargets::count(
+            &prefix_sets.account_prefix_set,
+            &prefix_sets.storage_prefix_sets,
+        );
 
         trace!(
             target: "trie::parallel_proof",

crates/trie/parallel/src/storage_root_targets.rs

@@ -24,6 +24,23 @@ impl StorageRootTargets {
                 .collect(),
         )
     }
+
+    /// Returns the total number of unique storage root targets without allocating new maps.
+    pub fn count(
+        account_prefix_set: &PrefixSet,
+        storage_prefix_sets: &B256Map<PrefixSet>,
+    ) -> usize {
+        let mut count = storage_prefix_sets.len();
+
+        for nibbles in account_prefix_set.iter() {
+            let hashed_address = B256::from_slice(&nibbles.pack());
+            if !storage_prefix_sets.contains_key(&hashed_address) {
+                count += 1;
+            }
+        }
+
+        count
+    }
 }
 
 impl IntoIterator for StorageRootTargets {