Allow building without 'parallel' feature (#2)

(overeager) backport of https://github.com/AleoHQ/snarkVM/pull/1172

Cargo.toml

@@ -113,6 +113,7 @@ utilities = [ "snarkvm-utilities" ]
 parameters_no_std_out = [ "snarkvm-parameters/no_std_out" ]
 parallel = [
   "rayon",
+  "snarkvm-algorithms/parallel",
   "snarkvm-compiler/parallel",
   "snarkvm-fields/parallel",
   "snarkvm-utilities/parallel"
@@ -123,6 +124,7 @@ noconfig = [ ]
 path = "./algorithms"
 version = "0.9.0"
 optional = true
+default-features = false
 
 [dependencies.snarkvm-circuit]
 path = "./circuit"
@@ -167,6 +169,7 @@ optional = true
 [dependencies.snarkvm-compiler]
 path = "./compiler"
 version = "0.9.0"
+default-features = false
 
 [dependencies.anyhow]
 version = "1.0.64"

compiler/src/ledger/mod.rs

@@ -218,7 +218,11 @@ impl<N: Network, B: BlockStorage<N>, P: ProgramStorage<N>> Ledger<N, B, P> {
         ledger.block_tree.append(&hashes)?;
 
         // Safety check the existence of every block.
-        (0..=latest_height).into_par_iter().try_for_each(|height| {
+        #[cfg(feature = "parallel")]
+        let heights_iter = (0..=latest_height).into_par_iter();
+        #[cfg(not(feature = "parallel"))]
+        let mut heights_iter = (0..=latest_height).into_iter();
+        heights_iter.try_for_each(|height| {
             ledger.get_block(height)?;
             Ok::<_, Error>(())
         })?;
@@ -517,8 +521,12 @@ impl<N: Network, B: BlockStorage<N>, P: ProgramStorage<N>> Ledger<N, B, P> {
             bail!("Cannot validate a block with more than {} transactions", Transactions::<N>::MAX_TRANSACTIONS);
         }
 
-        // Ensure each transaction is well-formed.
-        if !block.transactions().par_iter().all(|(_, transaction)| self.vm.verify(transaction)) {
+        // Ensure each transaction is well-formed and unique.
+        #[cfg(feature = "parallel")]
+        let transactions_iter = block.transactions().par_iter();
+        #[cfg(not(feature = "parallel"))]
+        let mut transactions_iter = block.transactions().iter();
+        if !transactions_iter.all(|(_, transaction)| self.vm.verify(transaction)) {
             bail!("Invalid transaction found in the transactions list");
         }