Auto merge of #2183 - ebfull:rust-introduction, r=ebfull

Introduce Rust/librustzcash into the zcash build system

Introduces Rust into the build system and brings a trivial xor operation into the code (near where we will likely be working).

src/pow.cpp

@@ -16,6 +16,10 @@
 
 #include "sodium.h"
 
+#ifdef ENABLE_RUST
+#include "librustzcash.h"
+#endif // ENABLE_RUST
+
 unsigned int GetNextWorkRequired(const CBlockIndex* pindexLast, const CBlockHeader *pblock, const Consensus::Params& params)
 {
     unsigned int nProofOfWorkLimit = UintToArith256(params.powLimit).GetCompact();
@@ -96,6 +100,14 @@ bool CheckEquihashSolution(const CBlockHeader *pblock, const CChainParams& param
     // H(I||V||...
     crypto_generichash_blake2b_update(&state, (unsigned char*)&ss[0], ss.size());
 
+    #ifdef ENABLE_RUST
+    // Ensure that our Rust interactions are working in production builds. This is
+    // temporary and should be removed.
+    {
+        assert(librustzcash_xor(0x0f0f0f0f0f0f0f0f, 0x1111111111111111) == 0x1e1e1e1e1e1e1e1e);
+    }
+    #endif // ENABLE_RUST
+
     bool isValid;
     EhIsValidSolution(n, k, state, pblock->nSolution, isValid);
     if (!isValid)