Closes #2637. Make z_shieldcoinbase an experimental feature where it

can be enabled with: zcashd -experimentalfeatures -zshieldcoinbase.

src/wallet/rpcwallet.cpp

@@ -3525,9 +3525,16 @@ UniValue z_shieldcoinbase(const UniValue& params, bool fHelp)
     if (!EnsureWalletIsAvailable(fHelp))
         return NullUniValue;
 
+    bool fEnableShieldCoinbase = fExperimentalMode && GetBoolArg("-zshieldcoinbase", false);
+    std::string strDisabledMsg = "";
+    if (!fEnableShieldCoinbase) {
+        strDisabledMsg = "\nWARNING: z_shieldcoinbase is DISABLED but can be enabled as an experimental feature.\n";
+    }
+
     if (fHelp || params.size() < 2 || params.size() > 3)
         throw runtime_error(
             "z_shieldcoinbase \"fromaddress\" \"tozaddress\" ( fee )\n"
+            + strDisabledMsg +
             "\nShield transparent coinbase funds by sending to a shielded zaddr.  This is an asynchronous operation and utxos"
             "\nselected for shielding will be locked.  If there is an error, they are unlocked.  The RPC call `listlockunspent`"
             "\ncan be used to return a list of locked utxos.  The number of coinbase utxos selected for shielding is limited by"
@@ -3549,6 +3556,10 @@ UniValue z_shieldcoinbase(const UniValue& params, bool fHelp)
             "}\n"
         );
 
+    if (!fEnableShieldCoinbase) {
+        throw JSONRPCError(RPC_WALLET_ERROR, "Error: z_shieldcoinbase is disabled.");
+    }
+
     LOCK2(cs_main, pwalletMain->cs_wallet);
 
     // Validate the from address