dev: Remove usage of custom concat function in Solidity (#283)

* Remove use of concat entirely

* Reduce diff
Co-authored-by: Mark Tyneway <mark.tyneway@gmail.com>

packages/contracts/contracts/optimistic-ethereum/libraries/rlp/Lib_RLPWriter.sol

@@ -34,7 +34,7 @@ library Lib_RLPWriter {
         if (_in.length == 1 && uint8(_in[0]) < 128) {
             encoded = _in;
         } else {
-            encoded = Lib_BytesUtils.concat(_writeLength(_in.length, 128), _in);
+            encoded = abi.encodePacked(_writeLength(_in.length, 128), _in);
         }
 
         return encoded;
@@ -55,7 +55,7 @@ library Lib_RLPWriter {
         )
     {
         bytes memory list = _flatten(_in);
-        return Lib_BytesUtils.concat(_writeLength(list.length, 192), list);
+        return abi.encodePacked(_writeLength(list.length, 192), list);
     }
 
     /**

packages/contracts/contracts/optimistic-ethereum/libraries/trie/Lib_MerkleTrie.sol

@@ -897,7 +897,7 @@ library Lib_MerkleTrie {
         uint8 offset = uint8(_key.length % 2);
         bytes memory prefixed = new bytes(2 - offset);
         prefixed[0] = bytes1(prefix + offset);
-        return Lib_BytesUtils.concat(prefixed, _key);
+        return abi.encodePacked(prefixed, _key);
     }
 
     /**

packages/contracts/contracts/optimistic-ethereum/libraries/utils/Lib_BytesUtils.sol

@@ -10,84 +10,6 @@ library Lib_BytesUtils {
      * Internal Functions *
      **********************/
 
-    function concat(
-        bytes memory _preBytes,
-        bytes memory _postBytes
-    )
-        internal
-        pure
-        returns (bytes memory)
-    {
-        bytes memory tempBytes;
-
-        assembly {
-            // Get a location of some free memory and store it in tempBytes as
-            // Solidity does for memory variables.
-            tempBytes := mload(0x40)
-
-            // Store the length of the first bytes array at the beginning of
-            // the memory for tempBytes.
-            let length := mload(_preBytes)
-            mstore(tempBytes, length)
-
-            // Maintain a memory counter for the current write location in the
-            // temp bytes array by adding the 32 bytes for the array length to
-            // the starting location.
-            let mc := add(tempBytes, 0x20)
-            // Stop copying when the memory counter reaches the length of the
-            // first bytes array.
-            let end := add(mc, length)
-
-            for {
-                // Initialize a copy counter to the start of the _preBytes data,
-                // 32 bytes into its memory.
-                let cc := add(_preBytes, 0x20)
-            } lt(mc, end) {
-                // Increase both counters by 32 bytes each iteration.
-                mc := add(mc, 0x20)
-                cc := add(cc, 0x20)
-            } {
-                // Write the _preBytes data into the tempBytes memory 32 bytes
-                // at a time.
-                mstore(mc, mload(cc))
-            }
-
-            // Add the length of _postBytes to the current length of tempBytes
-            // and store it as the new length in the first 32 bytes of the
-            // tempBytes memory.
-            length := mload(_postBytes)
-            mstore(tempBytes, add(length, mload(tempBytes)))
-
-            // Move the memory counter back from a multiple of 0x20 to the
-            // actual end of the _preBytes data.
-            mc := end
-            // Stop copying when the memory counter reaches the new combined
-            // length of the arrays.
-            end := add(mc, length)
-
-            for {
-                let cc := add(_postBytes, 0x20)
-            } lt(mc, end) {
-                mc := add(mc, 0x20)
-                cc := add(cc, 0x20)
-            } {
-                mstore(mc, mload(cc))
-            }
-
-            // Update the free-memory pointer by padding our last write location
-            // to 32 bytes: add 31 bytes to the end of tempBytes to move to the
-            // next 32 byte block, then round down to the nearest multiple of
-            // 32. If the sum of the length of the two arrays is zero then add
-            // one before rounding down to leave a blank 32 bytes (the length block with 0).
-            mstore(0x40, and(
-              add(add(end, iszero(add(length, mload(_preBytes)))), 31),
-              not(31) // Round down to the nearest 32 bytes.
-            ))
-        }
-
-        return tempBytes;
-    }
-
     function slice(
         bytes memory _bytes,
         uint256 _start,

packages/contracts/contracts/test-libraries/utils/TestLib_BytesUtils.sol

@@ -19,7 +19,7 @@ contract TestLib_BytesUtils {
         pure
         returns (bytes memory)
     {
-        return Lib_BytesUtils.concat(
+        return abi.encodePacked(
             _preBytes,
             _postBytes
         );

packages/contracts/test/data/json/libraries/utils/Lib_BytesUtils.test.json

 {
     "tests": {
-        "concat": {
-            "two non-empty inputs": {
-                "in": ["0x1234", "0x5678"],
-                "out": ["0x12345678"]
-            },
-            "two empty inputs": {
-                "in": ["0x", "0x"],
-                "out": ["0x"]
-            },
-            "first empty, second non-empty": {
-                "in": ["0x1234", "0x"],
-                "out": ["0x1234"]
-            },
-            "first non-empty, second empty": {
-                "in": ["0x", "0x5678"],
-                "out": ["0x5678"]
-            }
-        },
         "slice": {
             "start zero, length = 0": {
                 "in": ["0x12345678", 0, 0],