cannon: Port step post-checks to MIPS2.sol (#12815)

packages/contracts-bedrock/snapshots/abi/MIPS2.json

@@ -45,7 +45,7 @@
     "outputs": [
       {
         "internalType": "bytes32",
-        "name": "",
+        "name": "postState_",
         "type": "bytes32"
       }
     ],

packages/contracts-bedrock/snapshots/semver-lock.json

@@ -144,8 +144,8 @@
     "sourceCodeHash": "0xd8467700c80b3e62fa37193dc6513bac35282094b686b50e162e157f704dde00"
   },
   "src/cannon/MIPS2.sol": {
-    "initCodeHash": "0xaedf0d0b0e94a0c5e7d987331d2fdba84230f5704a6ca33677e70cde7051b17e",
-    "sourceCodeHash": "0x9fa2d1297ad1e93b4d3c5c0fed08bedcd8f746807589f0fd3369e79347c6a027"
+    "initCodeHash": "0xc38c76ab3aad78c81ca01b3235b402614972d6604b22fda1e870f1bf47be1194",
+    "sourceCodeHash": "0x3d38b1924669d1bde756f1306601c764a6d31f428ac72667a3dd194b3388210d"
   },
   "src/cannon/MIPS64.sol": {
     "initCodeHash": "0x93aa8d7f9fd3c22276c0d303a3fefdf8f73cc55807b35e483bba64c92d02aaef",

packages/contracts-bedrock/src/cannon/MIPS2.sol

@@ -60,8 +60,8 @@ contract MIPS2 is ISemver {
     }
 
     /// @notice The semantic version of the MIPS2 contract.
-    /// @custom:semver 1.0.0-beta.21
-    string public constant version = "1.0.0-beta.21";
+    /// @custom:semver 1.0.0-beta.22
+    string public constant version = "1.0.0-beta.22";
 
     /// @notice The preimage oracle contract.
     IPreimageOracle internal immutable ORACLE;
@@ -102,7 +102,39 @@ contract MIPS2 is ISemver {
     /// the current thread stack.
     /// @param _localContext The local key context for the preimage oracle. Optional, can be set as a constant
     ///                      if the caller only requires one set of local keys.
-    function step(bytes calldata _stateData, bytes calldata _proof, bytes32 _localContext) public returns (bytes32) {
+    /// @return postState_ The hash of the post state witness after the state transition.
+    function step(
+        bytes calldata _stateData,
+        bytes calldata _proof,
+        bytes32 _localContext
+    )
+        public
+        returns (bytes32 postState_)
+    {
+        postState_ = doStep(_stateData, _proof, _localContext);
+        assertPostStateChecks();
+    }
+
+    function assertPostStateChecks() internal pure {
+        State memory state;
+        assembly {
+            state := STATE_MEM_OFFSET
+        }
+
+        bytes32 activeStack = state.traverseRight ? state.rightThreadStack : state.leftThreadStack;
+        if (activeStack == EMPTY_THREAD_ROOT) {
+            revert("MIPS2: post-state active thread stack is empty");
+        }
+    }
+
+    function doStep(
+        bytes calldata _stateData,
+        bytes calldata _proof,
+        bytes32 _localContext
+    )
+        internal
+        returns (bytes32)
+    {
         unchecked {
             State memory state;
             ThreadState memory thread;

packages/contracts-bedrock/src/cannon/interfaces/IMIPS2.sol

@@ -46,7 +46,13 @@ interface IMIPS2 is ISemver {
     error InvalidRMWInstruction();
 
     function oracle() external view returns (IPreimageOracle oracle_);
-    function step(bytes memory _stateData, bytes memory _proof, bytes32 _localContext) external returns (bytes32);
+    function step(
+        bytes memory _stateData,
+        bytes memory _proof,
+        bytes32 _localContext
+    )
+        external
+        returns (bytes32 postState_);
 
     function __constructor__(IPreimageOracle _oracle) external;
 }