add snapLastCall

.forge-snapshots/singleSstoreLastCall.snap

@@ -0,0 +1 @@
+43429

src/GasSnapshot.sol

@@ -55,7 +55,6 @@ contract GasSnapshot is Script {
     }
 
     /// @notice Snapshot the given external closure
-    /// @dev most accurate as storage cost semantics are not involved
     function snap(string memory name, function() external fn) internal {
         uint256 gasBefore = gasleft();
         fn();
@@ -68,7 +67,6 @@ contract GasSnapshot is Script {
     }
 
     /// @notice Snapshot the given internal closure
-    /// @dev most accurate as storage cost semantics are not involved
     function snap(string memory name, function() internal fn) internal {
         uint256 gasBefore = gasleft();
         fn();
@@ -80,6 +78,17 @@ contract GasSnapshot is Script {
         }
     }
 
+    /// @notice Snapshot using forge isolate of gas of the previous call
+    /// @dev most accurate as this uses a complete transaction and no storage semantics
+    function snapLastCall(string memory name) internal {
+        uint256 gasUsed = vm.lastCallGas().gasTotalUsed;
+        if (check) {
+            _checkSnapshot(name, gasUsed);
+        } else {
+            _writeSnapshot(name, gasUsed);
+        }
+    }
+
     /// @notice Start a snapshot with the given name
     /// @dev The next call to `snapEnd` will end the snapshot
     function snapStart(string memory name) internal {

src/test/SimpleOperations.sol

@@ -16,6 +16,10 @@ contract SimpleOperations {
         }
     }
 
+    function singleSstore() public {
+        test = block.timestamp + 3;
+    }
+
     function manySstore() public {
         for (uint256 i = 0; i < 100; i++) {
             test = i + 2;

test/GasSnapshot.t.sol

@@ -29,6 +29,15 @@ contract GasSnapshotTest is Test, GasSnapshot {
         assertEq(value, "5247");
     }
 
+    function testAddLastCall() public {
+        simpleOperations.singleSstore();
+        snapLastCall("singleSstoreLastCall");
+
+        string memory value = vm.readLine(".forge-snapshots/singleSstoreLastCall.snap");
+        // includes 21,000 overhead for transaction, 20,000 clean SSTORE
+        assertEq(value, "43429");
+    }
+
     function testAddClosure() public {
         snap("addClosure", simpleOperations.add);