ctb: Add echidna tests for Burn

.changeset/pretty-chicken-rest.md

+---
+'@eth-optimism/ci-builder': patch
+'@eth-optimism/contracts-bedrock': patch
+---
+
+Add echidna tests for Burn

.circleci/config.yml

@@ -817,6 +817,10 @@ workflows:
           echidna_target: aliasing
           requires:
             - bedrock-echidna-build
+      - bedrock-echidna-run:
+          echidna_target: burn
+          requires:
+            - bedrock-echidna-build
       - op-bindings-build:
           requires:
             - yarn-monorepo

packages/contracts-bedrock/contracts/echidna/FuzzBurn.sol

+pragma solidity 0.8.15;
+
+import { Burn } from "../libraries/Burn.sol";
+
+contract EchidnaFuzzBurn {
+    bool failedEthBurn;
+    bool failedGasBurn;
+
+    /**
+     * @notice Takes an integer amount of eth to burn through the Burn library and
+     * updates the contract state if an incorrect amount of eth moved from the contract
+     */
+    function testBurn(uint256 _value) public {
+        // cache the contract's eth balance
+        uint256 preBurnBalance = address(this).balance;
+
+        // execute a burn of _value eth
+        // (may way to add guardrails to this value rather than a truly unbounded uint256)
+        Burn.eth(_value);
+
+        // check that exactly _value eth was transfered from the contract
+        if (address(this).balance != preBurnBalance - _value) {
+            failedEthBurn = true;
+        }
+    }
+
+    /**
+     * @notice Takes an integer amount of gas to burn through the Burn library and
+     * updates the contract state if at least that amount of gas was not burned
+     * by the library
+     */
+    function testGas(uint256 _value) public {
+        // cache the contract's current remaining gas
+        uint256 preBurnGas = gasleft();
+
+        // execute the gas burn
+        Burn.gas(_value);
+
+        // cache the remaining gas post burn
+        uint256 postBurnGas = gasleft();
+
+        // check that at least _value gas was burnt
+        if (postBurnGas > preBurnGas - _value) {
+            failedGasBurn = true;
+        }
+    }
+
+    function echidna_burn_eth() public view returns (bool) {
+        // ASSERTION: The amount burned should always match the amount passed exactly
+        return !failedEthBurn;
+    }
+
+    function echidna_burn_gas() public view returns (bool) {
+        // ASSERTION: The amount of gas burned should be strictly greater than the
+        // the amount passed as _value (minimum _value + whatever minor overhead to
+        // the value after the call)
+        return !failedGasBurn;
+    }
+}