test: add sablier fees tests

src/SablierFees.sol

@@ -9,6 +9,8 @@ import { Adminable } from "./Adminable.sol";
 /// @title SablierFees
 /// @notice See the documentation in {ISablierFees}.
 abstract contract SablierFees is Adminable, ISablierFees {
+    constructor(address initialAdmin) Adminable(initialAdmin) { }
+
     /// @inheritdoc ISablierFees
     function collectFees() external override {
         uint256 feeAmount = address(this).balance;

tests/mocks/Receive.sol

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity >=0.8.22 <0.9.0;
+
+contract ContractWithoutReceive { }
+
+contract ContractWithReceive {
+    receive() external payable { }
+}

tests/mocks/SablierFeesMock.sol

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity >=0.8.22;
+
+import { SablierFees } from "src/SablierFees.sol";
+
+contract SablierFeesMock is SablierFees {
+    constructor(address initialAdmin) SablierFees(initialAdmin) { }
+}

tests/unit/Unit.t.sol

@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity >=0.8.22 <0.9.0;
+
+import { CommonBase } from "../Base.t.sol";
+import { ContractWithoutReceive, ContractWithReceive } from "../mocks/Receive.sol";
+
+abstract contract Unit_Test is CommonBase {
+    address internal admin;
+    address internal eve;
+
+    ContractWithoutReceive internal contractWithoutReceive;
+    ContractWithReceive internal contractWithReceive;
+
+    function setUp() public virtual override {
+        CommonBase.setUp();
+
+        admin = createUser("admin");
+        eve = createUser("eve");
+        contractWithoutReceive = new ContractWithoutReceive();
+        contractWithReceive = new ContractWithReceive();
+
+        resetPrank(admin);
+    }
+}

tests/unit/sablier-fees/collectFees.t.sol

@@ -0,0 +1,81 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity >=0.8.22 <0.9.0;
+
+import { ISablierFees } from "src/interfaces/ISablierFees.sol";
+import { Errors } from "src/libraries/Errors.sol";
+
+import { Unit_Test } from "../Unit.t.sol";
+import { SablierFeesMock } from "../../mocks/SablierFeesMock.sol";
+
+contract CollectFees_Unit_Concrete_Test is Unit_Test {
+    SablierFeesMock internal sablierFeesMock;
+
+    function setUp() public virtual override {
+        Unit_Test.setUp();
+
+        sablierFeesMock = new SablierFeesMock(admin);
+    }
+
+    function test_GivenAdminIsNotContract() external {
+        _test_CollectFees(admin);
+    }
+
+    modifier givenAdminIsContract() {
+        _;
+    }
+
+    function test_RevertGiven_AdminDoesNotImplementReceiveFunction() external givenAdminIsContract {
+        // Transfer the admin to a contract that does not implement the receive function.
+        resetPrank({ msgSender: admin });
+        sablierFeesMock.transferAdmin(address(contractWithoutReceive));
+
+        // Make the contract the caller.
+        resetPrank({ msgSender: address(contractWithoutReceive) });
+
+        // Expect a revert.
+        vm.expectRevert(
+            abi.encodeWithSelector(
+                Errors.SablierFees_FeeTransferFail.selector,
+                address(contractWithoutReceive),
+                address(sablierFeesMock).balance
+            )
+        );
+
+        // Collect the fees.
+        sablierFeesMock.collectFees();
+    }
+
+    function test_GivenAdminImplementsReceiveFunction() external givenAdminIsContract {
+        // Transfer the admin to a contract that implements the receive function.
+        resetPrank({ msgSender: admin });
+        sablierFeesMock.transferAdmin(address(contractWithReceive));
+
+        // Make the contract the caller.
+        resetPrank({ msgSender: address(contractWithReceive) });
+
+        // Run the tests.
+        _test_CollectFees(address(contractWithReceive));
+    }
+
+    function _test_CollectFees(address admin) private {
+        uint256 fee = 1 ether;
+
+        // Set the contract balance to 1 ETH.
+        vm.deal({ account: address(sablierFeesMock), newBalance: 1 ether });
+
+        // Load the initial ETH balance of the admin.
+        uint256 initialAdminBalance = admin.balance;
+
+        // It should emit a {CollectFees} event.
+        vm.expectEmit({ emitter: address(sablierFeesMock) });
+        emit ISablierFees.CollectFees({ admin: admin, feeAmount: fee });
+
+        sablierFeesMock.collectFees();
+
+        // It should transfer the fee.
+        assertEq(admin.balance, initialAdminBalance + fee, "admin ETH balance");
+
+        // It should decrease contract balance to zero.
+        assertEq(address(sablierFeesMock).balance, 0, "sablierFeesMock ETH balance");
+    }
+}

tests/unit/sablier-fees/collectFees.tree

@@ -0,0 +1,12 @@
+CollectFees_Unit_Concrete_Test
+├── given admin is not contract
+│  ├── it should transfer fee
+│  ├── it should decrease contract balance to zero
+│  └── it should emit a {CollectFees} event
+└── given admin is contract
+   ├── given admin does not implement receive function
+   │  └── it should revert
+   └── given admin implements receive function
+      ├── it should transfer fee
+      ├── it should decrease contract balance to zero
+      └── it should emit a {CollectFees} event