https://mirror.xyz/horsefacts.eth/Jex2YVaO65dda6zEyfM_-DXlXhOWCAoSpOx5PLocYgw

https://docs.soliditylang.org/en/v0.8.18/types.html#function-types

https://book.getfoundry.sh/forge/invariant-testing#actor-management

Call summary

$ forge test -vv -m invariant_callSummary

Running 1 test for test/WETH9.invariants.t.sol:WETH9Invariants
[PASS] invariant_callSummary() (runs: 1000, calls: 15000, reverts: 11)
Logs:
  Call summary:
  deposit 5
  withdraw 7
  sendFallback 3

Test result: ok. 1 passed; 0 failed; finished in 4.42s

Although we performed 1000 runs, the summary printed here is a snapshot of calls made during the final run.

The total number of calls in our summary should always be the same as the depth parameter set for invariant tests in foundry.toml

Reusing actors

Running 1 test for test/WETH9.invariants.t.sol:WETH9Invariants
[PASS] invariant_callSummary() (runs: 2000, calls: 50000, reverts: 2)
Logs:
  Call summary:
  deposit 5
  withdraw 8
  sendFallback 12
  zero withdrawals 3

Including transfers

Note that we call bound twice in transferFrom to ensure the transfer value is less than the from account's balance and that currentActor has a sufficient allowance to perform the third-party transfer.

If you look carefully at this, you may notice we have a similar problem to the zero amount issue we just solved for withdraw: even though we're reusing known callers, most of the time amount will be zero, since it's unlikely the caller has an approval from the from account. (You can use the same call summary process to debug yourself if you're interested).

Running 5 tests for test/WETH9.invariants.t.sol:WETH9Invariants
[PASS] invariant_callSummary() (runs: 1000, calls: 25000, reverts: 49)
[PASS] invariant_conservationOfETH() (runs: 1000, calls: 25000, reverts: 49)
[PASS] invariant_depositorBalances() (runs: 1000, calls: 25000, reverts: 49)
[PASS] invariant_solvencyBalances() (runs: 1000, calls: 25000, reverts: 49)
[PASS] invariant_solvencyDeposits() (runs: 1000, calls: 25000, reverts: 49)
Test result: ok. 5 passed; 0 failed; finished in 21.35s

Testing our tests

We can introduce bugs manually like so:

    function deposit() public payable {
        // balanceOf[msg.sender] += msg.value;
        balanceOf[msg.sender] += 1;
        emit Deposit(msg.sender, msg.value);
    }

https://en.wikipedia.org/wiki/Mutation_testing

foundry-rs/foundry#478

Accounting for selfdestruct

Running 5 tests for test/WETH9.invariants.t.sol:WETH9Invariants
[PASS] invariant_callSummary() (runs: 1000, calls: 25000, reverts: 55)
[PASS] invariant_conservationOfETH() (runs: 1000, calls: 25000, reverts: 55)
[PASS] invariant_depositorBalances() (runs: 1000, calls: 25000, reverts: 55)
[PASS] invariant_solvencyBalances() (runs: 1000, calls: 25000, reverts: 55)
[PASS] invariant_solvencyDeposits() (runs: 1000, calls: 25000, reverts: 55)
Test result: ok. 5 passed; 0 failed; finished in 19.99s

More resources

Maple Finance invariant tests repo

invariant-examples repo

Invariant Testing in the Foundry Book