Assert state change

Severity: Med ID: assert-state-change

Flags expressions inside assert() that modify contract state.

What it does

Warns when an assert() argument contains a state-mutating operation: a pre- or post-increment/decrement (++/--) on a state variable, an assignment (=, +=,etc.) to a state variable, a delete of a state variable, or a call to a function that writes state variables.

Why is this bad?

assert() is meant for invariant checking, conditions that should never be false if the contract is correct. require() is for input validation and conditions that may legitimately fail at runtime. Mixing a state mutation into an assert() argument conflates these two concerns:

• If the assertion fails, the mutation is reverted, but the failure indicates a bug in the contract itself, not a recoverable condition.
• If the assertion passes, the mutation happens as a side effect of the check, making the code harder to reason about.

The correct pattern is to perform the mutation first, then assert the post-condition as a true invariant. For user-facing validation use require() instead.

Example

Bad

uint256 public counter;

// Side effect buried inside an invariant check
function increment(uint256 expected) external {
    assert(++counter == expected);
}

// _deposit() writes state as a side effect of the assert
function depositAndAssert() external payable {
    assert(_deposit());
}

Good

uint256 public counter;

// Mutate first, then assert the post-condition
function increment(uint256 expected) external {
    counter++;
    assert(counter == expected);
}

// Use require() for validation that also performs work
function deposit() external payable {
    bool ok = _deposit();
    require(ok, "deposit failed");
}