InheritedStateMutation

An optional detector that highlights all instances where inherited trait variables are directly modified.

Why is it bad?

Traits should provide setter methods to ensure that invariants related to their state are preserved. Directly modifying trait variables (e.g., self.traitVar = 42) can violate these invariants, leading to potential bugs or security vulnerabilities. This detector warns when such direct modifications occur, prompting further review by auditors.

Example

trait T {
  balance: Int;
}

contract C with T {
  balance: Int = 42;
  fun updateBalance() {
    self.balance = 100; // Suspicious: Highlighted by the detector
  }
}

Use instead:

trait T {
  balance: Int;
  fun setBalance(newBalance: Int) {
    require(newBalance > 0, "balance cannot be negative"); // Invariant check
    self.balance = newBalance;
  }
}

contract C with T {
  balance: Int = 42;
  fun updateBalance() {
    self.setBalance(100); // OK: Invariant preserved
  }
}