ArgCopyMutation

A detector that highlights cases where function argument mutations are ineffective due to call-by-value semantics in Tact.

Why is it bad?

In Tact, function arguments are passed by value, meaning that any mutations applied to these arguments will only affect the local copy of the variable within the function. Such mutations are unobservable outside the function, except for potentially increasing gas consumption or causing exceptions.

Example

fun addEntry(m: map<Int,Int>) {
  m.set(1, 10); // Bad: Mutating the copy
}

Use instead:

fun addEntry() {
  self.m.set(1, 10); // OK: Changing contract's state
}

Alternatively, you could redesign the method:

fun generateNewValue(): Int {
  // ... produce new value for the map
  return self.nextValue + 1;
}

m.set(self.nextKey, self.generateNewValue()); // OK