Declare an object `const` or `constexpr` unless you want to modify its value later on

Reason

That way you can't change the value by mistake. That way might offer the compiler optimization opportunities.

Example

void f(int n)
{
    const int bufmax = 2 * n + 2;  // good: we can't change bufmax by accident
    int xmax = n;                  // suspicious: is xmax intended to change?
    // ...
}

Enforcement

Look to see if a variable is actually mutated, and flag it if not. Unfortunately, it might be impossible to detect when a non-const was not intended to vary (vs when it merely did not vary).