**= and //= evaluate complex targets twice

[ylluminate]

**= and //= expand to target = func(target, val), which evaluates the target expression twice. Fine for simple variables, but wrong if the target has side effects.

def f():
    print("called")
    return 0
arr = [1, 2, 3]
arr[f()] **= 2

transpiles to:

arr[f()] = math.powi(arr[f()], 2)

f() gets called twice. Should use a temp variable for complex targets:

tmp := f()
arr[tmp] = math.powi(arr[tmp], 2)

Only affects **= and //= since those are the operators that expand to function calls. Regular +=, -= etc. use V's native operators and don't have this issue.