Finds places where you're using QString::left(1) instead of QString::at(0).
The later form is cheaper, as it doesn't deep-copy the string.
There's however another difference between the two: left(1) will return an empty
string if the string is empty, while QString::at(0) will assert. So be sure
that the string can't be empty, or add a if (!str.isEmpty() guard, which is still
faster than calling left() for the cases which deep-copy.