You can use the RTL viewer. Any attempt to find "idx" will turn up nothing. The variable "idx" never actually makes it into the netlist.
If you right click on the warning message and click "Help" you'll get the help info for the warning message. Part of the help file says the following:
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In some cases, you may receive this warning unexpectedly. These unexpected, and sometimes false, warnings occur because Quartus II Integrated Synthesis does not eliminate false paths prior to checking for potential combinational loops.
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Jake