When I run XST for the first time it works fine, but when I run it a second time the following error occurs:
"ERROR:HDLParsers:3354 - my_file.vhd" Current unit uses a unit which was discarded. (LRM 11.1)"
This error occurs because of an issue with the VHDL Parser. To work around the problem, remove the "work" directory before running synthesis again.
This problem will be fixed in 11.1.