The limitation is rust specific, because there are a lot of places in the code which assume that `/usr/bin/rustc`, `/usr/lib/` and `/usr/lib/rustc/` all share the same prefix (`/usr` in this case). I don't really know what the point is of setting this to a location outside of the standard prefix. Also see my comment about the GNU specs here: https://github.com/mozilla/rust/issues/5223#issuecomment-30455086