Surviving the expat upgrade on Gentoo February 28th, 2007
Moving to ~x86 on Gentoo Linux requires an upgrade to dev-libs/expat-2.0.0 from dev-libs/expat-1.95.8.
Unfortunately, the expat maintainers broke its ABI between 1.95.8 and 2.0.0 without bumping the library version number. This means the Gentoo ebuild maintainer can't easily slot the versions for happy coexistence.
Whether you've already upgraded dev-libs/expat and are now stuck, or you're still using a version earlier than 2.0.0 and are about to upgrade, this will get you over the hump:
cd /usr/lib emerge -1 '<expat-2' cp libexpat.so.1.5.0 libexpat-1.95.x.so.1.5.0 emerge -1 '>=expat-2' ln -fs libexpat-1.95.x.so.1.5.0 libexpat.so.0
Then, one day when your initial ~x86 upgrade is complete, you can go back and remove libexpat-1.95.x.so.1.5.0 and libexpat.so.0 from /usr/lib once you've used revdep-rebuild --library=libexpat.so.0 -pv to verify that the pre-release library has no remaining dependencies.
This helped me a lot. In my setup the previous version of expat produced a library with the number 0.5.0 instead of 1.5.0 (perhaps because I use the stable branch of portage instead of the testing branch). So I had to adapt the suggestions above just slightly. Thank you!