I was emerging world after the harvester merge. I encountered a blocker related to boost and was trying to reduce the number of packages to emerge by running a few individually for a cleaner bug report on that issue.
>>> Running pre-merge checks for sys-kernel/debian-sources-6.4.13_p1 * Checking for at least 6 GiB disk space at "/var/tmp/portage/portage/sys-kernel/debian-sources-6.4.13_p1/temp" ... [ ok ] * ERROR: sys-kernel/debian-sources-6.4.13_p1::core-kit failed (pretend phase): * Currently, btrfs is unsupported in our binary kernel/initramfs. * * Call stack: * ebuild.sh, line 93: Called pkg_pretend * debian-sources-6.4.13_p1.ebuild, line 91: Called die * The specific snippet of code: * use $unsupported && die "Currently, $unsupported is unsupported in our binary kernel/initramfs."