Created attachment 451518 [details, diff] rust-1.12.1.ebuild patch dev-lang/rust could've been build with llvm:-multitarget for a while (https://github.com/gentoo/gentoo-rust/issues/196) and with the recent LLVM_TARGETS change this requirement complicates updates.
Thanks for the report. Fixed in http://gitweb.gentoo.org/repo/gentoo.git/commit/?id=4f6cd6184c7dd680a1e74dda900b39b2fc03170a