Created attachment 445402 [details, diff] rust-1.12.0_beta20160824.ebuild.patch Please find attached an ebuild that builds dev-lang/rust-1.12.0_beta20160824 (which supports sys-devel/llvm-3.9) successfully. I had to adapt the stage0 injection code, as it changed in 1.12, compared to 1.11. I also added a useflag "system-rust", which allows to build using the system installed rust compiler as stage0.
Well 1.12.1 is now in the tree so I think this is obsolete.