Install to /usr/lib/rust/<ver>

ver=<ver>
cp /usr/lib/rust/$ver/bin/{rustc,rustc-$ver}
ln -s /usr/lib/rust/$ver/bin/rustc-$ver /usr/bin/rustc-$ver
touch /etc/env.d/rust/provider-rust-$ver
emerge -1 app-eselect/eselect-rust
eselect rust set rust-$ver
emerge -O1 dev-lang/rust
(might need to set CC=clang and CXX=clang++ for initial build...)