ver=<ver>
sudo rsync -rvP build/install-rustc-$ver/ /opt/rust-bin-$ver

sudo emerge -1 app-eselect/eselect-rust
sudo ln -s /opt/rust-bin-$ver/bin/rustc /usr/bin/rustc-bin-$ver
sudo mkdir -p /etc/env.d/rust
sudo touch /etc/env.d/rust/provider-rust-bin-$ver
sudo eselect rust set rust-bin-$ver
sudo mkdir -p /var/db/pkg/dev-lang/rust-bin-$ver
echo 0 | sudo tee /var/db/pkg/dev-lang/rust-bin-$ver/SLOT > /dev/null

sudo emerge dev-lang/rust
(might need to set CC=clang and CXX=clang++ for initial build...)

sudo rm -rf /opt/rust-bin-$ver /usr/bin/rustc-bin-$ver
sudo rm -rf /etc/env.d/rust/provider-rust-bin-$ver
sudo rm -rf /var/db/pkg/dev-lang/rust-bin-$ver