As the error message states, you cannot compile that code with stable Rust. You need to install nightly Rust and then use it to compile the program:
rustup install nightly
cargo +nightly install racer
See also:
- How to execute cargo test using the nightly channel?
- Is it possible to have multiple coexisting Rust installations?
- Rocket requires a minimum version of Rust nightly, but a higher stable version is already installed