error[E0554]: #![feature] may not be used on the stable release channel Couldn't install racer using cargo

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?

I received error 0554 when trying to compile source code using the stable channel for armv7-unknown-linux-gnueabihf.

It failed because the application uses features not available in the stable channel.

The solution was to install the nightly channel with:

rustup install nightly

And then to compile with:

cargo +nightly build --target=armv7-unknown-linux-gnueabihf

That did it for me.

Don't be tempted to follow the syntax offered when rustup installs the nightly channel, because it won't work:

cargo build --target=nightly-armv7-unknown-linux-gnueabihf