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

I'm trying to install racer using cargo, so I executed the command cargo install racer in the terminal and it resulted in the error:

error[E0554]: #![feature] may not be used on the stable release channel
--> /home/rajkumar/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-0.1.2/src/lib.rs:47:34
|
47 | #![cfg_attr(feature = "nightly", feature(macro_vis_matcher))]
|                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0554]: #![feature] may not be used on the stable release channel
--> /home/rajkumar/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-0.1.2/src/lib.rs:48:34
|
48 | #![cfg_attr(feature = "nightly", feature(allow_internal_unstable))]
|                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0554`.
error: failed to compile `racer v2.1.10`, intermediate artifacts can be found at `/tmp/cargo-install5YWPWW`

Caused by:
Could not compile `scoped-tls`.

To learn more, run the command again with --verbose.

Below are my Rust details:

$rustc --version
rustc 1.30.0 (da5f414c2 2018-10-24)

> rustup --version 
rustup 1.14.0 (1e51b07cc 2018-10-04)

> cargo --version 
cargo 1.30.0 (36d96825d 2018-10-24)

Below is my opensuse version details:

> cat /usr/lib/os-release 
NAME="openSUSE Tumbleweed"
# VERSION="20181029"
ID="opensuse-tumbleweed"
ID_LIKE="opensuse suse"
VERSION_ID="20181029"
PRETTY_NAME="openSUSE Tumbleweed"
ANSI_COLOR="0;32"
CPE_NAME="cpe:/o:opensuse:tumbleweed:20181029"
BUG_REPORT_URL="https://bugs.opensuse.org"
HOME_URL="https://www.opensuse.org/"

Why am I not able to install racer using cargo? Am I missing anything?

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

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