racer: rust-racer failed to install: (error el-get: cargo el-get could not build rust-racer [cargo build --release])

• Arch linux

• rustc 1.26.2 (594fb253c 2018-06-01)

• cargo 1.26.0 (0e7c5a931 2018-04-06)

• stable channel

• emacs package manager: el-get

   Compiling cfg-if v0.1.2
   Compiling nodrop v0.1.12
   Compiling memoffset v0.2.1
   Compiling scopeguard v0.3.3
   Compiling lazy_static v1.0.0
   Compiling libc v0.2.39
   Compiling rustc-rayon-core v0.1.0
   Compiling pkg-config v0.3.11
   Compiling cc v1.0.15
   Compiling smallvec v0.6.1
   Compiling stable_deref_trait v1.0.0
   Compiling unicode-xid v0.0.4
   Compiling bitflags v1.0.1
   Compiling byteorder v1.2.3
   Compiling void v1.0.2
   Compiling matches v0.1.6
   Compiling quote v0.3.15
   Compiling either v1.5.0
   Compiling unicode-xid v0.1.0
   Compiling regex v0.2.10
   Compiling serde v1.0.62
   Compiling unicode-normalization v0.1.7
   Compiling ucd-util v0.1.1
   Compiling rustc-ap-serialize v151.0.0
   Compiling foreign-types-shared v0.1.1
error[E0554]: #![feature] may not be used on the stable release channel
  --> /home/crandel/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-ap-serialize-151.0.0/lib.rs:23:1
   |
23 | #![feature(rustc_private, box_syntax)]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0554]: #![feature] may not be used on the stable release channel
  --> /home/crandel/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-ap-serialize-151.0.0/lib.rs:24:1
   |
24 | #![feature(core_intrinsics)]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error[E0554]: #![feature] may not be used on the stable release channel
  --> /home/crandel/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-ap-serialize-151.0.0/lib.rs:25:1
   |
25 | #![feature(specialization)]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to 3 previous errors

For more information about this error, try `rustc --explain E0554`.
error: Could not compile `rustc-ap-serialize`.
warning: build failed, waiting for other jobs to finish...
error: build failed