#![feature] may not be used on the stable release channel

[lelongg]

ArrayDeque used to compile but suddenly I'm stuck with this error. Does anyone know where it came from ?

error[E0554]: `#![feature]` may not be used on the stable release channel
  --> /home/sisyphe/.cargo/registry/src/github.com-1ecc6299db9ec823/arraydeque-0.4.5/src/lib.rs:57:32
   |
57 | #![cfg_attr(has_union_feature, feature(untagged_unions))]
   |