compiler/rustc_error_codes/src/error_codes/E0554.md

   1 Feature attributes are only allowed on the nightly release channel. Stable or
   2 beta compilers will not comply.
   3
   4 Erroneous code example:
   5
   6 ```ignore (depends on release channel)
   7 #![feature(non_ascii_idents)] // error: `#![feature]` may not be used on the
   8                               //        stable release channel
   9 ```
  10
  11 If you need the feature, make sure to use a nightly release of the compiler
  12 (but be warned that the feature may be removed or altered in the future).