| error[E0658]: the `#[no_core]` attribute is an experimental feature | 
 |   --> $DIR/cfg-attr-multi-invalid-2.rs:4:29 | 
 |    | | 
 | LL | #![cfg_attr(broken, no_std, no_core)] | 
 |    |                             ^^^^^^^ | 
 |    | | 
 |    = note: see issue #29639 <https://github.com/rust-lang/rust/issues/29639> for more information | 
 |    = help: add `#![feature(no_core)]` to the crate attributes to enable | 
 |    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date | 
 |  | 
 | error: aborting due to 1 previous error | 
 |  | 
 | For more information about this error, try `rustc --explain E0658`. |