error[E0658]: unadjusted ABI is an implementation detail and perma-unstable
  --> $DIR/feature-gate-abi_unadjusted.rs:1:8
   |
LL | extern "unadjusted" fn foo() {
   |        ^^^^^^^^^^^^
   |
   = help: add `#![feature(abi_unadjusted)]` to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0658`.