warning: `extern` declarations without an explicit ABI are deprecated
  --> $DIR/feature-gate-explicit-extern-abis.rs:27:1
   |
LL | extern fn _foo() {}
   | ^^^^^^ help: explicitly specify the "C" ABI: `extern "C"`
   |
   = note: `#[warn(missing_abi)]` on by default

warning: `extern` declarations without an explicit ABI are deprecated
  --> $DIR/feature-gate-explicit-extern-abis.rs:33:8
   |
LL | unsafe extern fn _bar() {}
   |        ^^^^^^ help: explicitly specify the "C" ABI: `extern "C"`

warning: `extern` declarations without an explicit ABI are deprecated
  --> $DIR/feature-gate-explicit-extern-abis.rs:39:8
   |
LL | unsafe extern {}
   |        ^^^^^^ help: explicitly specify the "C" ABI: `extern "C"`

warning: 3 warnings emitted