error[E0580]: main function has wrong type
  --> $DIR/extern-main-fn.rs:1:1
   |
LL | extern fn main() {}
   | ^^^^^^^^^^^^^^^^ expected "Rust" fn, found "C" fn
   |
   = note: expected type `fn()`
              found type `extern "C" fn()`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0580`.