src/test/ui/parser/mod_file_not_exist.rs

// ignore-windows

mod not_a_real_file; //~ ERROR file not found for module `not_a_real_file`
//~^ HELP name the file either not_a_real_file.rs or not_a_real_file/mod.rs inside the directory

fn main() {
    assert_eq!(mod_file_aux::bar(), 10);
}