error[E0603]: module `sys` is private
  --> $DIR/stability-in-private-module.rs:2:18
   |
LL |     let _ = std::sys::os::errno();
   |                  ^^^      ----- function `errno` is not publicly re-exported
   |                  |
   |                  private module
   |
note: the module `sys` is defined here
  --> $SRC_DIR/std/src/lib.rs:LL:COL

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0603`.