error: environment variable `CARGO__HOPEFULLY_NOT_DEFINED__` not defined at compile time
  --> $DIR/extenv-not-defined-default.rs:2:5
   |
LL |     env!("CARGO__HOPEFULLY_NOT_DEFINED__");
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: Cargo sets build script variables at run time. Use `std::env::var("CARGO__HOPEFULLY_NOT_DEFINED__")` instead
   = note: this error originates in the macro `env` (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to 1 previous error