error: my error message
  --> $DIR/env-not-defined-custom.rs:1:13
   |
LL | fn main() { env!("__HOPEFULLY_NOT_DEFINED__", "my error message"); }
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = 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