error[E0796]: reference of mutable static is disallowed
  --> $DIR/reference-of-mut-static-safe.rs:9:14
   |
LL |     let _x = &X;
   |              ^^ reference of mutable static
   |
   = note: mutable statics can be written to by multiple threads: aliasing violations or data races will cause undefined behavior
help: shared references are dangerous since if there's any kind of mutation of that static while the reference lives, that's UB; use `addr_of!` instead to create a raw pointer
   |
LL |     let _x = addr_of!(X);
   |              ~~~~~~~~~~~

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0796`.