error: any use of this value will cause an error
  --> $DIR/issue-51559.rs:4:33
   |
LL | pub const FOO: usize = unsafe { BAR as usize };
   | --------------------------------^^^^^^^^^^^^---
   |                                 |
   |                                 "pointer-to-integer cast" needs an rfc before being allowed inside constants
   |
   = note: `#[deny(const_err)]` on by default

error: aborting due to previous error