error[E0015]: cannot call non-const fn `xyz` in constants
  --> $DIR/issue-43105.rs:3:17
   |
LL | const NUM: u8 = xyz();
   |                 ^^^^^
   |
   = note: calls in constants are limited to constant functions, tuple structs and tuple variants

error: could not evaluate constant pattern
  --> $DIR/issue-43105.rs:8:9
   |
LL |         NUM => unimplemented!(),
   |         ^^^

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0015`.