error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants
  --> $DIR/issue-43105.rs:13:17
   |
LL | const NUM: u8 = xyz();
   |                 ^^^^^

error[E0080]: could not evaluate constant pattern
  --> $DIR/issue-43105.rs:18:9
   |
LL | const NUM: u8 = xyz();
   |                 ----- calling non-const fn `xyz`
...
LL |         NUM => unimplemented!(),
   |         ^^^

error: aborting due to 2 previous errors

Some errors occurred: E0015, E0080.
For more information about an error, try `rustc --explain E0015`.