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: could not evaluate constant pattern --> $DIR/issue-43105.rs:8:9 | LL | NUM => unimplemented!(), | ^^^ error: aborting due to 3 previous errors For more information about this error, try `rustc --explain E0015`.