fn _unstable_fn() {}
#[rustc_const_unstable(feature = "_unstable_const_fn", issue = "0")] // invalid
-fn _unstable_const_fn() {}
+const fn _unstable_const_fn() {}
```
To fix this issue, you need to provide a correct value in the `issue` field.
fn _unstable_fn() {}
#[rustc_const_unstable(feature = "_unstable_const_fn", issue = "1")] // ok!
-fn _unstable_const_fn() {}
+const fn _unstable_const_fn() {}
```
See the [How Rust is Made and “Nightly Rust”][how-rust-made-nightly] appendix