fn _unstable_fn() {}
#[rustc_const_unstable(feature = "_unstable_const_fn")] // invalid
-fn _unstable_const_fn() {}
+const fn _unstable_const_fn() {}
```
To fix this issue, you need to provide the `issue` field. Example:
feature = "_unstable_const_fn",
issue = "none"
)] // ok!
-fn _unstable_const_fn() {}
+const fn _unstable_const_fn() {}
```
See the [How Rust is Made and “Nightly Rust”][how-rust-made-nightly] appendix