Enable feature gate for deny(non_exhaustive_omitted_patterns)

    error: unknown lint: `non_exhaustive_omitted_patterns`
       --> impl/src/declaration.rs:297:43
        |
    297 |         #[cfg_attr(all(test, exhaustive), deny(non_exhaustive_omitted_patterns))]
        |                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: the `non_exhaustive_omitted_patterns` lint is unstable
        = note: see issue #89554 <rust-lang/rust#89554> for more information
        = help: add `#![feature(non_exhaustive_omitted_patterns_lint)]` to the crate attributes to enable
        = note: `-D unknown-lints` implied by `-D warnings`

Author: dtolnay

impl/src/lib.rs

@@ -1,3 +1,4 @@
+#![cfg_attr(all(test, exhaustive), feature(non_exhaustive_omitted_patterns_lint))]
 #![allow(
     clippy::cast_possible_truncation, // https://github.com/rust-lang/rust-clippy/issues/7486
     clippy::needless_pass_by_value,