Revert "Remove never_type feature gate"

This reverts commit 533fb9c.

Author: dtolnay

serde/src/lib.rs

@@ -82,7 +82,7 @@
 // discussion of these features please refer to this issue:
 //
 //    https://github.com/serde-rs/serde/issues/812
-#![cfg_attr(feature = "unstable", feature(specialization))]
+#![cfg_attr(feature = "unstable", feature(specialization, never_type))]
 #![allow(unknown_lints, bare_trait_objects, deprecated)]
 #![cfg_attr(feature = "cargo-clippy", allow(renamed_and_removed_lints))]
 #![cfg_attr(feature = "cargo-clippy", deny(clippy, clippy_pedantic))]

test_suite/tests/test_de.rs

@@ -1,4 +1,5 @@
 #![allow(clippy::decimal_literal_representation, clippy::unreadable_literal)]
+#![cfg_attr(feature = "unstable", feature(never_type))]
 
 use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
 use std::default::Default;

test_suite/tests/test_ser.rs

@@ -1,4 +1,5 @@
 #![allow(clippy::unreadable_literal)]
+#![cfg_attr(feature = "unstable", feature(never_type))]
 
 use std::cell::RefCell;
 use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};