Merge pull request #730 from nikomatsakis/never-type-feature-gate

introduce feature gate into never-type test

Author: Centril

src/types/never.md

@@ -8,7 +8,15 @@ computations that never complete. Expressions of type `!` can be coerced into
 any other type.
 
 ```rust,should_panic
+#![feature(never_type)]
 let x: ! = panic!();
 // Can be coerced into any type.
 let y: u32 = x;
 ```
+
+**NB.** The never type was expected to be stabilized in 1.41, but due
+to some last minute regressions detected the stabilization was
+temporarily reverted. The `!` type can only appear in function return
+types presently. See [the tracking
+issue](https://github.com/rust-lang/rust/issues/35121) for more
+details.