Deprecate unreachable function

This is designed to merged after servo#162, as it deprecates
a function that should be no longer necessary on newer
Rust versions.

.travis.yml

@@ -1,6 +1,6 @@
 language: rust
 rust:
-  - 1.20.0
+  - 1.36.0
   - nightly
   - beta
   - stable

lib.rs

@@ -132,10 +132,9 @@ macro_rules! smallvec {
 ///
 /// Equivalent to `std::hint::unreachable_unchecked` but works in older versions of Rust.
 #[inline]
+#[deprecated(note = "Use std::hint::unreachable_unchecked instead")]
 pub unsafe fn unreachable() -> ! {
-    enum Void {}
-    let x: &Void = mem::transmute(1usize);
-    match *x {}
+    std::hint::unreachable_unchecked()
 }
 
 /// `panic!()` in debug builds, optimization hint in release.
@@ -144,7 +143,7 @@ macro_rules! debug_unreachable {
     () => { debug_unreachable!("entered unreachable code") };
     ($e:expr) => {
         if cfg!(not(debug_assertions)) {
-            unreachable();
+            std::hint::unreachable_unchecked();
         } else {
             panic!($e);
         }