Allow explicit matches on ! without warning

src/test/ui/unreachable/unwarned-match-on-never.rs

@@ -0,0 +1,18 @@
+#![deny(unreachable_code)]
+#![allow(dead_code)]
+
+#![feature(never_type)]
+
+fn foo(x: !) -> bool {
+    // Explicit matches on the never type are unwarned.
+    match x {}
+    // But matches in unreachable code are warned.
+    match x {} //~ ERROR: unreachable expression
+}
+
+fn main() {
+    return;
+    match () { //~ ERROR: unreachable expression
+        () => (),
+    }
+}

src/test/ui/unreachable/unwarned-match-on-never.stderr

@@ -0,0 +1,22 @@
+error: unreachable expression
+  --> $DIR/unwarned-match-on-never.rs:10:5
+   |
+LL |     match x {} //~ ERROR: unreachable expression
+   |     ^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/unwarned-match-on-never.rs:1:9
+   |
+LL | #![deny(unreachable_code)]
+   |         ^^^^^^^^^^^^^^^^
+
+error: unreachable expression
+  --> $DIR/unwarned-match-on-never.rs:15:5
+   |
+LL | /     match () { //~ ERROR: unreachable expression
+LL | |         () => (),
+LL | |     }
+   | |_____^
+
+error: aborting due to 2 previous errors
+