Add a test for uninhabitedness changes

Author: varkor

src/test/ui/always-inhabited-union-ref.rs

@@ -0,0 +1,29 @@
+#![feature(exhaustive_patterns)]
+#![feature(never_type)]
+
+#![allow(dead_code)]
+#![allow(unreachable_code)]
+
+pub union Foo {
+    foo: !,
+}
+
+fn uninhab_ref() -> &'static ! {
+    unimplemented!()
+}
+
+fn uninhab_union() -> Foo {
+    unimplemented!()
+}
+
+fn match_on_uninhab() {
+    match uninhab_ref() {
+        //~^ ERROR non-exhaustive patterns: type &'static ! is non-empty
+    }
+
+    match uninhab_union() {
+        //~^ ERROR non-exhaustive patterns: type Foo is non-empty
+    }
+}
+
+fn main() {}

src/test/ui/always-inhabited-union-ref.stderr

@@ -0,0 +1,27 @@
+error[E0004]: non-exhaustive patterns: type &'static ! is non-empty
+  --> $DIR/always-inhabited-union-ref.rs:20:11
+   |
+LL |     match uninhab_ref() {
+   |           ^^^^^^^^^^^^^
+   |
+help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+  --> $DIR/always-inhabited-union-ref.rs:20:11
+   |
+LL |     match uninhab_ref() {
+   |           ^^^^^^^^^^^^^
+
+error[E0004]: non-exhaustive patterns: type Foo is non-empty
+  --> $DIR/always-inhabited-union-ref.rs:24:11
+   |
+LL |     match uninhab_union() {
+   |           ^^^^^^^^^^^^^^^
+   |
+help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
+  --> $DIR/always-inhabited-union-ref.rs:24:11
+   |
+LL |     match uninhab_union() {
+   |           ^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0004`.