Make is_useful handle empty types properly

src/librustc/lint/builtin.rs

@@ -70,6 +70,12 @@ declare_lint! {
     "detects unreachable code paths"
 }
 
+declare_lint! {
+    pub UNREACHABLE_PATTERNS,
+    Warn,
+    "detects unreachable patterns"
+}
+
 declare_lint! {
     pub WARNINGS,
     Warn,
@@ -239,6 +245,7 @@ impl LintPass for HardwiredLints {
             UNUSED_ASSIGNMENTS,
             DEAD_CODE,
             UNREACHABLE_CODE,
+            UNREACHABLE_PATTERNS,
             WARNINGS,
             UNUSED_FEATURES,
             STABLE_FEATURES,