Add unstable-options flag to stage!=0

src/bootstrap/bin/rustc.rs

@@ -316,6 +316,11 @@ fn main() {
         }
     }
 
+    // This is required for internal lints.
+    if stage != "0" {
+        cmd.arg("-Zunstable-options");
+    }
+
     // Force all crates compiled by this compiler to (a) be unstable and (b)
     // allow the `rustc_private` feature to link to other unstable crates
     // also in the sysroot. We also do this for host crates, since those