rustc: Output a note about how to debug unexpected failures

Author: brson

src/comp/driver/rustc.rs

@@ -164,6 +164,12 @@ fn monitor(f: fn~(diagnostic::emitter)) {
                     none,
                     diagnostic::ice_msg("unexpected failure"),
                     diagnostic::error);
+                let note = "The compiler hit an unexpected failure path. \
+                            This is a bug. Try running with \
+                            RUST_LOG=rustc=0,::rt::backtrace \
+                            to get further details and report the results \
+                            to github.com/mozilla/rust/issues";
+                diagnostic::emit(none, note, diagnostic::note);
             }
             // Fail so the process returns a failure code
             fail;