Automatically enable -Zorbit if -Zincremental is specified.

src/librustc/session/config.rs

@@ -1143,7 +1143,15 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
         })
     });
 
-    let debugging_opts = build_debugging_options(matches, error_format);
+    let mut debugging_opts = build_debugging_options(matches, error_format);
+
+    // Incremental compilation only works reliably when translation is done via
+    // MIR, so let's enable -Z orbit if necessary (see #34973).
+    if debugging_opts.incremental.is_some() && !debugging_opts.orbit {
+        early_warn(error_format, "Automatically enabling `-Z orbit` because \
+                                  `-Z incremental` was specified");
+        debugging_opts.orbit = true;
+    }
 
     let parse_only = debugging_opts.parse_only;
     let no_trans = debugging_opts.no_trans;

src/test/compile-fail/enable-orbit-for-incr-comp.rs

@@ -0,0 +1,20 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// ignore-pretty
+// compile-flags:-Zincremental=tmp/cfail-tests/enable-orbit-for-incr-comp -Zorbit=off
+// error-pattern:Automatically enabling `-Z orbit` because `-Z incremental` was specified
+
+#![deny(warnings)]
+
+fn main() {
+    FAIL! // We just need some compilation error. What we really care about is
+          // that the error pattern above is checked.
+}