dump statistics about re-use w/ -Z time-passes

It's nice to get a rough idea of how much work we're
saving.

src/librustc_trans/back/write.rs

@@ -724,6 +724,10 @@ pub fn run_passes(sess: &Session,
         work_items.push(work);
     }
 
+    if sess.time_passes() && sess.opts.incremental.is_some() {
+        dump_incremental_data(&trans);
+    }
+
     // Process the work items, optionally using worker threads.
     // NOTE: This code is not really adapted to incremental compilation where
     //       the compiler decides the number of codegen units (and will
@@ -901,6 +905,17 @@ pub fn run_passes(sess: &Session,
     }
 }
 
+fn dump_incremental_data(trans: &CrateTranslation) {
+    let mut reuse = 0;
+    for mtrans in trans.modules.iter() {
+        match mtrans.source {
+            ModuleSource::Preexisting(..) => reuse += 1,
+            ModuleSource::Translated(..) => (),
+        }
+    }
+    println!("incremental: re-using {} out of {} modules", reuse, trans.modules.len());
+}
+
 struct WorkItem {
     mtrans: ModuleTranslation,
     config: ModuleConfig,