add a -Z incremental-info flag

src/librustc/session/config.rs

@@ -724,6 +724,8 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
           "attempt to recover from parse errors (experimental)"),
     incremental: Option<String> = (None, parse_opt_string,
           "enable incremental compilation (experimental)"),
+    incremental_info: bool = (false, parse_bool,
+        "print high-level information about incremental reuse (or the lack thereof)"),
     dump_dep_graph: bool = (false, parse_bool,
           "dump the dependency graph to $RUST_DEP_GRAPH (default: /tmp/dep_graph.gv)"),
     query_dep_graph: bool = (false, parse_bool,

src/librustc_trans/back/write.rs

@@ -724,7 +724,7 @@ pub fn run_passes(sess: &Session,
         work_items.push(work);
     }
 
-    if sess.time_passes() && sess.opts.incremental.is_some() {
+    if sess.opts.debugging_opts.incremental_info {
         dump_incremental_data(&trans);
     }