[incremental] Don't panic if decoding the cache fails

src/librustc_driver/driver.rs

@@ -901,7 +901,9 @@ pub fn phase_2_configure_and_expand_inner<'a, F>(sess: &'a Session,
         Some(future) => {
             let prev_graph = time(sess, "blocked while dep-graph loading finishes", || {
                 future.open()
-                      .expect("Could not join with background dep_graph thread")
+                      .unwrap_or_else(|e| rustc_incremental::LoadResult::Error {
+                          message: format!("could not decode incremental cache: {:?}", e)
+                      })
                       .open(sess)
             });
             DepGraph::new(prev_graph)

src/librustc_incremental/lib.rs

@@ -39,6 +39,7 @@ pub use assert_dep_graph::assert_dep_graph;
 pub use persist::dep_graph_tcx_init;
 pub use persist::load_dep_graph;
 pub use persist::load_query_result_cache;
+pub use persist::LoadResult;
 pub use persist::save_dep_graph;
 pub use persist::save_trans_partition;
 pub use persist::save_work_products;

src/librustc_incremental/persist/load.rs

@@ -89,7 +89,8 @@ impl LoadResult<PreviousDepGraph> {
     pub fn open(self, sess: &Session) -> PreviousDepGraph {
         match self {
             LoadResult::Error { message } => {
-                sess.fatal(&message) /* never returns */
+                sess.warn(&message);
+                PreviousDepGraph::new(SerializedDepGraph::new())
             },
             LoadResult::DataOutOfDate => {
                 if let Err(err) = delete_all_session_dir_contents(sess) {

src/librustc_incremental/persist/mod.rs

@@ -27,6 +27,7 @@ pub use self::fs::prepare_session_directory;
 pub use self::load::dep_graph_tcx_init;
 pub use self::load::load_dep_graph;
 pub use self::load::load_query_result_cache;
+pub use self::load::LoadResult;
 pub use self::save::save_dep_graph;
 pub use self::save::save_work_products;
 pub use self::work_product::save_trans_partition;