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

If the cached data can't be loaded from disk, just issue a warning to
the user so they know why compilation is taking longer than usual but
don't fail the entire compilation since we can recover by ignorning the
on disk cache.

In the same way, if the disk cache can't be deserialized (because it has
been corrupted for some reason), report the issue as a warning and
continue without failing the compilation. `Decodable::decode()` tends to
panic with various errors like "entered unreachable code" or "index out
of range" if the input data is corrupted. Work around this by catching
panics from the `decode()` calls when joining the thread and continuing
without the cached data.

Fixes #48847

4 files changed, 7 insertions, 2 deletions

diff --git a/src/librustc_driver/driver.rs b/src/librustc_driver/driver.rs
index a3115544f30..c6ebc992680 100644
--- a/src/librustc_driver/driver.rs
+++ b/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)
diff --git a/src/librustc_incremental/lib.rs b/src/librustc_incremental/lib.rs
index d7ccf9d5562..4b524287ca1 100644
--- a/src/librustc_incremental/lib.rs
+++ b/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;
diff --git a/src/librustc_incremental/persist/load.rs b/src/librustc_incremental/persist/load.rs
index 38468e29427..44d6e532f79 100644
--- a/src/librustc_incremental/persist/load.rs
+++ b/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) {
diff --git a/src/librustc_incremental/persist/mod.rs b/src/librustc_incremental/persist/mod.rs
index 2f864aaefba..755a550b5bc 100644
--- a/src/librustc_incremental/persist/mod.rs
+++ b/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;