From 455e6140d81f48d99d43573382c67f804434a949 Mon Sep 17 00:00:00 2001
From: Bastian Kauschke <bastian_kauschke@hotmail.de>
Date: Sun, 19 Jul 2020 15:45:44 +0200
Subject: [PATCH] do not try fetching the ancestors of errored trait impls

---
 src/librustc_middle/traits/specialization_graph.rs       | 4 +++-
 .../min_specialization/impl-on-nonexisting.rs            | 7 +++++++
 .../min_specialization/impl-on-nonexisting.stderr        | 9 +++++++++
 3 files changed, 19 insertions(+), 1 deletion(-)
 create mode 100644 src/test/ui/specialization/min_specialization/impl-on-nonexisting.rs
 create mode 100644 src/test/ui/specialization/min_specialization/impl-on-nonexisting.stderr

diff --git a/src/librustc_middle/traits/specialization_graph.rs b/src/librustc_middle/traits/specialization_graph.rs
index f4961617b81c6..c9aae8980076f 100644
--- a/src/librustc_middle/traits/specialization_graph.rs
+++ b/src/librustc_middle/traits/specialization_graph.rs
@@ -1,5 +1,6 @@
 use crate::ich::{self, StableHashingContext};
 use crate::ty::fast_reject::SimplifiedType;
+use crate::ty::fold::TypeFoldable;
 use crate::ty::{self, TyCtxt};
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
@@ -226,7 +227,8 @@ pub fn ancestors(
     start_from_impl: DefId,
 ) -> Result<Ancestors<'tcx>, ErrorReported> {
     let specialization_graph = tcx.specialization_graph_of(trait_def_id);
-    if specialization_graph.has_errored {
+
+    if specialization_graph.has_errored || tcx.type_of(start_from_impl).references_error() {
         Err(ErrorReported)
     } else {
         Ok(Ancestors {
diff --git a/src/test/ui/specialization/min_specialization/impl-on-nonexisting.rs b/src/test/ui/specialization/min_specialization/impl-on-nonexisting.rs
new file mode 100644
index 0000000000000..77a64320d6f37
--- /dev/null
+++ b/src/test/ui/specialization/min_specialization/impl-on-nonexisting.rs
@@ -0,0 +1,7 @@
+#![feature(min_specialization)]
+
+trait Trait {}
+impl Trait for NonExistent {}
+//~^ ERROR cannot find type `NonExistent` in this scope
+
+fn main() {}
diff --git a/src/test/ui/specialization/min_specialization/impl-on-nonexisting.stderr b/src/test/ui/specialization/min_specialization/impl-on-nonexisting.stderr
new file mode 100644
index 0000000000000..b032ccbe53ffc
--- /dev/null
+++ b/src/test/ui/specialization/min_specialization/impl-on-nonexisting.stderr
@@ -0,0 +1,9 @@
+error[E0412]: cannot find type `NonExistent` in this scope
+  --> $DIR/impl-on-nonexisting.rs:4:16
+   |
+LL | impl Trait for NonExistent {}
+   |                ^^^^^^^^^^^ not found in this scope
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0412`.