From 5aeb6c756e4b6e28366dee28802022430c66605e Mon Sep 17 00:00:00 2001
From: "Felix S. Klock II" <pnkfelix@pnkfx.org>
Date: Sat, 3 Nov 2018 02:24:50 +0100
Subject: [PATCH 1/2] Sidestep an ICE by providing *some* description for
 `ReEmpty` when it arises.

---
 src/librustc/infer/error_reporting/mod.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/librustc/infer/error_reporting/mod.rs b/src/librustc/infer/error_reporting/mod.rs
index d19c495af3b96..6c5f10332956d 100644
--- a/src/librustc/infer/error_reporting/mod.rs
+++ b/src/librustc/infer/error_reporting/mod.rs
@@ -178,6 +178,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
                 self.msg_span_from_early_bound_and_free_regions(region)
             }
             ty::ReStatic => ("the static lifetime".to_owned(), None),
+            ty::ReEmpty => ("an empty lifetime".to_owned(), None),
             _ => bug!("{:?}", region),
         }
     }

From cc33aecb683b1cb08bd60ba1f24ef797827fda94 Mon Sep 17 00:00:00 2001
From: "Felix S. Klock II" <pnkfelix@pnkfx.org>
Date: Sat, 3 Nov 2018 02:29:26 +0100
Subject: [PATCH 2/2] Regression test for issue 55608.

---
 .../issue-55608-captures-empty-region.rs      | 22 +++++++++++++++++++
 .../issue-55608-captures-empty-region.stderr  | 11 ++++++++++
 2 files changed, 33 insertions(+)
 create mode 100644 src/test/ui/impl-trait/issue-55608-captures-empty-region.rs
 create mode 100644 src/test/ui/impl-trait/issue-55608-captures-empty-region.stderr

diff --git a/src/test/ui/impl-trait/issue-55608-captures-empty-region.rs b/src/test/ui/impl-trait/issue-55608-captures-empty-region.rs
new file mode 100644
index 0000000000000..7ebc348996f5e
--- /dev/null
+++ b/src/test/ui/impl-trait/issue-55608-captures-empty-region.rs
@@ -0,0 +1,22 @@
+// This used to ICE because it creates an `impl Trait` that captures a
+// hidden empty region.
+
+#![feature(conservative_impl_trait)]
+
+fn server() -> impl FilterBase2 { //~ ERROR [E0700]
+    segment2(|| { loop { } }).map2(|| "")
+}
+
+trait FilterBase2 {
+    fn map2<F>(self, _fn: F) -> Map2<F> where Self: Sized { loop { } }
+}
+
+struct Map2<F> { _func: F }
+
+impl<F> FilterBase2 for Map2<F> { }
+
+fn segment2<F>(_fn: F) -> Map2<F> where F: Fn() -> Result<(), ()> {
+    loop { }
+}
+
+fn main() { server(); }
diff --git a/src/test/ui/impl-trait/issue-55608-captures-empty-region.stderr b/src/test/ui/impl-trait/issue-55608-captures-empty-region.stderr
new file mode 100644
index 0000000000000..d1f147834d2ef
--- /dev/null
+++ b/src/test/ui/impl-trait/issue-55608-captures-empty-region.stderr
@@ -0,0 +1,11 @@
+error[E0700]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
+  --> $DIR/issue-55608-captures-empty-region.rs:6:16
+   |
+LL | fn server() -> impl FilterBase2 { //~ ERROR [E0700]
+   |                ^^^^^^^^^^^^^^^^
+   |
+   = note: hidden type `Map2<[closure@$DIR/issue-55608-captures-empty-region.rs:7:36: 7:41]>` captures an empty lifetime
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0700`.