Rollup merge of #49730 - sinkuu:fix_ice_49556, r=cramertj

Fix ICE with impl Trait

Fixes https://github.com/rust-lang/rust/issues/49556#issuecomment-379154713. May or may not fix 49556 itself. Closures like `|x: &'a _| x` has `ClosureSubsts` of `fn(&'a _) -> &'(ReScope) _`, so `tcx.note_and_explain_free_region` (called [here](a143462783/src/librustc/infer/anon_types/mod.rs (L572))) panics.

src/librustc/infer/anon_types/mod.rs

@@ -533,10 +533,14 @@ impl<'cx, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for ReverseMapper<'cx, 'gcx, 'tcx>
         match r {
             // ignore bound regions that appear in the type (e.g., this
             // would ignore `'r` in a type like `for<'r> fn(&'r u32)`.
-            ty::ReLateBound(..) => return r,
+            ty::ReLateBound(..) |
 
             // ignore `'static`, as that can appear anywhere
-            ty::ReStatic => return r,
+            ty::ReStatic |
+
+            // ignore `ReScope`, as that can appear anywhere
+            // See `src/test/run-pass/issue-49556.rs` for example.
+            ty::ReScope(..) => return r,
 
             _ => { }
         }

src/librustc/infer/error_reporting/mod.rs

@@ -181,7 +181,7 @@ 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),
-            _ => bug!(),
+            _ => bug!("{:?}", region),
         }
     }