Dont ICE on an attempt to use GAT without feature gate

src/librustc/middle/region.rs

@@ -658,12 +658,15 @@ impl<'tcx> ScopeTree {
             // The lifetime was defined on node that doesn't own a body,
             // which in practice can only mean a trait or an impl, that
             // is the parent of a method, and that is enforced below.
-            assert_eq!(Some(param_owner_id), self.root_parent,
-                       "free_scope: {:?} not recognized by the \
-                        region scope tree for {:?} / {:?}",
-                       param_owner,
-                       self.root_parent.map(|id| tcx.hir().local_def_id_from_hir_id(id)),
-                       self.root_body.map(|hir_id| DefId::local(hir_id.owner)));
+            if Some(param_owner_id) != self.root_parent {
+                tcx.sess.delay_span_bug(
+                    DUMMY_SP,
+                    &format!("free_scope: {:?} not recognized by the \
+                              region scope tree for {:?} / {:?}",
+                             param_owner,
+                             self.root_parent.map(|id| tcx.hir().local_def_id_from_hir_id(id)),
+                             self.root_body.map(|hir_id| DefId::local(hir_id.owner))));
+            }
 
             // The trait/impl lifetime is in scope for the method's body.
             self.root_body.unwrap().local_id

src/librustc/ty/subst.rs

@@ -479,21 +479,22 @@ impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for SubstFolder<'a, 'gcx, 'tcx> {
         // the specialized routine `ty::replace_late_regions()`.
         match *r {
             ty::ReEarlyBound(data) => {
-                let r = self.substs.get(data.index as usize).map(|k| k.unpack());
-                match r {
+                let rk = self.substs.get(data.index as usize).map(|k| k.unpack());
+                match rk {
                     Some(UnpackedKind::Lifetime(lt)) => {
                         self.shift_region_through_binders(lt)
                     }
                     _ => {
                         let span = self.span.unwrap_or(DUMMY_SP);
-                        span_bug!(
-                            span,
+                        let msg = format!(
                             "Region parameter out of range \
                              when substituting in region {} (root type={:?}) \
                              (index={})",
                             data.name,
                             self.root_ty,
                             data.index);
+                        self.tcx.sess.delay_span_bug(span, &msg);
+                        r
                     }
                 }
             }

src/test/ui/rfc1598-generic-associated-types/gat-dont-ice-on-absent-feature.rs

@@ -0,0 +1,14 @@
+// rust-lang/rust#60654: Do not ICE on an attempt to use GATs that is
+// missing the feature gate.
+
+struct Foo;
+
+impl Iterator for Foo {
+    type Item<'b> = &'b Foo; //~ ERROR generic associated types are unstable [E0658]
+
+    fn next(&mut self) -> Option<Self::Item> {
+        None
+    }
+}
+
+fn main() { }

src/test/ui/rfc1598-generic-associated-types/gat-dont-ice-on-absent-feature.stderr

@@ -0,0 +1,12 @@
+error[E0658]: generic associated types are unstable
+  --> $DIR/gat-dont-ice-on-absent-feature.rs:7:5
+   |
+LL |     type Item<'b> = &'b Foo;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/44265
+   = help: add #![feature(generic_associated_types)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.