+
+ fn fold_closure_substs(
+ &mut self,
+ def_id: DefId,
+ substs: ty::SubstsRef<'tcx>,
+ ) -> ty::SubstsRef<'tcx> {
+ // I am a horrible monster and I pray for death. When
+ // we encounter a closure here, it is always a closure
+ // from within the function that we are currently
+ // type-checking -- one that is now being encapsulated
+ // in an opaque type. Ideally, we would
+ // go through the types/lifetimes that it references
+ // and treat them just like we would any other type,
+ // which means we would error out if we find any
+ // reference to a type/region that is not in the
+ // "reverse map".
+ //
+ // **However,** in the case of closures, there is a
+ // somewhat subtle (read: hacky) consideration. The
+ // problem is that our closure types currently include
+ // all the lifetime parameters declared on the
+ // enclosing function, even if they are unused by the
+ // closure itself. We can't readily filter them out,
+ // so here we replace those values with `'empty`. This
+ // can't really make a difference to the rest of the
+ // compiler; those regions are ignored for the
+ // outlives relation, and hence don't affect trait
+ // selection or auto traits, and they are erased
+ // during codegen.
+
+ let generics = self.tcx.generics_of(def_id);
+ self.tcx.mk_substs(substs.iter().enumerate().map(|(index, kind)| {
+ if index < generics.parent_count {
+ // Accommodate missing regions in the parent kinds...
+ self.fold_kind_no_missing_regions_error(kind)
+ } else {
+ // ...but not elsewhere.
+ self.fold_kind_normally(kind)
+ }
+ }))
+ }