explain why pick-constraints can be an empty vector

src/librustc_mir/borrow_check/nll/type_check/mod.rs

@@ -2514,7 +2514,14 @@ fn prove_closure_bounds(
         if let Some(closure_region_requirements) = tcx.mir_borrowck(def_id).closure_requirements {
             let closure_constraints = QueryRegionConstraints {
                 outlives: closure_region_requirements.apply_requirements(tcx, def_id, substs),
-                pick_constraints: vec![], // TODO
+
+                // Presently, closures never propagate pick
+                // constraints to their parents -- they are enforced
+                // locally.  This is largely a non-issue as pick
+                // constraints only come from `-> impl Trait` and
+                // friends which don't appear (thus far...) in
+                // closures.
+                pick_constraints: vec![],
             };
 
             let bounds_mapping = closure_constraints