Limit from_anon_const to AnonConsts.

Author: oli-obk

src/librustc/ty/sty.rs

@@ -2409,7 +2409,10 @@ impl<'tcx> Const<'tcx> {
 
         let hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 
-        let body_id = tcx.hir().body_owned_by(hir_id);
+        let body_id = match tcx.hir().get(hir_id) {
+            hir::Node::AnonConst(ac) => ac.body,
+            _ => span_bug!(tcx.def_span(def_id.to_def_id()), "from_anon_const can only process anonymous constants"),
+        };
 
         let expr = &tcx.hir().body(body_id).value;