Assign structured errors to min_const_fn checks

Author: ecstatic-morse

src/librustc_mir/transform/check_consts/mod.rs

@@ -111,3 +111,10 @@ impl fmt::Display for ConstKind {
 pub fn is_lang_panic_fn(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
     Some(def_id) == tcx.lang_items().panic_fn() || Some(def_id) == tcx.lang_items().begin_panic_fn()
 }
+
+/// Returns `true` if the constness of this item is not stabilized, either because it is declared
+/// with `#![rustc_const_unstable]` or because the item itself is unstable.
+fn is_const_unstable(tcx: TyCtxt<'tcx>, def_id: DefId) -> bool {
+    tcx.lookup_const_stability(def_id).map_or(false, |stab| stab.level.is_unstable())
+        || tcx.lookup_stability(def_id).map_or(false, |stab| stab.level.is_unstable())
+}