Prevent const trait methods from being marked stable

Author: ecstatic-morse

src/librustc_mir/transform/qualify_min_const_fn.rs

@@ -10,6 +10,14 @@ use std::borrow::Cow;
 type McfResult = Result<(), (Span, Cow<'static, str>)>;
 
 pub fn is_min_const_fn(tcx: TyCtxt<'tcx>, def_id: DefId, body: &'a Body<'tcx>) -> McfResult {
+    // Prevent const trait methods from being annotated as `stable`.
+    if tcx.features().staged_api {
+        let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
+        if crate::const_eval::is_parent_const_impl_raw(tcx, hir_id) {
+            return Err((body.span, "trait methods cannot be stable const fn".into()));
+        }
+    }
+
     let mut current = def_id;
     loop {
         let predicates = tcx.predicates_of(current);