From 5f06ce2c0f3938dd69a0676c7abe2243046c2f09 Mon Sep 17 00:00:00 2001
From: Dylan MacKenzie <ecstaticmorse@gmail.com>
Date: Tue, 18 Feb 2020 15:45:39 -0800
Subject: [PATCH] Prevent const trait methods from being marked stable

---
 src/librustc_mir/transform/qualify_min_const_fn.rs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/librustc_mir/transform/qualify_min_const_fn.rs b/src/librustc_mir/transform/qualify_min_const_fn.rs
index b12f4ce3269..5a99ee27301 100644
--- a/src/librustc_mir/transform/qualify_min_const_fn.rs
+++ b/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);