Auto merge of #41494 - withoutboats:associated-consts-are-not-object-safe, r=eddyb

Associated consts are not object safe.

fixes #26847

r? @eddyb

src/librustc/traits/object_safety.rs

@@ -37,6 +37,9 @@ pub enum ObjectSafetyViolation {
 
     /// Method has something illegal
     Method(ast::Name, MethodViolationCode),
+
+    /// Associated const
+    AssociatedConst(ast::Name),
 }
 
 impl ObjectSafetyViolation {
@@ -54,6 +57,8 @@ impl ObjectSafetyViolation {
                          in its arguments or return type", name).into(),
             ObjectSafetyViolation::Method(name, MethodViolationCode::Generic) =>
                 format!("method `{}` has generic type parameters", name).into(),
+            ObjectSafetyViolation::AssociatedConst(name) =>
+                format!("the trait cannot contain associated consts like `{}`", name).into(),
         }
     }
 }
@@ -141,6 +146,10 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
             violations.push(ObjectSafetyViolation::SupertraitSelf);
         }
 
+        violations.extend(self.associated_items(trait_def_id)
+            .filter(|item| item.kind == ty::AssociatedKind::Const)
+            .map(|item| ObjectSafetyViolation::AssociatedConst(item.name)));
+
         debug!("object_safety_violations_for_trait(trait_def_id={:?}) = {:?}",
                trait_def_id,
                violations);

src/test/compile-fail/object-safety-associated-consts.rs

@@ -0,0 +1,28 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Check that we correctly prevent users from making trait objects
+// from traits with associated consts.
+
+#![feature(associated_consts)]
+
+trait Bar {
+    const X: usize;
+}
+
+fn make_bar<T:Bar>(t: &T) -> &Bar {
+    //~^ ERROR E0038
+    //~| NOTE the trait cannot contain associated consts like `X`
+    //~| NOTE the trait `Bar` cannot be made into an object
+    t
+}
+
+fn main() {
+}