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Associated consts are not object safe. #41494

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9 changes: 9 additions & 0 deletions src/librustc/traits/object_safety.rs
Original file line number Diff line number Diff line change
Expand Up @@ -37,6 +37,9 @@ pub enum ObjectSafetyViolation {

/// Method has something illegal
Method(ast::Name, MethodViolationCode),

/// Associated const
AssociatedConst(ast::Name),
}

impl ObjectSafetyViolation {
Expand All @@ -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(),
}
}
}
Expand Down Expand Up @@ -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);
Expand Down
28 changes: 28 additions & 0 deletions src/test/compile-fail/object-safety-associated-consts.rs
Original file line number Diff line number Diff line change
@@ -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() {
}