Require that marker impls are empty, but allow them to overlap

src/librustc/ty/mod.rs

@@ -2652,23 +2652,32 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
             as Box<dyn Iterator<Item = AssociatedItem> + 'a>
     }
 
-    /// Returns true if the impls are the same polarity and are implementing
-    /// a trait which contains no items
+    /// Returns true if the impls are the same polarity and the trait either
+    /// has no items or is annotated #[marker] and prevents item overrides.
     pub fn impls_are_allowed_to_overlap(self, def_id1: DefId, def_id2: DefId) -> bool {
-        if !self.features().overlapping_marker_traits {
-            return false;
+        if self.features().overlapping_marker_traits {
+            let trait1_is_empty = self.impl_trait_ref(def_id1)
+                .map_or(false, |trait_ref| {
+                    self.associated_item_def_ids(trait_ref.def_id).is_empty()
+                });
+            let trait2_is_empty = self.impl_trait_ref(def_id2)
+                .map_or(false, |trait_ref| {
+                    self.associated_item_def_ids(trait_ref.def_id).is_empty()
+                });
+            self.impl_polarity(def_id1) == self.impl_polarity(def_id2)
+                && trait1_is_empty
+                && trait2_is_empty
+        } else if self.features().marker_trait_attr {
+            let is_marker_impl = |def_id: DefId| -> bool {
+                let trait_ref = self.impl_trait_ref(def_id);
+                trait_ref.map_or(false, |tr| self.trait_def(tr.def_id).is_marker)
+            };
+            self.impl_polarity(def_id1) == self.impl_polarity(def_id2)
+                && is_marker_impl(def_id1)
+                && is_marker_impl(def_id2)
+        } else {
+            false
         }
-        let trait1_is_empty = self.impl_trait_ref(def_id1)
-            .map_or(false, |trait_ref| {
-                self.associated_item_def_ids(trait_ref.def_id).is_empty()
-            });
-        let trait2_is_empty = self.impl_trait_ref(def_id2)
-            .map_or(false, |trait_ref| {
-                self.associated_item_def_ids(trait_ref.def_id).is_empty()
-            });
-        self.impl_polarity(def_id1) == self.impl_polarity(def_id2)
-            && trait1_is_empty
-            && trait2_is_empty
     }
 
     // Returns `ty::VariantDef` if `def` refers to a struct,

src/librustc_typeck/coherence/mod.rs

@@ -46,6 +46,7 @@ fn check_impl<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, node_id: ast::NodeId) {
         }
 
         enforce_trait_manually_implementable(tcx, impl_def_id, trait_ref.def_id);
+        enforce_empty_impls_for_marker_traits(tcx, impl_def_id, trait_ref.def_id);
     }
 }
 
@@ -99,6 +100,25 @@ fn enforce_trait_manually_implementable(tcx: TyCtxt, impl_def_id: DefId, trait_d
         .emit();
 }
 
+/// We allow impls of marker traits to overlap, so they can't override impls
+/// as that could make it ambiguous which associated item to use.
+fn enforce_empty_impls_for_marker_traits(tcx: TyCtxt, impl_def_id: DefId, trait_def_id: DefId) {
+    if !tcx.trait_def(trait_def_id).is_marker {
+        return;
+    }
+
+    if tcx.associated_item_def_ids(trait_def_id).is_empty() {
+        return;
+    }
+
+    let span = tcx.sess.source_map().def_span(tcx.span_of_impl(impl_def_id).unwrap());
+    struct_span_err!(tcx.sess,
+                     span,
+                     E0715,
+                     "impls for marker traits cannot contain items")
+        .emit();
+}
+
 pub fn provide(providers: &mut Providers) {
     use self::builtin::coerce_unsized_info;
     use self::inherent_impls::{crate_inherent_impls, inherent_impls};

src/librustc_typeck/diagnostics.rs

@@ -4833,4 +4833,5 @@ register_diagnostics! {
     E0641, // cannot cast to/from a pointer with an unknown kind
     E0645, // trait aliases not finished
     E0698, // type inside generator must be known in this context
+    E0715, // impls for marker traits cannot contain items
 }

src/test/run-pass/overlap-permitted-for-annotated-marker-traits.rs

@@ -0,0 +1,35 @@
+// Copyright 2017 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.
+
+// Tests for RFC 1268: we allow overlapping impls of marker traits,
+// that is, traits with #[marker]. In this case, a type `T` is
+// `MyMarker` if it is either `Debug` or `Display`.
+
+#![feature(marker_trait_attr)]
+
+use std::fmt::{Debug, Display};
+
+#[marker] trait MyMarker {}
+
+impl<T: Debug> MyMarker for T {}
+impl<T: Display> MyMarker for T {}
+
+fn foo<T: MyMarker>(t: T) -> T {
+    t
+}
+
+fn main() {
+    // Debug && Display:
+    assert_eq!(1, foo(1));
+    assert_eq!(2.0, foo(2.0));
+
+    // Debug && !Display:
+    assert_eq!(vec![1], foo(vec![1]));
+}

src/test/ui/marker_trait_attr/overlap-marker-trait.rs

@@ -0,0 +1,38 @@
+// Copyright 2018 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.
+
+// Test for RFC 1268: we allow overlapping impls of marker traits,
+// that is, traits with #[marker]. In this case, a type `T` is
+// `MyMarker` if it is either `Debug` or `Display`. This test just
+// checks that we don't consider **all** types to be `MyMarker`.
+
+#![feature(marker_trait_attr)]
+
+use std::fmt::{Debug, Display};
+
+#[marker] trait Marker {}
+
+impl<T: Debug> Marker for T {}
+impl<T: Display> Marker for T {}
+
+fn is_marker<T: Marker>() { }
+
+struct NotDebugOrDisplay;
+
+fn main() {
+    // Debug && Display:
+    is_marker::<i32>();
+
+    // Debug && !Display:
+    is_marker::<Vec<i32>>();
+
+    // !Debug && !Display
+    is_marker::<NotDebugOrDisplay>(); //~ ERROR
+}

src/test/ui/marker_trait_attr/overlap-marker-trait.stderr

@@ -0,0 +1,15 @@
+error[E0277]: the trait bound `NotDebugOrDisplay: Marker` is not satisfied
+  --> $DIR/overlap-marker-trait.rs:37:5
+   |
+LL |     is_marker::<NotDebugOrDisplay>(); //~ ERROR
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Marker` is not implemented for `NotDebugOrDisplay`
+   |
+note: required by `is_marker`
+  --> $DIR/overlap-marker-trait.rs:25:1
+   |
+LL | fn is_marker<T: Marker>() { }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.

src/test/ui/marker_trait_attr/override-item-on-marker-trait.rs

@@ -0,0 +1,33 @@
+// Copyright 2018 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.
+
+#![feature(marker_trait_attr)]
+
+#[marker]
+trait Marker {
+    const N: usize = 0;
+    fn do_something() {}
+}
+
+struct OverrideConst;
+impl Marker for OverrideConst {
+//~^ ERROR impls for marker traits cannot contain items
+    const N: usize = 1;
+}
+
+struct OverrideFn;
+impl Marker for OverrideFn {
+//~^ ERROR impls for marker traits cannot contain items
+    fn do_something() {
+        println!("Hello world!");
+    }
+}
+
+fn main() {}

src/test/ui/marker_trait_attr/override-item-on-marker-trait.stderr

@@ -0,0 +1,15 @@
+error[E0715]: impls for marker traits cannot contain items
+  --> $DIR/override-item-on-marker-trait.rs:20:1
+   |
+LL | impl Marker for OverrideConst {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0715]: impls for marker traits cannot contain items
+  --> $DIR/override-item-on-marker-trait.rs:26:1
+   |
+LL | impl Marker for OverrideFn {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0715`.