Put overlapping impls behind feature gate, add tests

I've added some explicit tests that negative impls are allowed to
overlap, and also to make sure that the feature doesn't interfere with
specialization. I've not added an explicit test for positive overlapping
with negative, as that's already tested elsewhere.

7 files changed, 59 insertions, 0 deletions

diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index 2ae77046a90..3da9383762b 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -2230,6 +2230,9 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     /// Returns true if the impls are the same polarity and are implementing
     /// a trait which contains no items
     pub fn impls_are_allowed_to_overlap(self, def_id1: DefId, def_id2: DefId) -> bool {
+        if !self.sess.features.borrow().overlapping_marker_traits {
+            return 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()
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 8b62416dcbd..6e455234196 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -349,6 +349,9 @@ declare_features! (
 
     // Allows module-level inline assembly by way of global_asm!()
     (active, global_asm, "1.18.0", Some(35119)),
+
+    // Allows overlapping impls of marker traits
+    (active, overlapping_marker_traits, "1.18.0", Some(29864)),
 );
 
 declare_features! (
diff --git a/src/test/compile-fail/coherence-conflicting-negative-trait-impl.rs b/src/test/compile-fail/coherence-conflicting-negative-trait-impl.rs
index d841e8c41d9..8e9d1eff345 100644
--- a/src/test/compile-fail/coherence-conflicting-negative-trait-impl.rs
+++ b/src/test/compile-fail/coherence-conflicting-negative-trait-impl.rs
@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(optin_builtin_traits)]
+#![feature(overlapping_marker_traits)]
 
 trait MyTrait {}
 
diff --git a/src/test/compile-fail/coherence-impls-send.rs b/src/test/compile-fail/coherence-impls-send.rs
index d0e6bc6a1c6..9caaee41aeb 100644
--- a/src/test/compile-fail/coherence-impls-send.rs
+++ b/src/test/compile-fail/coherence-impls-send.rs
@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(optin_builtin_traits)]
+#![feature(overlapping_marker_traits)]
 
 use std::marker::Copy;
 
diff --git a/src/test/compile-fail/overlapping-impls-requires-feature-gate.rs b/src/test/compile-fail/overlapping-impls-requires-feature-gate.rs
new file mode 100644
index 00000000000..bf2b06dd8ba
--- /dev/null
+++ b/src/test/compile-fail/overlapping-impls-requires-feature-gate.rs
@@ -0,0 +1,17 @@
+// 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.
+
+trait MyMarker {}
+
+impl<T> MyMarker for T {}
+impl<T> MyMarker for Vec<T> {}
+//~^ ERROR E0119
+
+fn main() {}
diff --git a/src/test/run-pass/overlap-doesnt-conflict-with-specialization.rs b/src/test/run-pass/overlap-doesnt-conflict-with-specialization.rs
new file mode 100644
index 00000000000..ed45d81c0d6
--- /dev/null
+++ b/src/test/run-pass/overlap-doesnt-conflict-with-specialization.rs
@@ -0,0 +1,27 @@
+// 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.
+
+#![feature(overlapping_marker_traits)]
+#![feature(specialization)]
+
+trait MyMarker {}
+
+impl<T> MyMarker for T {}
+impl<T> MyMarker for Vec<T> {}
+
+fn foo<T: MyMarker>(t: T) -> T {
+    t
+}
+
+fn main() {
+    assert_eq!(1, foo(1));
+    assert_eq!(2.0, foo(2.0));
+    assert_eq!(vec![1], foo(vec![1]));
+}
diff --git a/src/test/run-pass/overlap-permitted-for-marker-traits.rs b/src/test/run-pass/overlap-permitted-for-marker-traits.rs
index b0b1930d274..45085c093fc 100644
--- a/src/test/run-pass/overlap-permitted-for-marker-traits.rs
+++ b/src/test/run-pass/overlap-permitted-for-marker-traits.rs
@@ -8,11 +8,18 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(overlapping_marker_traits)]
+#![feature(optin_builtin_traits)]
+
 trait MyMarker {}
 
 impl<T: Copy> MyMarker for T {}
 impl<T: Eq> MyMarker for T {}
 
+struct MyStruct;
+impl !Send for MyStruct {}
+impl !Send for MyStruct {}
+
 fn foo<T: MyMarker>(t: T) -> T {
     t
 }