Rollup merge of #41309 - frewsxcv:sg-implement-rfc-1268, r=nikomatsakis

Implement RFC 1268.

Rebased version of #40097.

Tracking issue: #29864.

Author: frewsxcv

src/doc/unstable-book/src/SUMMARY.md

@@ -136,6 +136,7 @@
 - [optin_builtin_traits](optin-builtin-traits.md)
 - [option_entry](option-entry.md)
 - [osstring_shrink_to_fit](osstring-shrink-to-fit.md)
+- [overlapping_marker_traits](overlapping-marker-traits.md)
 - [panic_abort](panic-abort.md)
 - [panic_runtime](panic-runtime.md)
 - [panic_unwind](panic-unwind.md)

src/doc/unstable-book/src/overlapping-marker-traits.md

@@ -0,0 +1,7 @@
+# `overlapping_marker_traits`
+
+The tracking issue for this feature is: [#29864]
+
+[#29864]: https://github.com/rust-lang/rust/issues/29864
+
+------------------------

src/librustc/traits/select.rs

@@ -1736,7 +1736,8 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
                 if other.evaluation == EvaluatedToOk {
                     if let ImplCandidate(victim_def) = victim.candidate {
                         let tcx = self.tcx().global_tcx();
-                        return traits::specializes(tcx, other_def, victim_def);
+                        return traits::specializes(tcx, other_def, victim_def) ||
+                            tcx.impls_are_allowed_to_overlap(other_def, victim_def);
                     }
                 }
 

src/librustc/traits/specialize/specialization_graph.rs

@@ -113,6 +113,10 @@ impl<'a, 'gcx, 'tcx> Children {
                                                         possible_sibling,
                                                         impl_def_id);
                 if let Some(impl_header) = overlap {
+                    if tcx.impls_are_allowed_to_overlap(impl_def_id, possible_sibling) {
+                        return Ok((false, false));
+                    }
+
                     let le = specializes(tcx, impl_def_id, possible_sibling);
                     let ge = specializes(tcx, possible_sibling, impl_def_id);
 

src/librustc/ty/mod.rs

@@ -2227,6 +2227,25 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         queries::impl_trait_ref::get(self, DUMMY_SP, id)
     }
 
+    /// 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()
+            });
+        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.trait_impl_polarity(def_id1) == self.trait_impl_polarity(def_id2)
+            && trait1_is_empty
+            && trait2_is_empty
+    }
+
     // Returns `ty::VariantDef` if `def` refers to a struct,
     // or variant or their constructors, panics otherwise.
     pub fn expect_variant_def(self, def: Def) -> &'tcx VariantDef {

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! (

src/test/compile-fail/E0120.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-trait MyTrait {}
+trait MyTrait { fn foo() {} }
 
 impl Drop for MyTrait {
               //~^ ERROR E0120

src/test/compile-fail/auxiliary/trait_impl_conflict.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 pub trait Foo {
+    fn foo() {}
 }
 
 impl Foo for isize {

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 {}
 
@@ -20,8 +21,8 @@ impl<T: MyTrait> !Send for TestType<T> {}
 //~^ ERROR conflicting implementations of trait `std::marker::Send`
 
 unsafe impl<T:'static> Send for TestType<T> {}
-//~^ ERROR conflicting implementations of trait `std::marker::Send`
 
 impl !Send for TestType<i32> {}
+//~^ ERROR conflicting implementations of trait `std::marker::Send`
 
 fn main() {}

src/test/compile-fail/coherence-default-trait-impl.rs

@@ -10,7 +10,7 @@
 
 #![feature(optin_builtin_traits)]
 
-trait MyTrait {}
+trait MyTrait { fn foo() {} }
 
 impl MyTrait for .. {}
 //~^ ERROR redundant default implementations of trait `MyTrait`