[EXPERIMENT] disable orphan check for marker traits

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -592,6 +592,11 @@ fn orphan_check_trait_ref<'tcx>(
 ) -> Result<(), OrphanCheckErr<'tcx>> {
     debug!("orphan_check_trait_ref(trait_ref={:?}, in_crate={:?})", trait_ref, in_crate);
 
+    // Allow arbitrary impls of marker traits.
+    if tcx.trait_def(trait_ref.def_id).is_marker {
+        return Ok(());
+    }
+
     if trait_ref.needs_infer() && trait_ref.needs_subst() {
         bug!(
             "can't orphan check a trait ref with both params and inference variables {:?}",

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -223,6 +223,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // who might care about this case, like coherence, should use
         // that function).
         if candidates.is_empty() {
+            // It's always possible to add a marker trait impl so
+            // we can never consider any `T: Marker` bound to never apply.
+            if self
+                .tcx()
+                .trait_def(stack.obligation.predicate.skip_binder().trait_ref.def_id)
+                .is_marker
+            {
+                return Ok(None);
+            }
             // If there's an error type, 'downgrade' our result from
             // `Err(Unimplemented)` to `Ok(None)`. This helps us avoid
             // emitting additional spurious errors, since we're guaranteed

compiler/rustc_typeck/src/check/wfcheck.rs

@@ -140,9 +140,36 @@ pub fn check_item_well_formed(tcx: TyCtxt<'_>, def_id: LocalDefId) {
                         )
                         .emit();
                     }
+
+                    if let Some(trait_ref) = tcx.impl_trait_ref(item.def_id)
+                        && tcx.trait_def(trait_ref.def_id).is_marker
+                    {
+                        tcx
+                            .sess
+                            .struct_span_err(span, "negative impls are not allowed for marker traits")
+                            .span_note(tcx.def_ident_span(trait_ref.def_id).unwrap(), "marker trait")
+                            .emit();
+                    }
                 }
                 (ty::ImplPolarity::Reservation, _) => {
-                    // FIXME: what amount of WF checking do we need for reservation impls?
+                    if let Some(trait_ref) = tcx.impl_trait_ref(item.def_id)
+                        && tcx.trait_def(trait_ref.def_id).is_marker
+                    {
+                        let span = tcx
+                            .sess
+                            .source_map()
+                            .guess_head_span(tcx.span_of_impl(item.def_id.to_def_id()).unwrap());
+                        tcx.sess
+                            .struct_span_err(
+                                span,
+                                "reservation impls are not allowed for marker traits",
+                            )
+                            .span_note(
+                                tcx.def_ident_span(trait_ref.def_id).unwrap(),
+                                "marker trait",
+                            )
+                            .emit();
+                    }
                 }
                 _ => unreachable!(),
             }

src/test/ui/coherence/coherence-conflicting-negative-trait-impl.rs

@@ -12,6 +12,6 @@ impl<T: MyTrait> !Send for TestType<T> {} //~ ERROR found both positive and nega
 
 unsafe impl<T: 'static> Send for TestType<T> {} //~ ERROR conflicting implementations
 
-impl !Send for TestType<i32> {}
+impl !Send for TestType<i32> {} //~ ERROR found both positive and negative implementation
 
 fn main() {}

src/test/ui/coherence/coherence-conflicting-negative-trait-impl.stderr

@@ -16,7 +16,16 @@ LL | unsafe impl<T: MyTrait + 'static> Send for TestType<T> {}
 LL | unsafe impl<T: 'static> Send for TestType<T> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `TestType<_>`
 
-error: aborting due to 2 previous errors
+error[E0751]: found both positive and negative implementation of trait `std::marker::Send` for type `TestType<i32>`:
+  --> $DIR/coherence-conflicting-negative-trait-impl.rs:15:1
+   |
+LL | unsafe impl<T: MyTrait + 'static> Send for TestType<T> {}
+   | ------------------------------------------------------ positive implementation here
+...
+LL | impl !Send for TestType<i32> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ negative implementation here
+
+error: aborting due to 3 previous errors
 
 Some errors have detailed explanations: E0119, E0751.
 For more information about an error, try `rustc --explain E0119`.

src/test/ui/coherence/negative-impl-marker-traits.rs

@@ -0,0 +1,10 @@
+#![feature(marker_trait_attr, negative_impls)]
+#[marker]
+trait Trait {}
+
+impl !Trait for () {} //~ ERROR negative impls are not allowed for marker traits
+
+struct Local;
+impl !Trait for Local {} //~ ERROR negative impls are not allowed for marker traits
+
+fn main() {}

src/test/ui/coherence/negative-impl-marker-traits.stderr

@@ -0,0 +1,26 @@
+error: negative impls are not allowed for marker traits
+  --> $DIR/negative-impl-marker-traits.rs:5:6
+   |
+LL | impl !Trait for () {}
+   |      ^
+   |
+note: marker trait
+  --> $DIR/negative-impl-marker-traits.rs:3:7
+   |
+LL | trait Trait {}
+   |       ^^^^^
+
+error: negative impls are not allowed for marker traits
+  --> $DIR/negative-impl-marker-traits.rs:8:6
+   |
+LL | impl !Trait for Local {}
+   |      ^
+   |
+note: marker trait
+  --> $DIR/negative-impl-marker-traits.rs:3:7
+   |
+LL | trait Trait {}
+   |       ^^^^^
+
+error: aborting due to 2 previous errors
+

src/test/ui/marker_trait_attr/forbidden-fruit.rs

@@ -0,0 +1,20 @@
+#![feature(marker_trait_attr)]
+#[marker]
+pub trait Marker {}
+
+pub struct B;
+
+trait NotMarker {
+    type Assoc;
+}
+
+impl NotMarker for B {
+    type Assoc = usize;
+}
+
+impl<T: Marker> NotMarker for T {
+    //~^ ERROR conflicting implementations of trait `NotMarker` for type `B`
+    type Assoc = Box<String>;
+}
+
+fn main() {}

src/test/ui/marker_trait_attr/forbidden-fruit.stderr

@@ -0,0 +1,14 @@
+error[E0119]: conflicting implementations of trait `NotMarker` for type `B`
+  --> $DIR/forbidden-fruit.rs:15:1
+   |
+LL | impl NotMarker for B {
+   | -------------------- first implementation here
+...
+LL | impl<T: Marker> NotMarker for T {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `B`
+   |
+   = note: downstream crates may implement trait `Marker` for type `B`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.

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

@@ -1,9 +1,10 @@
-error[E0277]: the trait bound `NotDebugOrDisplay: Marker` is not satisfied
-  --> $DIR/overlap-marker-trait.rs:27:17
+error[E0283]: type annotations needed
+  --> $DIR/overlap-marker-trait.rs:27:5
    |
 LL |     is_marker::<NotDebugOrDisplay>();
-   |                 ^^^^^^^^^^^^^^^^^ the trait `Marker` is not implemented for `NotDebugOrDisplay`
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type for struct `NotDebugOrDisplay`
    |
+   = note: cannot satisfy `NotDebugOrDisplay: Marker`
 note: required by a bound in `is_marker`
   --> $DIR/overlap-marker-trait.rs:15:17
    |
@@ -12,4 +13,4 @@ LL | fn is_marker<T: Marker>() { }
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0283`.