forbid manually impl'ing one of an object type's marker traits

This shouldn't break compatibility for crates that do not use
`feature(optin_builtin_traits)`, because as the test shows, it is
only possible to impl a marker trait for a trait object in the crate the
marker trait is defined in, which must define
`feature(optin_builtin_traits)`.

Fixes #56934

src/librustc_typeck/coherence/mod.rs

@@ -171,13 +171,23 @@ fn check_impl_overlap<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, node_id: ast::NodeI
         // This is something like impl Trait1 for Trait2. Illegal
         // if Trait1 is a supertrait of Trait2 or Trait2 is not object safe.
 
-        if let Some(principal_def_id) = data.principal_def_id() {
-            if !tcx.is_object_safe(principal_def_id) {
+        let component_def_ids = data.iter().flat_map(|predicate| {
+            match predicate.skip_binder() {
+                ty::ExistentialPredicate::Trait(tr) => Some(tr.def_id),
+                ty::ExistentialPredicate::AutoTrait(def_id) => Some(*def_id),
+                // An associated type projection necessarily comes with
+                // an additional `Trait` requirement.
+                ty::ExistentialPredicate::Projection(..) => None,
+            }
+        });
+
+        for component_def_id in component_def_ids {
+            if !tcx.is_object_safe(component_def_id) {
                 // This is an error, but it will be reported by wfcheck.  Ignore it here.
                 // This is tested by `coherence-impl-trait-for-trait-object-safe.rs`.
             } else {
                 let mut supertrait_def_ids =
-                    traits::supertrait_def_ids(tcx, principal_def_id);
+                    traits::supertrait_def_ids(tcx, component_def_id);
                 if supertrait_def_ids.any(|d| d == trait_def_id) {
                     let sp = tcx.sess.source_map().def_span(tcx.span_of_impl(impl_def_id).unwrap());
                     struct_span_err!(tcx.sess,
@@ -193,6 +203,5 @@ fn check_impl_overlap<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, node_id: ast::NodeI
                 }
             }
         }
-        // FIXME: also check auto-trait def-ids? (e.g. `impl Sync for Foo+Sync`)?
     }
 }

src/test/ui/coherence/coherence-impl-trait-for-marker-trait-negative.rs

@@ -0,0 +1,29 @@
+#![feature(optin_builtin_traits)]
+
+// Test for issue #56934 - that it is impossible to redundantly
+// implement an auto-trait for a trait object type that contains it.
+
+// Negative impl variant.
+
+auto trait Marker1 {}
+auto trait Marker2 {}
+
+trait Object: Marker1 {}
+
+// A supertrait marker is illegal...
+impl !Marker1 for dyn Object + Marker2 { }   //~ ERROR E0371
+// ...and also a direct component.
+impl !Marker2 for dyn Object + Marker2 { }   //~ ERROR E0371
+
+// But implementing a marker if it is not present is OK.
+impl !Marker2 for dyn Object {} // OK
+
+// A non-principal trait-object type is orphan even in its crate.
+impl !Send for dyn Marker2 {} //~ ERROR E0117
+
+// And impl'ing a remote marker for a local trait object is forbidden
+// by one of these special orphan-like rules.
+impl !Send for dyn Object {} //~ ERROR E0321
+impl !Send for dyn Object + Marker2 {} //~ ERROR E0321
+
+fn main() { }

src/test/ui/coherence/coherence-impl-trait-for-marker-trait-negative.stderr

@@ -0,0 +1,37 @@
+error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker1`
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:14:1
+   |
+LL | impl !Marker1 for dyn Object + Marker2 { }   //~ ERROR E0371
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker1`
+
+error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker2`
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:16:1
+   |
+LL | impl !Marker2 for dyn Object + Marker2 { }   //~ ERROR E0371
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker2`
+
+error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:22:1
+   |
+LL | impl !Send for dyn Marker2 {} //~ ERROR E0117
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ impl doesn't use types inside crate
+   |
+   = note: the impl does not reference any types defined in this crate
+   = note: define and implement a trait or new type instead
+
+error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `(dyn Object + 'static)`
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:26:1
+   |
+LL | impl !Send for dyn Object {} //~ ERROR E0321
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
+
+error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `(dyn Object + Marker2 + 'static)`
+  --> $DIR/coherence-impl-trait-for-marker-trait-negative.rs:27:1
+   |
+LL | impl !Send for dyn Object + Marker2 {} //~ ERROR E0321
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
+
+error: aborting due to 5 previous errors
+
+Some errors occurred: E0117, E0321, E0371.
+For more information about an error, try `rustc --explain E0117`.

src/test/ui/coherence/coherence-impl-trait-for-marker-trait-positive.rs

@@ -0,0 +1,29 @@
+#![feature(optin_builtin_traits)]
+
+// Test for issue #56934 - that it is impossible to redundantly
+// implement an auto-trait for a trait object type that contains it.
+
+// Positive impl variant.
+
+auto trait Marker1 {}
+auto trait Marker2 {}
+
+trait Object: Marker1 {}
+
+// A supertrait marker is illegal...
+impl Marker1 for dyn Object + Marker2 { }   //~ ERROR E0371
+// ...and also a direct component.
+impl Marker2 for dyn Object + Marker2 { }   //~ ERROR E0371
+
+// But implementing a marker if it is not present is OK.
+impl Marker2 for dyn Object {} // OK
+
+// A non-principal trait-object type is orphan even in its crate.
+unsafe impl Send for dyn Marker2 {} //~ ERROR E0117
+
+// And impl'ing a remote marker for a local trait object is forbidden
+// by one of these special orphan-like rules.
+unsafe impl Send for dyn Object {} //~ ERROR E0321
+unsafe impl Send for dyn Object + Marker2 {} //~ ERROR E0321
+
+fn main() { }

src/test/ui/coherence/coherence-impl-trait-for-marker-trait-positive.stderr

@@ -0,0 +1,37 @@
+error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker1`
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:14:1
+   |
+LL | impl Marker1 for dyn Object + Marker2 { }   //~ ERROR E0371
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker1`
+
+error[E0371]: the object type `(dyn Object + Marker2 + 'static)` automatically implements the trait `Marker2`
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:16:1
+   |
+LL | impl Marker2 for dyn Object + Marker2 { }   //~ ERROR E0371
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `(dyn Object + Marker2 + 'static)` automatically implements trait `Marker2`
+
+error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:22:1
+   |
+LL | unsafe impl Send for dyn Marker2 {} //~ ERROR E0117
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ impl doesn't use types inside crate
+   |
+   = note: the impl does not reference any types defined in this crate
+   = note: define and implement a trait or new type instead
+
+error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `(dyn Object + 'static)`
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:26:1
+   |
+LL | unsafe impl Send for dyn Object {} //~ ERROR E0321
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
+
+error[E0321]: cross-crate traits with a default impl, like `std::marker::Send`, can only be implemented for a struct/enum type, not `(dyn Object + Marker2 + 'static)`
+  --> $DIR/coherence-impl-trait-for-marker-trait-positive.rs:27:1
+   |
+LL | unsafe impl Send for dyn Object + Marker2 {} //~ ERROR E0321
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
+
+error: aborting due to 5 previous errors
+
+Some errors occurred: E0117, E0321, E0371.
+For more information about an error, try `rustc --explain E0117`.