Rollup merge of #57352 - arielb1:no-manual-markers, r=nikomatsakis

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.

r? @nikomatsakis

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 only 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 only 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`.