Remove the `overlapping_marker_traits` feature

See #29864

This has been replaced by `#[feature(marker_trait_attr)]`

A few notes:

* Due to PR #68057 not yet being in the bootstrap compiler, it's
  necessary to continue using `#![feature(overlapping_marker_traits)]`
  under `#[cfg(bootstrap)]` to work around type inference issues.
* I've updated tests that used `overlapping_marker_traits` to now use
  `marker_trait_attr` where applicable

The test `src/test/ui/overlap-marker-trait.rs` doesn't make any sense
now that `overlapping_marker_traits`, so I removed it.

The test `src/test/ui/traits/overlap-permitted-for-marker-traits-neg.rs`
now fails, since it's no longer possible to have multiple overlapping
negative impls of `Send`. I believe that this is the behavior we want
(assuming that `Send` is not going to become a `#[marker]` trait, so I
renamed the test to `overlap-permitted-for-marker-traits-neg`

src/librustc/arena.rs

@@ -216,6 +216,7 @@ arena_types!(declare_arena, [], 'tcx);
 
 arena_types!(impl_arena_allocatable, [], 'tcx);
 
+#[marker]
 pub trait ArenaAllocatable {}
 
 impl<T: Copy> ArenaAllocatable for T {}

src/librustc/lib.rs

@@ -36,7 +36,7 @@
 #![feature(drain_filter)]
 #![feature(never_type)]
 #![feature(exhaustive_patterns)]
-#![feature(overlapping_marker_traits)]
+#![feature(marker_trait_attr)]
 #![feature(extern_types)]
 #![feature(nll)]
 #![feature(optin_builtin_traits)]

src/librustc/ty/mod.rs

@@ -2642,9 +2642,7 @@ impl<'tcx> ::std::ops::Deref for Attributes<'tcx> {
 pub enum ImplOverlapKind {
     /// These impls are always allowed to overlap.
     Permitted {
-        /// Whether or not the impl is permitted due to the trait being
-        /// a marker trait (a trait with #[marker], or a trait with
-        /// no associated items and #![feature(overlapping_marker_traits)] enabled)
+        /// Whether or not the impl is permitted due to the trait being a `#[marker]` trait
         marker: bool,
     },
     /// These impls are allowed to overlap, but that raises
@@ -2791,15 +2789,7 @@ impl<'tcx> TyCtxt<'tcx> {
             | (ImplPolarity::Negative, ImplPolarity::Negative) => {}
         };
 
-        let is_marker_overlap = if self.features().overlapping_marker_traits {
-            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()
-            });
-            trait1_is_empty && trait2_is_empty
-        } else {
+        let is_marker_overlap = {
             let is_marker_impl = |def_id: DefId| -> bool {
                 let trait_ref = self.impl_trait_ref(def_id);
                 trait_ref.map_or(false, |tr| self.trait_def(tr.def_id).is_marker)

src/librustc_feature/active.rs

@@ -344,9 +344,6 @@ declare_features! (
     /// Allows `extern "x86-interrupt" fn()`.
     (active, abi_x86_interrupt, "1.17.0", Some(40180), None),
 
-    /// Allows overlapping impls of marker traits.
-    (active, overlapping_marker_traits, "1.18.0", Some(29864), None),
-
     /// Allows a test to fail without failing the whole suite.
     (active, allow_fail, "1.19.0", Some(46488), None),
 

src/librustc_feature/removed.rs

@@ -108,7 +108,9 @@ declare_features! (
     /// Allows using `#[on_unimplemented(..)]` on traits.
     /// (Moved to `rustc_attrs`.)
     (removed, on_unimplemented, "1.40.0", None, None, None),
-
+    /// Allows overlapping impls of marker traits.
+    (removed, overlapping_marker_traits, "1.42.0", Some(29864), None,
+     Some("removed in favor of `#![feature(marker_trait_attr)]`")),
     // -------------------------------------------------------------------------
     // feature-group-end: removed features
     // -------------------------------------------------------------------------

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

@@ -1,6 +1,7 @@
 #![feature(optin_builtin_traits)]
-#![feature(overlapping_marker_traits)]
+#![feature(marker_trait_attr)]
 
+#[marker]
 trait MyTrait {}
 
 struct TestType<T>(::std::marker::PhantomData<T>);
@@ -8,11 +9,11 @@ struct TestType<T>(::std::marker::PhantomData<T>);
 unsafe impl<T: MyTrait+'static> Send for TestType<T> {}
 
 impl<T: MyTrait> !Send for TestType<T> {}
-//~^ ERROR E0119
+//~^ ERROR conflicting implementations
 
 unsafe impl<T:'static> Send for TestType<T> {}
+//~^ ERROR conflicting implementations
 
 impl !Send for TestType<i32> {}
-//~^ ERROR E0119
 
 fn main() {}

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

@@ -1,5 +1,5 @@
 error[E0119]: conflicting implementations of trait `std::marker::Send` for type `TestType<_>`:
-  --> $DIR/coherence-conflicting-negative-trait-impl.rs:10:1
+  --> $DIR/coherence-conflicting-negative-trait-impl.rs:11:1
    |
 LL | unsafe impl<T: MyTrait+'static> Send for TestType<T> {}
    | ---------------------------------------------------- first implementation here
@@ -7,14 +7,14 @@ LL |
 LL | impl<T: MyTrait> !Send for TestType<T> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `TestType<_>`
 
-error[E0119]: conflicting implementations of trait `std::marker::Send` for type `TestType<i32>`:
-  --> $DIR/coherence-conflicting-negative-trait-impl.rs:15:1
+error[E0119]: conflicting implementations of trait `std::marker::Send` for type `TestType<_>`:
+  --> $DIR/coherence-conflicting-negative-trait-impl.rs:14:1
    |
+LL | unsafe impl<T: MyTrait+'static> Send for TestType<T> {}
+   | ---------------------------------------------------- first implementation here
+...
 LL | unsafe impl<T:'static> Send for TestType<T> {}
-   | ------------------------------------------- first implementation here
-LL | 
-LL | impl !Send for TestType<i32> {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `TestType<i32>`
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `TestType<_>`
 
 error: aborting due to 2 previous errors
 

src/test/ui/coherence/coherence-impls-send.rs

@@ -1,5 +1,4 @@
 #![feature(optin_builtin_traits)]
-#![feature(overlapping_marker_traits)]
 
 use std::marker::Copy;
 
@@ -24,7 +23,8 @@ unsafe impl Send for [MyType] {}
 //~^ ERROR E0117
 
 unsafe impl Send for &'static [NotSync] {}
-//~^ ERROR E0117
+//~^ ERROR conflicting implementations of trait
+//~| ERROR only traits defined in the current crate
 
 fn main() {
 }

src/test/ui/coherence/coherence-impls-send.stderr

@@ -1,5 +1,16 @@
+error[E0119]: conflicting implementations of trait `std::marker::Send` for type `&[NotSync]`:
+  --> $DIR/coherence-impls-send.rs:25:1
+   |
+LL | unsafe impl Send for &'static [NotSync] {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: conflicting implementation in crate `core`:
+           - impl<T> std::marker::Send for &T
+             where T: std::marker::Sync, T: ?Sized;
+   = note: upstream crates may add a new impl of trait `std::marker::Sync` for type `[NotSync]` in future versions
+
 error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
-  --> $DIR/coherence-impls-send.rs:17:1
+  --> $DIR/coherence-impls-send.rs:16:1
    |
 LL | unsafe impl Send for (MyType, MyType) {}
    | ^^^^^^^^^^^^^^^^^^^^^----------------
@@ -10,13 +21,13 @@ LL | unsafe impl Send for (MyType, MyType) {}
    = 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 `&'static NotSync`
-  --> $DIR/coherence-impls-send.rs:20:1
+  --> $DIR/coherence-impls-send.rs:19:1
    |
 LL | unsafe impl Send for &'static NotSync {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't implement cross-crate trait with a default impl for non-struct/enum type
 
 error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
-  --> $DIR/coherence-impls-send.rs:23:1
+  --> $DIR/coherence-impls-send.rs:22:1
    |
 LL | unsafe impl Send for [MyType] {}
    | ^^^^^^^^^^^^^^^^^^^^^--------
@@ -27,7 +38,7 @@ LL | unsafe impl Send for [MyType] {}
    = note: define and implement a trait or new type instead
 
 error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
-  --> $DIR/coherence-impls-send.rs:26:1
+  --> $DIR/coherence-impls-send.rs:25:1
    |
 LL | unsafe impl Send for &'static [NotSync] {}
    | ^^^^^^^^^^^^^^^^^^^^^------------------
@@ -37,7 +48,7 @@ LL | unsafe impl Send for &'static [NotSync] {}
    |
    = note: define and implement a trait or new type instead
 
-error: aborting due to 4 previous errors
+error: aborting due to 5 previous errors
 
-Some errors have detailed explanations: E0117, E0321.
+Some errors have detailed explanations: E0117, E0119, E0321.
 For more information about an error, try `rustc --explain E0117`.

src/test/ui/overlap-doesnt-conflict-with-specialization.rs

@@ -1,8 +1,9 @@
 // run-pass
 
-#![feature(overlapping_marker_traits)]
+#![feature(marker_trait_attr)]
 #![feature(specialization)]
 
+#[marker]
 trait MyMarker {}
 
 impl<T> MyMarker for T {}

src/test/ui/overlap-marker-trait.rs

@@ -1,31 +0,0 @@
-// Test for RFC 1268: we allow overlapping impls of marker traits,
-// that is, traits without items. In this case, a type `T` is
-// `MyMarker` if it is either `Debug` or `Display`. This test just
-// checks that we don't consider **all** types to be `MyMarker`.  See
-// also the companion test in
-// `run-pass/overlap-permitted-for-marker-traits.rs`.
-
-#![feature(overlapping_marker_traits)]
-#![feature(optin_builtin_traits)]
-
-use std::fmt::{Debug, Display};
-
-trait Marker {}
-
-impl<T: Debug> Marker for T {}
-impl<T: Display> Marker for T {}
-
-fn is_marker<T: Marker>() { }
-
-struct NotDebugOrDisplay;
-
-fn main() {
-    // Debug && Display:
-    is_marker::<i32>();
-
-    // Debug && !Display:
-    is_marker::<Vec<i32>>();
-
-    // !Debug && !Display
-    is_marker::<NotDebugOrDisplay>(); //~ ERROR
-}

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

@@ -1,12 +0,0 @@
-error[E0277]: the trait bound `NotDebugOrDisplay: Marker` is not satisfied
-  --> $DIR/overlap-marker-trait.rs:30:17
-   |
-LL | fn is_marker<T: Marker>() { }
-   |    ---------    ------ required by this bound in `is_marker`
-...
-LL |     is_marker::<NotDebugOrDisplay>();
-   |                 ^^^^^^^^^^^^^^^^^ the trait `Marker` is not implemented for `NotDebugOrDisplay`
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0277`.

src/test/ui/traits/overlap-not-permitted-for-builtin-trait.rs

@@ -1,12 +1,11 @@
-// run-pass
 #![allow(dead_code)]
-#![feature(overlapping_marker_traits)]
 #![feature(optin_builtin_traits)]
 
-// Overlapping negative impls for `MyStruct` are permitted:
+// Overlapping negative impls for `MyStruct` are not permitted:
 struct MyStruct;
 impl !Send for MyStruct {}
 impl !Send for MyStruct {}
+//~^ ERROR conflicting implementations of trait
 
 fn main() {
 }

src/test/ui/traits/overlap-not-permitted-for-builtin-trait.stderr

@@ -0,0 +1,11 @@
+error[E0119]: conflicting implementations of trait `std::marker::Send` for type `MyStruct`:
+  --> $DIR/overlap-not-permitted-for-builtin-trait.rs:7:1
+   |
+LL | impl !Send for MyStruct {}
+   | ----------------------- first implementation here
+LL | impl !Send for MyStruct {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `MyStruct`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.

src/test/ui/traits/overlap-permitted-for-marker-traits.rs

@@ -1,27 +0,0 @@
-// run-pass
-// Tests for RFC 1268: we allow overlapping impls of marker traits,
-// that is, traits without items. In this case, a type `T` is
-// `MyMarker` if it is either `Debug` or `Display`.
-
-#![feature(overlapping_marker_traits)]
-#![feature(optin_builtin_traits)]
-
-use std::fmt::{Debug, Display};
-
-trait MyMarker {}
-
-impl<T: Debug> MyMarker for T {}
-impl<T: Display> MyMarker for T {}
-
-fn foo<T: MyMarker>(t: T) -> T {
-    t
-}
-
-fn main() {
-    // Debug && Display:
-    assert_eq!(1, foo(1));
-    assert_eq!(2.0, foo(2.0));
-
-    // Debug && !Display:
-    assert_eq!(vec![1], foo(vec![1]));
-}