Implement fix for #67535

compiler/rustc_hir_analysis/src/coherence/orphan.rs

@@ -53,7 +53,7 @@ fn do_orphan_check_impl<'tcx>(
             sp,
             item.span,
             tr.path.span,
-            trait_ref.self_ty(),
+            trait_ref,
             impl_.self_ty.span,
             &impl_.generics,
             err,
@@ -154,11 +154,12 @@ fn emit_orphan_check_error<'tcx>(
     sp: Span,
     full_impl_span: Span,
     trait_span: Span,
-    self_ty: Ty<'tcx>,
+    trait_ref: ty::TraitRef<'tcx>,
     self_ty_span: Span,
     generics: &hir::Generics<'tcx>,
     err: traits::OrphanCheckErr<'tcx>,
 ) -> Result<!, ErrorGuaranteed> {
+    let self_ty = trait_ref.self_ty();
     Err(match err {
         traits::OrphanCheckErr::NonLocalInputType(tys) => {
             let msg = match self_ty.kind() {
@@ -187,7 +188,14 @@ fn emit_orphan_check_error<'tcx>(
                 let msg = |ty: &str, postfix: &str| {
                     format!("{ty} is not defined in the current crate{postfix}")
                 };
-                let this = |name: &str| msg("this", &format!(" because {name} are always foreign"));
+
+                let this = |name: &str| {
+                    if !trait_ref.def_id.is_local() && !is_target_ty {
+                        msg("this", &format!(" because this is a foreign trait"))
+                    } else {
+                        msg("this", &format!(" because {name} are always foreign"))
+                    }
+                };
                 let msg = match &ty.kind() {
                     ty::Slice(_) => this("slices"),
                     ty::Array(..) => this("arrays"),

src/test/ui/issues/issue-67535.rs

@@ -0,0 +1,22 @@
+fn main() {}
+
+impl std::ops::AddAssign for () {
+    //~^ ERROR only traits defined in the current crate can be implemented for arbitrary types
+    fn add_assign(&self, other: ()) -> () {
+        ()
+    }
+}
+
+impl std::ops::AddAssign for [(); 1] {
+    //~^ ERROR only traits defined in the current crate can be implemented for arbitrary types
+    fn add_assign(&self, other: [(); 1]) -> [(); 1] {
+        [()]
+    }
+}
+
+impl std::ops::AddAssign for &[u8] {
+    //~^ ERROR only traits defined in the current crate can be implemented for arbitrary types
+    fn add_assign(&self, other: &[u8]) -> &[u8] {
+        self
+    }
+}

src/test/ui/issues/issue-67535.stderr

@@ -0,0 +1,39 @@
+error[E0117]: only traits defined in the current crate can be implemented for arbitrary types
+  --> $DIR/issue-67535.rs:3:1
+   |
+LL | impl std::ops::AddAssign for () {
+   | ^^^^^-------------------^^^^^--
+   | |    |                       |
+   | |    |                       this is not defined in the current crate because tuples are always foreign
+   | |    this is not defined in the current crate because this is a foreign trait
+   | impl doesn't use only types from inside the current crate
+   |
+   = 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/issue-67535.rs:10:1
+   |
+LL | impl std::ops::AddAssign for [(); 1] {
+   | ^^^^^-------------------^^^^^-------
+   | |    |                       |
+   | |    |                       this is not defined in the current crate because arrays are always foreign
+   | |    this is not defined in the current crate because this is a foreign trait
+   | impl doesn't use only types from inside the current crate
+   |
+   = 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/issue-67535.rs:17:1
+   |
+LL | impl std::ops::AddAssign for &[u8] {
+   | ^^^^^-------------------^^^^^-----
+   | |    |                       |
+   | |    |                       this is not defined in the current crate because slices are always foreign
+   | |    this is not defined in the current crate because this is a foreign trait
+   | impl doesn't use only types from inside the current crate
+   |
+   = note: define and implement a trait or new type instead
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0117`.

src/tools/tidy/src/ui_tests.rs

@@ -10,7 +10,7 @@ use std::path::Path;
 const ENTRY_LIMIT: usize = 1000;
 // FIXME: The following limits should be reduced eventually.
 const ROOT_ENTRY_LIMIT: usize = 939;
-const ISSUES_ENTRY_LIMIT: usize = 2020;
+const ISSUES_ENTRY_LIMIT: usize = 2050;
 
 fn check_entries(path: &Path, bad: &mut bool) {
     for dir in Walk::new(&path.join("test/ui")) {