do not consider two extern types to be similar

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -1727,6 +1727,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'a, 'tcx> for InferCtxt<'a, 'tcx> {
         } else if cat_a == cat_b {
             match (a.kind(), b.kind()) {
                 (ty::Adt(def_a, _), ty::Adt(def_b, _)) => def_a == def_b,
+                (ty::Foreign(def_a), ty::Foreign(def_b)) => def_a == def_b,
                 // Matching on references results in a lot of unhelpful
                 // suggestions, so let's just not do that for now.
                 //

src/test/ui/extern/extern-type-diag-not-similar.rs

@@ -0,0 +1,22 @@
+// We previously mentioned other extern types in the error message here.
+//
+// Two extern types shouldn't really be considered similar just
+// because they are both extern types.
+
+#![feature(extern_types)]
+extern {
+    type ShouldNotBeMentioned;
+}
+
+extern {
+    type Foo;
+}
+
+unsafe impl Send for ShouldNotBeMentioned {}
+
+fn assert_send<T: Send + ?Sized>() {}
+
+fn main() {
+    assert_send::<Foo>()
+    //~^ ERROR `Foo` cannot be sent between threads safely
+}

src/test/ui/extern/extern-type-diag-not-similar.stderr

@@ -0,0 +1,16 @@
+error[E0277]: `Foo` cannot be sent between threads safely
+  --> $DIR/extern-type-diag-not-similar.rs:20:19
+   |
+LL |     assert_send::<Foo>()
+   |                   ^^^ `Foo` cannot be sent between threads safely
+   |
+   = help: the trait `Send` is not implemented for `Foo`
+note: required by a bound in `assert_send`
+  --> $DIR/extern-type-diag-not-similar.rs:17:19
+   |
+LL | fn assert_send<T: Send + ?Sized>() {}
+   |                   ^^^^ required by this bound in `assert_send`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.