Add a test that emitting diagnostics does not require the crate to use

the corresponding feature.

tests/ui/diagnostic_namespace/on_unimplemented/auxiliary/other.rs

@@ -0,0 +1,8 @@
+#![feature(diagnostic_namespace)]
+
+#[diagnostic::on_unimplemented(
+    message = "Message",
+    note = "Note",
+    label = "label"
+)]
+pub trait Foo {}

tests/ui/diagnostic_namespace/on_unimplemented/error_is_shown_in_downstream_crates.rs

@@ -0,0 +1,12 @@
+// aux-build:other.rs
+
+extern crate other;
+
+use other::Foo;
+
+fn take_foo(_: impl Foo) {}
+
+fn main() {
+    take_foo(());
+    //~^ERROR Message
+}

tests/ui/diagnostic_namespace/on_unimplemented/error_is_shown_in_downstream_crates.stderr

@@ -0,0 +1,19 @@
+error[E0277]: Message
+  --> $DIR/error_is_shown_in_downstream_crates.rs:10:14
+   |
+LL |     take_foo(());
+   |     -------- ^^ label
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Foo` is not implemented for `()`
+   = note: Note
+note: required by a bound in `take_foo`
+  --> $DIR/error_is_shown_in_downstream_crates.rs:7:21
+   |
+LL | fn take_foo(_: impl Foo) {}
+   |                     ^^^ required by this bound in `take_foo`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.