Test define opaques in extern items

compiler/rustc_builtin_macros/src/define_opaque.rs

@@ -52,7 +52,10 @@ pub(crate) fn expand(
                 .collect(),
         );
     } else {
-        ecx.dcx().span_err(meta_item.span, "only functions and methods can define opaque types");
+        ecx.dcx().span_err(
+            meta_item.span,
+            "only functions, statics, and consts can define opaque types",
+        );
     }
 
     vec![item]

tests/ui/impl-trait/define-via-extern.rs

@@ -0,0 +1,16 @@
+#![feature(type_alias_impl_trait)]
+
+type Hi = impl Sized;
+
+extern "C" {
+    #[define_opaque(Hi)] fn foo();
+    //~^ ERROR only functions, statics, and consts can define opaque types
+
+    #[define_opaque(Hi)] static HI: Hi;
+    //~^ ERROR only functions, statics, and consts can define opaque types
+}
+
+#[define_opaque(Hi)]
+fn main() {
+    let _: Hi = 0;
+}

tests/ui/impl-trait/define-via-extern.stderr

@@ -0,0 +1,14 @@
+error: only functions, statics, and consts can define opaque types
+  --> $DIR/define-via-extern.rs:6:5
+   |
+LL |     #[define_opaque(Hi)] fn foo();
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: only functions, statics, and consts can define opaque types
+  --> $DIR/define-via-extern.rs:9:5
+   |
+LL |     #[define_opaque(Hi)] static HI: Hi;
+   |     ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+