Add ui test for unsupported doc(attribute = "...") case for attributes with namespace

compiler/rustc_passes/src/check_attr.rs

@@ -879,6 +879,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             s.is_reserved(|| edition::LATEST_STABLE_EDITION) || s.is_weak() || s == sym::SelfTy
         }
 
+        // FIXME: This should support attributes with namespace like `diagnostic::do_not_recommend`.
         fn is_builtin_attr(s: Symbol) -> bool {
             rustc_feature::BUILTIN_ATTRIBUTE_MAP.contains_key(&s)
         }

tests/rustdoc-ui/doc-attribute-unsupported.rs

@@ -0,0 +1,7 @@
+// This is currently not supported but should be!
+
+#![feature(rustdoc_internals)]
+
+#[doc(attribute = "diagnostic::do_not_recommend")] //~ ERROR
+/// bla
+mod yup {}

tests/rustdoc-ui/doc-attribute-unsupported.stderr

@@ -0,0 +1,10 @@
+error: nonexistent builtin attribute `diagnostic::do_not_recommend` used in `#[doc(attribute = "...")]`
+  --> $DIR/doc-attribute-unsupported.rs:5:19
+   |
+LL | #[doc(attribute = "diagnostic::do_not_recommend")]
+   |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: only existing builtin attributes are allowed in core/std
+
+error: aborting due to 1 previous error
+