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

Author: GuillaumeGomez

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 +1,5 @@
+// This is currently not supported but should be!
+
+#[doc(attribute = "diagnostic::do_not_recommend")] //~ ERROR
+/// bla
+mod yup {}

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

@@ -0,0 +1,13 @@
+error[E0658]: `#[doc(attribute)]` is meant for internal use only
+  --> $DIR/doc-attribute-unsupported.rs:3:1
+   |
+LL | #[doc(attribute = "diagnostic::do_not_recommend")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #90418 <https://github.com/rust-lang/rust/issues/90418> for more information
+   = help: add `#![feature(rustdoc_internals)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.