diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index 20e492dbd8a78..b9e183f48f44a 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/compiler/rustc_lint_defs/src/builtin.rs
@@ -4341,7 +4341,6 @@ declare_lint! {
     pub UNKNOWN_OR_MALFORMED_DIAGNOSTIC_ATTRIBUTES,
     Warn,
     "unrecognized or malformed diagnostic attribute",
-    @feature_gate = sym::diagnostic_namespace;
 }
 
 declare_lint! {
diff --git a/tests/ui/diagnostic_namespace/deny_malformed_attribute.rs b/tests/ui/diagnostic_namespace/deny_malformed_attribute.rs
new file mode 100644
index 0000000000000..1d946a14aff1a
--- /dev/null
+++ b/tests/ui/diagnostic_namespace/deny_malformed_attribute.rs
@@ -0,0 +1,7 @@
+#![deny(unknown_or_malformed_diagnostic_attributes)]
+
+#[diagnostic::unknown_attribute]
+//~^ERROR unknown diagnostic attribute
+struct Foo;
+
+fn main() {}
diff --git a/tests/ui/diagnostic_namespace/deny_malformed_attribute.stderr b/tests/ui/diagnostic_namespace/deny_malformed_attribute.stderr
new file mode 100644
index 0000000000000..a646d3613de78
--- /dev/null
+++ b/tests/ui/diagnostic_namespace/deny_malformed_attribute.stderr
@@ -0,0 +1,14 @@
+error: unknown diagnostic attribute
+  --> $DIR/deny_malformed_attribute.rs:3:15
+   |
+LL | #[diagnostic::unknown_attribute]
+   |               ^^^^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/deny_malformed_attribute.rs:1:9
+   |
+LL | #![deny(unknown_or_malformed_diagnostic_attributes)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+