diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index 03664324404..712fb5ac71f 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -1051,6 +1051,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
 
                     walk_list!(this, visit_assoc_item, items, AssocCtxt::Impl);
                 });
+                walk_list!(self, visit_attribute, &item.attrs);
                 return; // Avoid visiting again.
             }
             ItemKind::Impl(box Impl {
@@ -1168,7 +1169,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 });
                 walk_list!(self, visit_assoc_item, items, AssocCtxt::Trait);
                 walk_list!(self, visit_attribute, &item.attrs);
-                return;
+                return; // Avoid visiting again
             }
             ItemKind::Mod(unsafety, ref mod_kind) => {
                 if let Unsafe::Yes(span) = unsafety {
diff --git a/src/test/ui/on-unimplemented/issue-104140.rs b/src/test/ui/on-unimplemented/issue-104140.rs
new file mode 100644
index 00000000000..ade3f727004
--- /dev/null
+++ b/src/test/ui/on-unimplemented/issue-104140.rs
@@ -0,0 +1,8 @@
+#![feature(rustc_attrs)]
+
+trait Foo {}
+
+#[rustc_on_unimplemented] //~ ERROR malformed `rustc_on_unimplemented` attribute input
+impl Foo for u32 {}
+
+fn main() {}
diff --git a/src/test/ui/on-unimplemented/issue-104140.stderr b/src/test/ui/on-unimplemented/issue-104140.stderr
new file mode 100644
index 00000000000..ddb1f50f0bb
--- /dev/null
+++ b/src/test/ui/on-unimplemented/issue-104140.stderr
@@ -0,0 +1,15 @@
+error: malformed `rustc_on_unimplemented` attribute input
+  --> $DIR/issue-104140.rs:5:1
+   |
+LL | #[rustc_on_unimplemented]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: the following are the possible correct uses
+   |
+LL | #[rustc_on_unimplemented = "message"]
+   | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+LL | #[rustc_on_unimplemented(/*opt*/ message = "...", /*opt*/ label = "...", /*opt*/ note = "...")]
+   | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error: aborting due to previous error
+