Add ui test of unsatisfied serde trait bound

test_suite/tests/ui/on_unimplemented.rs

@@ -0,0 +1,23 @@
+use serde::de::Deserialize;
+use serde::ser::Serialize;
+
+fn to_string<T>(_: &T) -> String
+where
+    T: Serialize,
+{
+    unimplemented!()
+}
+
+fn from_str<'de, T>(_: &'de str) -> T
+where
+    T: Deserialize<'de>,
+{
+    unimplemented!()
+}
+
+struct MyStruct;
+
+fn main() {
+    to_string(&MyStruct);
+    let _: MyStruct = from_str("");
+}