tests/ui/tool-attributes/diagnostic_item.rs

#[rustc_diagnostic_item = "foomp"]
//~^ ERROR use of an internal attribute [E0658]
//~| NOTE the `#[rustc_diagnostic_item]` attribute is an internal implementation detail that will never be stable
//~| NOTE the `#[rustc_diagnostic_item]` attribute allows the compiler to reference types from the standard library for diagnostic purposes
struct Foomp;
fn main() {}