error: #[proc_macro_error] attribute can be used only with procedural macros

  = hint: if you are really sure that #[proc_macro_error] should be applied to this exact function, use #[proc_macro_error(allow_not_macro)]

 --> $DIR/not_proc_macro.rs:3:1
  |
3 | #[proc_macro_error]
  | ^^^^^^^^^^^^^^^^^^^
  |
  = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)