Specify macro is invalid in certain contexts

compiler/rustc_parse/src/errors.rs

@@ -1800,6 +1800,12 @@ pub struct UnknownPrefix<'a> {
     pub sugg: Option<UnknownPrefixSugg>,
 }
 
+#[derive(Subdiagnostic)]
+#[note(parse_macro_expands_to_adt_field)]
+pub struct MacroExpandsToAdtField<'a> {
+    pub adt_ty: &'a str,
+}
+
 #[derive(Subdiagnostic)]
 pub enum UnknownPrefixSugg {
     #[suggestion(