error: expected identifier, found keyword `fn`
  --> $DIR/incomplete-fn-in-struct-definition.rs:4:5
   |
LL | struct S {
   |        - while parsing this struct
LL |     fn: u8
   |     ^^ expected identifier, found keyword
   |
help: escape `fn` to use it as an identifier
   |
LL |     r#fn: u8
   |     ++

error: aborting due to previous error