src/librustc_error_codes/error_codes/E0639.md

   1 This error indicates that the struct, enum or enum variant cannot be
   2 instantiated from outside of the defining crate as it has been marked
   3 as `non_exhaustive` and as such more fields/variants may be added in
   4 future that could cause adverse side effects for this code.
   5
   6 It is recommended that you look for a `new` function or equivalent in the
   7 crate's documentation.