Doesn't build with rustc 1.73

[sdroege]

error[E0446]: crate-private type `Conn<'a>` in public interface
  --> src/rusttls/stream.rs:13:5
   |
13 |     pub conn: Conn<'a>,
   |     ^^^^^^^^^^^^^^^^^^ can't leak crate-private type
...
17 | pub(crate) enum Conn<'a> {
   | ------------------------ `Conn<'a>` declared as crate-private

error[E0446]: crate-private type `Conn<'a>` in public interface
   --> src/rusttls/stream.rs:127:5
    |
17  | pub(crate) enum Conn<'a> {
    | ------------------------ `Conn<'a>` declared as crate-private
...
127 |     pub fn new(io: &'a mut IO, conn: impl Into<Conn<'a>>) -> Self {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't leak crate-private type

This is just cargo +1.73 check in this repository.