Private type … in public interface (error E0446)

[regexident]

When using non-pub types in ascent macros the generated code emits a "private type … in public interface (error E0446)" warning that will turn into an error in future versions of Rust.

To reproduce compile this code:

fn main() {
    #[derive(Hash, Eq, PartialEq, Clone)]
    struct PrivateType;

    #[allow(clippy::all)]
    let _ = ascent::ascent_run! {
        // struct Program;

        relation dummy(PrivateType);
    };
}

warning: private type `PrivateType` in public interface (error E0446)
  --> src/main.rs:6:13
   |
6  |       let _ = ascent::ascent_run! {
   |  _____________^
7  | |         // struct Program;
8  | |
9  | |         relation dummy(PrivateType);
10 | |     };
   | |_____^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #34537 <https://github.com/rust-lang/rust/issues/34537>
   = note: `#[warn(private_in_public)]` on by default
   = note: this warning originates in the macro `ascent::ascent_run` (in Nightly builds, run with -Z macro-backtrace for more info)

Un-commenting the // struct Program; silences the warning.

I'm not sure if there's a good solution for this (which doesn't get in the way of scenarios for public types), but it might be a good idea to add a note in the docs for this, since the fix might not be obvious immediately?

[s-arash]

As you pointed out, the solution already exists: a struct declaration at the top of the Ascent program. I'll add a hint to the documentation on silencing this warning.

[regexident]

Alternatively the struct <PROGRAM_NAME>; pattern could be mandatory, which would then allow for specifying the visibility by use of pub struct <PROGRAM_NAME>;, pub(crate) struct <PROGRAM_NAME>;, or whatever other level is required.

This way future changes in the compiler turning the warning into an error won't break any existing ascent-using projects.

[s-arash]

I added a note to README.MD about this warning. I think it's good enough.