E0446 Doesn't Trigger For Types In Private Modules

[danii]

Declaring a public type and a public function in a private module, and then publicly reexporting the function doesn't trigger the E0446 error on using private types in public interfaces.

mod module {
  pub struct Private; // This isn't available outside of the crate because the module it is in is private.
  
  pub fn public() -> Private {Private} // This is available outside of the crate...
}

pub use module::public; // ...because of this reexport here.

I expected to see the E0446 error, because the above code is semantically equivalent to this code, from a downstream crate's point of view.

struct Private;

pub fn public() -> Private {Private} // error[E0446]: private type `Private2` in public interface

Meta

This bug occurs both on Rust version rustc 1.56.0-nightly (3d0774d0d 2021-08-18) and version rustc 1.54.0 (a178d0322 2021-07-26).

[ExpHP]

I don't think this is a bug. It is a central part of the sealed trait pattern. (Essentially, it allows one to define "Voldemort traits" akin to D's Voldemort types)

[pierwill]

Should there be a lint for this (compiler or clippy) in certain cases, if there are any outside the sealed trait pattern? 🤔

@rustbot label +A-diagnostics +A-lint -C-bug

[emilk]

This bites me all the time. I usually mark most my things pub in each moduile, and then in the top lib.rs I decide what will be public to crate users. Because of this bug, E0446 almost never prevents me from "half-exporting" things (exporting a function but not its return-value, for instance).