Lint possible leaked private structs

What it does

I discovered yesterday that it's possible to leak private struct types in public functions if that struct is public but lives in a private module. Those on the Rust discord considered this a bug in Rust itself but since it's a breaking change, it might need to wait until 2024. So perhaps this should be added to clippy for the time being.

Categories (optional)

• Kind: correctness?

Drawbacks

Perhaps this should instead just be added as a warning into rustc

The issue is that this pattern is supposedly intended and is a feature to implement sealed Traits, but it seems pretty much a bug to me for Structs.

Example

This is a minimal example showing the leaking of structs.

use foo::bbb;
// use foo::bar::Bar; // error since `bar` is private

fn main() {
    bbb();
}

mod foo {
    use bar::Bar;

    pub fn bbb() -> Bar {
        Bar
    }

    mod bar {
        pub struct Bar;
    }
} 

Start by compiling the minimal Rust example in the issue and confirm that a public function exposes a struct from a private module. No source files or tests are named; done means defining the intended Clippy behavior for this case and adding coverage showing that the leak is detected.