Link error when a public module exposes types from a private module

[kornelski]

Compilation of:

mod privmod {
    pub struct HiddenType;

    impl HiddenType {
        pub fn hidden_method(&self) {

        }
    }
}

pub mod pubmod {
    use privmod;

    pub fn expose() -> privmod::HiddenType {
        privmod::HiddenType
    }
}

pub use pubmod::expose;

produces the warning:

warning: method is never used: hidden_method, #[warn(dead_code)] on by default

I think it's not correct, because hidden_method can be called from outside the crate by getting an instance of the type via pubmod::expose.

I'd expect Rust to either stop with an error that a private type (private due to being in a private module) is exposed, or figure out that the method is exposed indirectly and assume it's used.

This problem creates another when linking across crates. To reproduce the problem: take the example code above and put it in src/lib.rs of a crate called "library", and then create a crate "tool" with it as a dependency:

tool/Cargo.toml +=

[dependencies.library]
path = "../library"

and src/bin/tool.rs as follows:

extern crate library;

fn main() {
    library::expose().hidden_method();
}

cargo run --bin tool will compile, but won't link:

Undefined symbols for architecture x86_64:
  "privmod::HiddenType::hidden_method::h4e1e7549e65f404ckaa", referenced from:
      main::hb631601ec0f76d56faa in tool.0.o

I'd prefer this to either work, or fail at compilation stage, because linker errors are scary.

---

rustc 1.4.0-nightly (8f1b0aa 2015-08-21); OS X 11.11

[alexcrichton]

I believe this is a duplicate of #16734, so closing in favor of that.