Rust 1.16 leaking private types out of a crate library through "pub use"

[erickt]

Hello!

We found a case where it's possible to leak a private type through the use of pub use. Consider this crate with a single file buffer.rs:

mod buffer {
    pub struct EnumeratePixels;
    pub struct ImageBuffer;

    impl ImageBuffer {
        pub fn enumerate_pixels(&self) -> EnumeratePixels {
            EnumeratePixels
        }
    }
}

pub use buffer::ImageBuffer;

Rust allows it to compile, even though EnumeratePixels is not publicly exposed from root:

% rustc --crate-type lib buffer.rs
%

It's possible to then use this crate like so:

extern crate buffer;

fn main() {
    let buffer = buffer::ImageBuffer;
    let _pixels = buffer.enumerate_pixels();
}

Rust 1.16 allows this to compile:

% rustc -L . bug.rs
%

However, it's impossible to name the type of enumerate_pixels():

extern crate buffer;

fn main() {
    let buffer = buffer::ImageBuffer;
    let _pixels: buffer::buffer::EnumeratePixels = buffer.enumerate_pixels();
}

This produces:

% rustc -L . bug.rs
error: module `buffer` is private
 --> bug.rs:5:18
  |
5 |     let _pixels: buffer::buffer::EnumeratePixels = buffer.enumerate_pixels();
  |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

I suspect this bug may have been reported in a few other tickets: #33077, #39437, #38844, #39437

[Thinkofname]

This seems to be intended according to #34537 which documents using private modules to leak private types.

[petrochenkov]

@Thinkofname is right, this is the intended behavior.

[retep998]

The fact that this is intended behavior is why I'm so opposed to the current private in public rules.