Unexpected warning with use statement

[Nashenas88]

Playground link for all of the examples.

Using this code:

trait Trait {
    fn func(&self);
}

pub struct Struct;

impl Trait for Struct {
    fn func(&self) { }
}

#[cfg(test)]
mod test_level_0 {
    use super::Trait;
    pub use super::*;

    mod test_level_1 {
        pub use super::*;

        #[test]
        fn test() {
            Struct.func();
        }
    }
}

Gives me this warning:

<anon>:13:9: 13:21 warning: unused import, #[warn(unused_imports)] on by default
<anon>:13     use super::Trait;
                  ^~~~~~~~~~~~

But, removing that line gives me this error:

<anon>:20:20: 20:26 error: no method named `func` found for type `Struct` in the current scope
<anon>:20             Struct.func();
                             ^~~~~~
<anon>:20:20: 20:26 help: items from traits can only be used if the trait is in scope; the following trait is implemented but not in scope, perhaps add a `use` for it:
<anon>:20:20: 20:26 help: candidate #1: use `Trait`
error: aborting due to previous error
playpen: application terminated with error code 101

Switching the code to use these use statements:

trait Trait {
    fn func(&self);
}

pub struct Struct;

impl Trait for Struct {
    fn func(&self) { }
}

#[cfg(test)]
mod test_level_0 {
    use super::Trait;
    pub use super::{Trait, Struct};

    mod test_level_1 {
        pub use super::*;

        #[test]
        fn test() {
            Struct.func();
        }
    }
}

GIves me:

<anon>:14:21: 14:26 error: `Trait` is private, and cannot be reexported [E0365]
<anon>:14     pub use super::{Trait, Struct};
                              ^~~~~
<anon>:14:21: 14:26 help: see the detailed explanation for E0365
<anon>:14:21: 14:26 note: Consider declaring module `Trait` as a `pub mod`
<anon>:14     pub use super::{Trait, Struct};
                              ^~~~~
<anon>:14:21: 14:26 error: a trait named `Trait` has already been imported in this module [E0252]
<anon>:14     pub use super::{Trait, Struct};
                              ^~~~~
<anon>:14:21: 14:26 help: see the detailed explanation for E0252
<anon>:13:5: 13:22 note: previous import of `Trait` here
<anon>:13     use super::Trait;
              ^~~~~~~~~~~~~~~~~
error: aborting due to 2 previous errors
playpen: application terminated with error code 101

And my last attempt:

trait Trait {
    fn func(&self);
}

pub struct Struct;

impl Trait for Struct {
    fn func(&self) { }
}

#[cfg(test)]
mod test_level_0 {
    use super::Trait;
    pub use super::Struct;

    mod test_level_1 {
        pub use super::*;

        #[test]
        fn test() {
            Struct.func();
        }
    }
}

Gives me:

<anon>:21:20: 21:26 error: no method named `func` found for type `Struct` in the current scope
<anon>:21             Struct.func();
                             ^~~~~~
<anon>:21:20: 21:26 help: items from traits can only be used if the trait is in scope; the following trait is implemented but not in scope, perhaps add a `use` for it:
<anon>:21:20: 21:26 help: candidate #1: use `Trait`
error: aborting due to previous error
playpen: application terminated with error code 101

So it seems that useing a private item followed by pub use super::* will export the private item publicly. I could be misunderstanding something here, but where I'm running into this, test_level_1 is actually a plugin that does pub use super::*, so the warning version is the only one that will compile. In other words, I can't just write the mods as:

#[cfg(test)]
mod test_level_0 {
    pub use super::*;

    mod test_level_1 {
        use Trait;
        pub use super::*;

        #[test]
        fn test() {
            Struct.func();
        }
    }
}

[bluss]

cc @petrochenkov, it's related to privacy

[bstrie]

@sanxiyn You mentioned on IRC that you would like to see this nominated, can you leave a comment describing why?

[sanxiyn]

I find it worrying that glob import is not equivalent to explicitly named import.

[petrochenkov]

This is some fun bug in glob imports (in name resolution, not in privacy, strictly speaking).
Glob imports normally import only pub items, but it seems like a pub glob import from a module suddenly marks all other imports from this module as public (effectively).
This compiles, but it shouldn't, because the name S shouldn't be in the module m at all. (Or at least it should be private, but it's not how non-pub imports work currently.)

struct S;

mod m {
    use super::S;
    pub use super::*;
}

fn main() {
    let s = m::S;
}

Slight variations like

    // use super::S;
    pub use super::*;

or

    use super::S;
    // pub use super::*;

or even

    pub use super::*;
    use super::S;

fail with unresolved namem::S`, i.e. the order of imports is significant! (cc @vberger)

The incorrect unused import warning may be a consequence of this problem, or a totally separate bug, I don't know.

[jseyfried]

@sanxiyn @petrochenkov the incorrect unused import is a consequence of that problem.
The problem is that the visibility of a glob import overwrites the visibilities of any existing imports that share a name with a private item in the module being glob imported from.
For example,

pub fn f() {}
pub mod foo {
    fn f() {}
}

mod bar {
    pub use f;
    pub use f as g;
    use foo::*; // This makes first import private but does not affect the second import.
}

mod baz {
    use f;
    pub use foo::*; // This makes the above import public
}

In the original example, the visibility of the first import use super::Trait; was made public by pub use super::*; since the first import shares a name (Trait) with a private item in the module being glob imported from (super). Were it not for this bug, Trait would be private, the second glob import would not import it, and the code would not compile.

This problem is due to a bug in merge_import_resolution, specifically this line, which overwrites the visibility of an imported name regardless of whether it gets duplicate checked and defined in either namespace.

I fixed this problem in PR #30294 while fixing a very closely related issue.