Contradictory unused imports on traits

[WiSaGaN]

Below generates an unused warning:

extern crate num;

use num::Zero;

pub mod foo {
    use super::*;

    pub fn foo_zero() -> u64 {
        u64::zero()
    }
}

warning: unused import: `num::Zero`
 --> src/lib.rs:3:5
  |
3 | use num::Zero;
  |     ^^^^^^^^^
  |
  = note: #[warn(unused_imports)] on by default

While removing the suggested offended line generates error:

error: no associated item named `zero` found for type `u64` in the current scope
 --> src/lib.rs:7:9
  |
7 |         u64::zero()
  |         ^^^^^^^^^
  |
  = help: items from traits can only be used if the trait is in scope; the following traits are implemented but not in scope, perhaps add a `use` for one of them:
  = help: candidate #1: `use core::fmt::num::Int;`
  = help: candidate #2: `use num::Zero;`

Tested on rustc 1.18.0 (03fc9d622 2017-06-06) and rustc 1.21.0-nightly (59ccba995 2017-08-17) with slightly different error message.

Seems similar to symptom of #30159

[oli-obk]

Why are private use addressable by inner modules at all?

The warning disappears if foo contains type Foo = Zero;

[Marwes]

Reproduction without any dependency

mod test {
    pub trait A {
        fn a();
    }

    impl A for () {
        fn a() { }
    }
}

use test::A;

mod test2 {
    use super::*;
    pub fn b() {
        let _ = <()>::a();
    }
}

fn main() {
    test2::b();
}

[ExpHP]

Same as #45268. (this issue is older, but I'm building a list of "linkbacks" there)

[sanxiyn]

Closing as a duplicate.