use Trait; misses trait use within submodule

[cramertj]

I've seen this a bunch of times and always just thought that trait used-ness detection was broken, but it's actually something much more specific:

(playground)

trait MyTrait {
    fn trait_method(&self) {}
}
impl<T> MyTrait for T {}

pub struct MyStruct;

pub mod outer_mod {
    use super::{MyStruct, MyTrait};
    pub mod inner_mod {
        use super::*;
        pub fn call_trait_method() {
            5i32.trait_method()
        }
        pub fn return_struct() -> MyStruct {
            MyStruct
        }
    }
}

fn main() {
    let _ = outer_mod::inner_mod::call_trait_method();
    let _ = outer_mod::inner_mod::return_struct();
}

The code above produces the following warning:

warning: unused import: `MyTrait`
 --> src/main.rs:9:27
  |
9 |     use super::{MyStruct, MyTrait};
  |                           ^^^^^^^
  |
  = note: #[warn(unused_imports)] on by default

The use of used items through use super::*; in the submodule is handled inconsistently for types vs. traits. The use of the struct from the submodule inner_mod is detected and counted as a use of the use statement in outer_mod, but the same is not true of the use of the trait-- rustc misses that it is used via use super::*; in inner_mod and reports it as unused.

[ExpHP]

Dupe of #45268 ?

[RalfJung]

I just ran into the same problem, my minimized example is

mod helpers {
    pub trait IntExt {
        fn id(self) -> Self;
    }
    
    impl IntExt for i32 {
        fn id(self) -> Self { self }
    }
}

use helpers::IntExt;

pub mod user {
    use super::*;
    
    pub fn foo(x: i32) -> i32 {
        x.id()
    }
}

[jespersm]

Yes, this is the same as #45268, which is fixed on master now. I've run your examples, no more wrong warnings.