Errors when importing traits

[nventuro]

Aim

I noticed some incorrect 'unused import' warnings, and after looking into it some more found some strange incorrect behavior when importing traits. It may extend to other imports.

Bug

The following program seems correct:

mod foo {
    pub trait Foo<let N: u32> {
    }
}

use foo::Foo;

pub trait Bar<T, let N: u32> where T: Foo<N> {}

fn main() {
}

but it compiles with a seemingly incorrect warning:

warning: unused import Foo
  ┌─ src/main.nr:6:10
  │
6 │ use foo::Foo;
  │          --- unused import

since Foo is clearly used in Bar's declaration.

Removing the use::foo:Foo line does not cause a compilation error, which I also found odd.

---

Curiously, adding a second trait causes for the warning to go away:

mod foo {
    pub trait Bar<let N: u32> {
    }
}

use foo::Bar;

trait Qux {}

pub trait Baz<T, let N: u32> where T: Bar<N> {}

fn main() {
}

but this depends on the order of the declarations - the following program does emit the original warning:

mod foo {
    pub trait Bar<let N: u32> {
    }
}

use foo::Bar;

pub trait Baz<T, let N: u32> where T: Bar<N> {}

trait Qux {}

fn main() {
}

---

Finally, placing the new trait inside a new module causes a compilation error instead:

mod foo {
    pub trait Bar<let N: u32> {
    }
}

use foo::Bar;

pub trait Baz<T, let N: u32> where T: Bar<N> {}

mod qux { 
    trait Qux {}
}

fn main() {
}

warning: unused import Bar
  ┌─ src/main.nr:6:10
  │
6 │ use foo::Bar;
  │          --- unused import
  │

error: Could not resolve 'Bar' in path
  ┌─ src/main.nr:8:39
  │
8 │ pub trait Baz<T, let N: u32> where T: Bar<N> {}
  │                                       ---
  │