How do I correctly use traits in a file separate to its implementation

Question

I'm having a bit of a headache trying to use traits defined in separate files to the implementation and was hoping somebody could point out where I am going wrong.

My file structure is this

main.rs
file1.rs
thing.rs

Contents of main.rs

mod file1;
mod thing;

fn main() {
    let item : Box<dyn file1::Trait1> = Box::new(thing::Thing {});
}

file1.rs

pub trait Trait1 {    
}

thing.rs

mod file1 {
    include!("file1.rs");
}

pub struct Thing {    
}

impl file1::Trait1 for Thing {    
}

The error on compilation is:

error[E0277]: the trait bound `thing::Thing: file1::Trait1` is not satisfied
 --> src/main.rs:9:41
  |
9 |     let item : Box<dyn file1::Trait1> = Box::new(thing::Thing {});
  |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `file1::Trait1` is not implemented for `thing::Thing`
  |
  = note: required for the cast to the object type `dyn file1::Trait1`

As far as I can tell file1::Trait1 is implemented. If not, what have I actually implemented?

Answer 1

mod file1 {
    include!("file1.rs");
}

By writing this in thing.rs , you have created a module, thing::file1 , which is distinct from the top-level module file1 . Thus, you have two distinct versions of your trait, thing::file1::Trait1 and file1::Trait1 .

This is almost never the right thing. As a general principle, every .rs file (except for main.rs , lib.rs , and other crate root files) should have exactly one mod declaration .

Delete the above code from thing.rs , and use use instead of mod , or a fully qualified path:

use crate::file1;
...
impl file1::Trait1 for Thing {
    ...

or

use crate::file1::Trait1;
...
impl Trait1 for Thing {
    ...

or

impl crate::file1::Trait1 for Thing {
    ...

In general, mod defines a module , and use brings items into scope . You write mod only once per module, and use wherever you want to refer to that module.