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?

Kevin Reid · Accepted Answer · 2021-07-18 21:13:19Z

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.

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