How to create a function that accepts a request guard as well as its state inner?

[elibroftw]

I have a function

pub fn users_coll(db: &MainDatabase) -> mongodb::Collection<User> {
    // usage with Connection: &*db
    app_db(db).collection::<User>("users")
}

which I call in a fairing like so:

pub async fn create_indexes(rocket: Rocket<Build>) -> fairing::Result {
    if let Some(db) = MainDatabase::fetch(&rocket) {
        users_coll(db)
            .create_index(
                IndexModel::builder()
                    .keys(doc! {"username": 1})
                    .options(IndexOptions::builder().unique(true).build())
                    .build(),
                None,
            )
            .await;
        return Ok(rocket);
    }
    Err(rocket)
}

But I also want to call the function in this route

#[post("/sign-up", data = "<form>")]
async fn post_sign_up(mut db: Connection<MainDatabase>, csrf_token: CsrfToken, form: Form<SignUpData>) -> Result<Redirect, Template> {
                users_coll(&db); // error
            // ...
}

The error I get is:

mismatched types
expected reference `&users::databases::MainDatabase`
   found reference `&rocket_db_pools::Connection<users::databases::MainDatabase>`

======================

I ended up using traits, but I don't like the fact that the type is extremely broad and to avoid this broadness I'd have to create a redundant struct again...

pub trait DatabaseUtils {
    fn app_db(&self) -> mongodb::Database;

    fn users_coll(&self) -> mongodb::Collection<User>;
}

impl DatabaseUtils for mongodb::Client {

[elibroftw]

I ended up using traits, but I don't like the fact that the type is extremely broad and to avoid this broadness I'd have to create a redundant struct again...

pub trait DatabaseUtils {
    fn app_db(&self) -> mongodb::Database;

    fn users_coll(&self) -> mongodb::Collection<User>;
}

impl DatabaseUtils for mongodb::Client {

[open-schnick]

Did you try this: https://api.rocket.rs/v0.5-rc/rocket_db_pools/struct.Connection.html#example ?
I didn't use mongo before but as seen in this example one can get the native db type

[elibroftw]

Well yeah I used that, but as I mentioned, I want to make functions that accept both the Struct and the connection.