Compiler bad error message: E0277 error in Diesel

[JCapucho]

I tried this code:

pub fn establish_connection(database_url: &str) -> Pool<PgConnection> {
        let manager = ConnectionManager::<PgConnection>::new(database_url);
        Pool::builder()
                .build(manager)
                .expect(&format!("Error connecting to {}", database_url))
}

And was expecting to get an error message something like:

Expected Pool<ConnectionManager<PgConnection>> got Pool<PgConnection>

Instead i got an misleading error:

the trait `diesel::r2d2::ManageConnection` is not implemented for `diesel::PgConnection`

[estebank]

CC @sgrif as per our all-hands conversation, this is something that can be addressed for nightly users using rustc_on_unimplemented.

[estebank]

We are getting closer to being able to deal with this on stable: #111780