Error message is unclear when compiling

[am0d]

I had a schema in Postgres, with a column of type DATE.
I created a struct to match this in Rust, and tried to map that DATE column to a chrono::NaiveDateTime type, like this:

#[derive(Clone, Debug, Deserialize, Eq, Ord, PartialEq, PartialOrd, Queryable)]
#[insertable_into(periods)]
#[changeset_for(periods)]
pub struct Period {
    pub id: i32,
    pub name: String,
    pub start_date: NaiveDateTime,
    pub end_date: NaiveDateTime,
}

However, this failed to compile (only when the insertable_into or changeset_for attributes were there), but the error message was very unhelpful, and didn't point to the root cause at all:

error: the trait bound `chrono::NaiveDateTime: diesel::Expression` is not satisfied

(and the error span was pointing at the #[insertable_into(...)] attribute.

[sgrif]

Unsure how much control we have here, but I'd like the error message to either be:

`NaiveDateTime: AsExpression<types::Date>` is not satisfied

or

`NaiveDateTime: ToSql<types::Date, Pg>` is not satisfied

Those are only marginally more clear, but at least they mention the fact that the column is of type date. Ideally we could give an even better message, explicitly calling out the mistake, but I don't believe we have the ability to do that at the moment.

[sgrif]

This is another instance of rust-lang/rust#28894 biting us. It's picking up on the impl<T: Expression> AsExpression<T::SqlType> for T blanket impl, and complaining about Expression missing even though the underlying problem is the missing AsExpression impl. I'm trying to see if there's a way for me to move this around to circumvent the problem, but this might just need to be fixed upstream.

[arielb1]

This should be better in rustc 1.10.