Porting code with Nullable<> to 2.0.0

[ensc]

I am about to port my existing code to work with the master branch and stumble about problems with Nullable<>. The following code compiles fine with 1.4.6 but fails on master:

use diesel::dsl::{Or, Eq};
use diesel::expression::bound::Bound;
use diesel::expression::{ Expression };
use diesel::sql_types::{ Bool, Text, Nullable };

type FiBound<FI> = Bound<<FI as Expression>::SqlType, &'static str>;
type BoolBound = Bound<Bool, bool>;

fn filter<FI>(field: FI) -> Or<BoolBound,Eq<FI, FiBound<FI>>>
where
    FI: diesel::expression_methods::ExpressionMethods<SqlType = Nullable<Text>>
{
    use diesel::expression::AsExpression;
    use diesel::BoolExpressionMethods;

    let fn_bool = |v| <bool as AsExpression<Bool>>::as_expression(v);

    fn_bool(true).or(field.eq(""))
}

fn main() {
}

Error on master is

error[E0271]: type mismatch resolving `<Grouped<diesel::expression::operators::Eq<FI, diesel::expression::bound::Bound<diesel::sql_types::Nullable<Text>, &'static str>>> as diesel::Expression>::SqlType == Bool`
  --> src/main.rs:9:1
   |
9  | / fn filter<FI>(field: FI) -> Or<BoolBound,Eq<FI, FiBound<FI>>>
10 | | where
11 | |     FI: diesel::expression_methods::ExpressionMethods<SqlType = Nullable<Text>>
12 | | {
...  |
18 | |     fn_bool(true).or(field.eq(""))
19 | | }
   | |_^ expected struct `diesel::sql_types::Nullable`, found struct `Bool`
   |
   = note: expected struct `diesel::sql_types::Nullable<Bool>`
              found struct `Bool`
   = note: required because of the requirements on the impl of `AsExpression<Bool>` for `Grouped<diesel::expression::operators::Eq<FI, diesel::expression::bound::Bound<diesel::sql_types::Nullable<Text>, &'static str>>>`

Replacing the Nullable<Text> with Text allows it to build (but I need Nullable here).

How can I express this with master?

For reference: real code at

• https://gitlab-ext.sigma-chemnitz.de/ensc/sstate-server/-/blob/8b492651d2b867502f23bcaefe233f0144f4f615/src/models/lookup/string.rs#L49-74
• https://gitlab-ext.sigma-chemnitz.de/ensc/sstate-server/-/blob/8b492651d2b867502f23bcaefe233f0144f4f615/src/models/user.rs#L412-415

[weiznich]

We changed our type tracking to be more in line with what database actually do. NULL OR some_expr evaluates to NULL, therefore we made sure that boolean expressions propagate nullability based on the arguments.
In consequence that means as soon as at least on of your arguments may be nullable diesel requires that the output can be nullable. Due to type inference issues this can require a third type parameter to diesel::dsl::Or, as outlined in the documentation, to specify if that expression is Nullable or not.

use diesel::dsl::{Or, Eq, AsExprOf};
use diesel::sql_types::{ Bool, Text, Nullable };
use diesel::{IntoSql, ExpressionMethods};

type FiBound<ST> = AsExprOf<&'static str, ST>;
type BoolBound = AsExprOf<bool, Bool>;

fn filter<FI>(field: FI) -> Or<BoolBound, Eq<FI, FiBound<FI::SqlType>>, Nullable<Bool>>
where
    FI: ExpressionMethods<SqlType = Nullable<Text>>,
{
    use diesel::BoolExpressionMethods;

    true.into_sql::<Bool>().or(field.eq(""))
}

(I also removed any usage of API's that are not part of our public API.)