Some(diesel::expression::now) fails to set Nullable<Timestamp> value

[Nemo157]

Judging by impl Expression<SqlType = Timestamp> for now + impl<T, ST> AsExpression<Nullable<ST>> for Option<T> where ST: NotNull it seems like this should work, and the error message makes me think it has made it past that stage of the type checking, but for some reason the resulting UpdateStatement doesn't implement ExecuteDsl.

A workaround is to just use diesel::expression::sql to bypass the type checking and inject CURRENT_TIMESTAMP directly: baz.eq(sql("CURRENT_TIMESTAMP")).

diesel = { version = "0.12.0", features = ["postgres"] }
cargo-0.17.0-nightly (f9e5481 2017-03-03)
rustc 1.16.0 (30cf806ef 2017-03-10)

#[macro_use]
extern crate diesel;

table! {
    foo {
        id -> Int4,
        bar -> Timestamp,
        baz -> Nullable<Timestamp>,
    }
}

pub fn foo(pg: &diesel::pg::PgConnection) {
    use diesel::prelude::*;
    use diesel::update;
    use foo::dsl::{foo, id, bar, baz};

    // This fails to compile because of the `baz.eq(Some(now))` line,
    // comment that out and it will work
    {
        use diesel::expression::now;
        update(foo.filter(id.eq(4)))
            .set((
                bar.eq(now),
                baz.eq(Some(now))
            ))
            .execute(pg)
            .unwrap();
    }
}

error: no method named `execute` found for type `diesel::query_builder::UpdateStatement<foo::table, diesel::query_builder::where_clause::WhereClause<diesel::expression::predicates::Eq<foo::columns::id, diesel::expression::bound::Bound<diesel::types::Integer, i32>>>, (diesel::expression::predicates::Eq<foo::columns::bar, diesel::expression::now>, diesel::expression::predicates::Eq<foo::columns::baz, diesel::expression::bound::Bound<diesel::types::Nullable<diesel::types::Timestamp>, std::option::Option<diesel::expression::now>>>)>` in the current scope
  --> src/lib.rs:26:14
   |
26 |             .execute(pg)
   |              ^^^^^^^
   |
   = note: the method `execute` exists but the following trait bounds were not satisfied: `diesel::query_builder::UpdateStatement<foo::table, diesel::query_builder::where_clause::WhereClause<diesel::expression::predicates::Eq<foo::columns::id, diesel::expression::bound::Bound<diesel::types::Integer, i32>>>, (diesel::expression::predicates::Eq<foo::columns::bar, diesel::expression::now>, diesel::expression::predicates::Eq<foo::columns::baz, diesel::expression::bound::Bound<diesel::types::Nullable<diesel::types::Timestamp>, std::option::Option<diesel::expression::now>>>)> : diesel::query_builder::QueryFragment<_>`, `&diesel::query_builder::UpdateStatement<foo::table, diesel::query_builder::where_clause::WhereClause<diesel::expression::predicates::Eq<foo::columns::id, diesel::expression::bound::Bound<diesel::types::Integer, i32>>>, (diesel::expression::predicates::Eq<foo::columns::bar, diesel::expression::now>, diesel::expression::predicates::Eq<foo::columns::baz, diesel::expression::bound::Bound<diesel::types::Nullable<diesel::types::Timestamp>, std::option::Option<diesel::expression::now>>>)> : diesel::query_builder::QueryFragment<_>`, `&mut diesel::query_builder::UpdateStatement<foo::table, diesel::query_builder::where_clause::WhereClause<diesel::expression::predicates::Eq<foo::columns::id, diesel::expression::bound::Bound<diesel::types::Integer, i32>>>, (diesel::expression::predicates::Eq<foo::columns::bar, diesel::expression::now>, diesel::expression::predicates::Eq<foo::columns::baz, diesel::expression::bound::Bound<diesel::types::Nullable<diesel::types::Timestamp>, std::option::Option<diesel::expression::now>>>)> : diesel::query_builder::QueryFragment<_>`, `&mut diesel::query_builder::UpdateStatement<foo::table, diesel::query_builder::where_clause::WhereClause<diesel::expression::predicates::Eq<foo::columns::id, diesel::expression::bound::Bound<diesel::types::Integer, i32>>>, (diesel::expression::predicates::Eq<foo::columns::bar, diesel::expression::now>, diesel::expression::predicates::Eq<foo::columns::baz, diesel::expression::bound::Bound<diesel::types::Nullable<diesel::types::Timestamp>, std::option::Option<diesel::expression::now>>>)> : diesel::query_builder::QueryId`

[sgrif]

Some is for use with Rust types. now corresponds to the SQL CURRENT_TIMESTAMP expression. Combining the two doesn't make sense. You probably want now.nullable()

[Nemo157]

Ok, I think I understand, there's actually sort of two parallel worlds of Rust values and SQL expressions which use either Option or Nullable for wrapping non-nullable entities into nullable ones. It would be great if the API docs could mention this somehow, but I guess putting something on the impl<T, ST> AsExpression<Nullable<ST>> for Option<T> where ST: NotNull needs rust-lang/rust#18701 still.