Specify type of diesel::row::NamedRow::get in QueryableByName macro.

[fiadliel]

Resolves #4131

diesel::row::NamedRow::get has the signature

fn get<ST, T>(&self, column_name: &str) -> Result<T>
where
    T: FromSql<ST, DB>

This can be inferred from the resulting value, if there is only one ST type in scope for FromSql<ST, DB>.

But if multiple types are in scope, e.g.

impl<__DB: diesel::backend::Backend> QueryableByName<__DB>
for StructWithTextAndCitext
where
    String: diesel::deserialize::FromSql<sql_types::Text, __DB>,
    String: diesel::deserialize::FromSql<sql_types::Citext, __DB>,
{

then ST cannot be inferred.

[fiadliel]

This looks like an issue, I'll look into it a bit more:

select_expression = (users::id + 45),
                    ^^^^^^^^^^^^^^^^ expected `id`, found `Add<id, Bound<Integer, i32>>

[weiznich]

That's related to the rust release yesterday. See https://github.com/diesel-rs/diesel/pull/4130/files for the fixes for CI.

Otherwise I would appreciate an added test for this case.

[fiadliel]

@weiznich Added two tests.

They use citext, which is Postgres-only, but it's the only example of the issue that doesn't need any extra features compiled.
Also, let me know if you have a better idea for how to install the citext extension. It appears that CREATE EXTENSION IF NOT EXISTS is racy when run on the different connections at around the same time, so I'm doing it in the tests rather than at connection setup.

[weiznich]

Thanks 👍

I changed the test case to not depend on citext anymore. This solves the parallel initialization issue. I verified that it still produces the issue without fix.

[fiadliel]

Thanks!