cfg_attr does not work in combination with diesel derive attribute

[Ten0]

Setup

Versions

• Rust: Latest stable
• Diesel: Latest master (8a53cb7)
• Database: Postgres
• Operating System Linux

Feature Flags

• diesel: postgres

Problem Description

cfg_attr does not work in combination with diesel derive attribute

What are you trying to accomplish?

#[cfg_attr(
	feature = "diesel-by-alpha3-as-char3",
	derive(diesel::AsExpression, diesel::FromSqlRow)
)]
#[cfg_attr(
	feature = "diesel-by-alpha3-as-char3",
	diesel(sql_type = diesel::sql_types::Text)
)]
pub enum Alpha3 {
	/// Aruba
	ABW,
	/// Afghanistan
	AFG,
}

What is the expected output?

compiles

What is the actual output?

At least one `sql_type` is needed for deriving `AsExpression` on a structure.

If however I remove the cfg_attr on the diesel attribute, this compiles (but then obviously fails to compile when the feature is disabled).

Steps to reproduce

Add to the code above a FromSql/ToSql implementation

#[cfg(feature = "diesel-by-alpha3-as-char3")]
impl<DB> diesel::deserialize::FromSql<diesel::sql_types::Text, DB> for Alpha3
where
	DB: diesel::backend::Backend,
	String: diesel::deserialize::FromSql<diesel::sql_types::Text, DB>,
{
	fn from_sql(bytes: diesel::backend::RawValue<DB>) -> diesel::deserialize::Result<Self> {
		todo!()
	}
}

#[cfg(feature = "diesel-by-alpha3-as-char3")]
impl<DB> diesel::serialize::ToSql<diesel::sql_types::Text, DB> for Alpha3
where
	DB: diesel::backend::Backend,
	str: diesel::serialize::ToSql<diesel::sql_types::Text, DB>,
{
	fn to_sql<'b>(&'b self, out: &mut diesel::serialize::Output<'b, '_, DB>) -> diesel::serialize::Result {
		todo!()
	}
}

Notes

Might be related to #2626 @pksunkara ?

[weiznich]

A first step to debug this would be to dump the attributes passed to diesel_derives here.

[pksunkara]

Looks to be more of a Rust macro expansion related issue. There was a ton of work recently done on this but I am not sure if that work included cfg_attr related expansion.

[pksunkara]

Now that we bumped the MSRV to 1.56, this might be fixed. @Ten0 can you check?

[pksunkara]

Confirmed that this has been fixed.

[Ten0]

Indeed! Sorry for missing your previous message.