Macro dbc_tables 

macro_rules! dbc_tables {
    (
        dir $dir:ident;
        csv { $( $ckind:ident $cfield:ident : $crow:ty = $csv:literal ; )+ }
        derived { $( $dfield:ident : $dty:ty = $dcompute:expr ; )+ }
    ) => { ... };
}

The caller-supplied dir identifier shares a hygiene context with the compute expressions.