Design suggestion: change ASTNode variants to contain concrete structs

[khionu]

As is, the example below can be a necessary evil. Using separate, concrete structs allows the data to be passed as a single value.

fn main() {
    // ...
    match sql_ast {
        SQLSelect { projection, relation, joins, selection, 
                    order_by, group_by, having, limit } =>  {
            select(projection, relation, joins, selection,
                   order_by, group_by, having, limit, &mut out_ts);
        },
        _ => {}
    }
}

fn select(projection: Vec<ASTNode>, relation: Option<Box<ASTNode>>, joins: Vec<Join>,
          selection: Option<Box<ASTNode>>, order_by: Option<Vec<SQLOrderByExpr>>,
          group_by: Option<Vec<ASTNode>>, having: Option<Box<ASTNode>>, limit: Option<Box<ASTNode>>,
          out_ts: &mut TokenStream) {
    // ...
}

[nickolay]

FWIW, I did this to SQLSelect locally in an experiment to make the AST more strongly typed, because it lets me to use the struct both from an ASTNode (for a subquery) and from an SQLStatement (as a top-level statement).

[nickolay]

The branch I mentioned above has been merged, so SQLSelect / SQLQuery are separate structs now. Some ASTNode variants are still inlined though, but I'm not sure it makes sense to mechanically create a separate struct for each variant. Are there any variants where this is still a problem?

[alamb]

Closing as a dupe of #311 as suggested in #311 (comment)