nom-sql: Parse arbitrary exprs in args to group_concat

I'm pretty sure the previous behavior of only parsing columns is a
legacy accident from before we had a more flexible expression AST -
regardless, MySQL accepts any expr here, so we should as well.

Change-Id: I505a780da514d216abdd97cea63c55980a5e6c8b
Reviewed-on: https://gerrit.readyset.name/c/readyset/+/5161
Tested-by: Buildkite CI
Reviewed-by: Dan Wilbanks <dan@readyset.io>

nom-sql/src/common.rs

@@ -417,13 +417,8 @@ fn group_concat_fx_helper(
 
 fn group_concat_fx(
     dialect: Dialect,
-) -> impl Fn(LocatedSpan<&[u8]>) -> NomSqlResult<&[u8], (Column, Option<String>)> {
-    move |i| {
-        pair(
-            column_identifier_no_alias(dialect),
-            opt(group_concat_fx_helper(dialect)),
-        )(i)
-    }
+) -> impl Fn(LocatedSpan<&[u8]>) -> NomSqlResult<&[u8], (Expr, Option<String>)> {
+    move |i| pair(expression(dialect), opt(group_concat_fx_helper(dialect)))(i)
 }
 
 fn agg_fx_args(
@@ -576,8 +571,8 @@ pub fn function_expr(
                         ),
                     ),
                 ),
-                |(col, separator)| FunctionExpr::GroupConcat {
-                    expr: Box::new(Expr::Column(col)),
+                |(expr, separator)| FunctionExpr::GroupConcat {
+                    expr: Box::new(expr),
                     separator,
                 },
             ),
@@ -896,9 +891,9 @@ mod tests {
         assert_eq!(res.unwrap().1, expected);
 
         assert_eq!(
-            test_parse!(function_expr(Dialect::MySQL), b"group_concat(a)"),
+            test_parse!(function_expr(Dialect::MySQL), b"group_concat('a')"),
             FunctionExpr::GroupConcat {
-                expr: Box::new(Expr::Column("a".into())),
+                expr: Box::new(Expr::Literal("a".into())),
                 separator: None
             }
         );