expectFor

fn expectFor(p: *Parse, comptime bodyParseFn: fn (p: *Parse) Error!Node.Index) !Node.Index {
    const for_token = p.assertToken(.keyword_for);

    const scratch_top = p.scratch.items.len;
    defer p.scratch.shrinkRetainingCapacity(scratch_top);
    const inputs = try p.forPrefix();

    const then_expr = try bodyParseFn(p);
    var has_else = false;
    if (p.eatToken(.keyword_else)) |_| {
        try p.scratch.append(p.gpa, then_expr);
        const else_expr = try bodyParseFn(p);
        try p.scratch.append(p.gpa, else_expr);
        has_else = true;
    } else if (inputs == 1) {
        return try p.addNode(.{
            .tag = .for_simple,
            .main_token = for_token,
            .data = .{ .node_and_node = .{
                p.scratch.items[scratch_top],
                then_expr,
            } },
        });
    } else {
        try p.scratch.append(p.gpa, then_expr);
    }
    return try p.addNode(.{
        .tag = .@"for",
        .main_token = for_token,
        .data = .{ .@"for" = .{
            (try p.listToSpan(p.scratch.items[scratch_top..])).start,
            .{ .inputs = @intCast(inputs), .has_else = has_else },
        } },
    });
}