Macro build_fix 

macro_rules! build_fix {
    ($builder:expr, $seq_out:expr, $dt:expr, $msg_type:expr $(,)?) => { ... };
    ($builder:expr, $seq_out:expr, $dt:expr, $msg_type:expr $(, $($rest:tt)+)?) => { ... };
    (@fields $msg:expr) => { ... };
    (@fields $msg:expr, ) => { ... };
    (@fields $msg:expr, $tag:expr => $val:expr ,) => { ... };
    (@fields $msg:expr, @$val:expr ,) => { ... };
    (@fields $msg:expr, $tag:expr, $val:expr ,) => { ... };
    (@fields $msg:expr, $tag:expr => $val:expr $(, $($rest:tt)+)?) => { ... };
    (@fields $msg:expr, @$val:expr $(, $($rest:tt)+)?) => { ... };
    (@fields $msg:expr, $tag:expr, $val:expr $(, $($rest:tt)+)?) => { ... };
}

Build a FIX message with a single macro invocation.

This expands to begin_with(...).field(...).finish() using FixBuilder. Supports tag => value pairs, tagged values (@value), and legacy tag, value pairs.