Macro sort_in 

sort_in!() { /* proc-macro */ }

Sort macro input tokens

Example

After sorting, it is easy to input keys in an unordered manner

macro_rules! foo {
    (@branch (a $a:literal) (b $b:literal)) => {
        concat!(stringify!($a), stringify!($b))
    };
    ($($name:ident : $val:literal)+) => {
        sort_macro::sort_in!({@branch} foo!($( ($name $val) )+))
    };
}
fn main() {
    assert_eq!(foo!(b:3 a:2), "23");
    assert_eq!(foo!(a:2 b:3), "23");
}

Grammar

Sort in last group, sort key is TokenTree::to_string

input  = [prefix] *tt group
prefix = "{" *tt "}"