Macro label_order 

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

Define a label ordering, an implementation of LabelOrder

Syntax:

$type: $( $label_order ),*

with

$label_order: $( $label_group )<*
$label_group: { $($variant),* } | $variant
$variant: $ident | '$'

For example

#[derive(Copy, Clone)]
enum Lbl {
    Def,
    Mod,
    Lex,
    Imp,
}

label_order!(Lbl: $ < { Def, Mod } < Lex, Imp < Lex);

Here, $ means the end-of-path label: i.e., the current scope.

Using { ... }, labels with equal priority can be grouped.

Multiple partial orders can be combined using ,-separators: X < Y, Z < Y.

Sequences of multiple < can be chained (X < Y < Z). This is equivalent to the 2-windowed decomposition: X < Y, Y < Z (which, by transitivity, also includes X < Z).

The macro will at compile time validate whether the order is a strict partial order, and report any symmetric or reflexive entries.