Macro buffers 

macro_rules! buffers {
    ($($(#[$meta:meta])* $vis:vis const $name:ident = $label:literal $($flag:ident)*;)+) => { ... };
}

Declares a model’s storage buffers and their indices in one place.

The index is the declaration’s position, so it is derived rather than written down. Flags are named rather than positional, as in agent_lanes!, and default off. Invoke it at module scope, next to the impl that forwards BUFFERS to BUFFER_SPECS.

buffers! {
    const POS = "pos" double_buffered drawable;
    const VEL = "vel" double_buffered;
    const SITES = "sites";
}