Macro params 

macro_rules! params {
    ($($(#[$meta:meta])* $vis:vis const $name:ident = $descriptor:expr;)+) => { ... };
}

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

The index is the declaration’s position, so it is derived rather than written down. Invoke it at module scope, next to the impl that forwards param_descriptors to descriptors.

params! {
    /// Reason for this default, if it needs saying.
    const DENSITY = f32_param("density", "Initial Density", 0.3, 0.0, 1.0, Some(0.01));
}