Macro impl_const_default 

macro_rules! impl_const_default {
    ($($(#[$doc:meta])* $(@$pub:ident)? $type:ty => $default:expr;)*) => { ... };
}

Available on crate features formatting or parsing only.

Implement Default for the given type. This also generates an inherent implementation of a default method that is const fn, permitting the default value to be used in const contexts.