Macro feature_gated_sink_mod 

macro_rules! feature_gated_sink_mod {
    ($mod_name:ident, $sink_name:literal) => { ... };
    ($mod_name:ident, $struct_prefix:ident, $sink_name:literal) => { ... };
}

Define a sink module that is gated by a feature.

This is to allow some heavy or unpopular sink implementations (and their dependencies) to be disabled at compile time, in order to decrease compilation time and binary size.