Macro declare_inbound 

macro_rules! declare_inbound {
    ($name:ident) => { ... };
    () => { ... };
}

Generate the optional code_module_set_inbound export.

A macro rather than a plain function in this crate, and that is load-bearing: #[no_mangle] symbols defined in a dependency are not reliably kept in the final cdylib, so the export has to be emitted in your crate. One invocation at the top level is all it takes:

code_native::declare_inbound!();

A module that never speaks first simply doesn’t invoke it — the export is optional, and the host checks for it rather than requiring it.