Function is_host_callable_type 

pub fn is_host_callable_type(ty: &DataType) -> bool

Whether a declared parameter type can be filled from a host Value.

This is the gate candela build applies when it decides which functions get a host-callable export, and it admits exactly the types value_matches_type can check an argument against. Struct, enum and function parameters are left out: a host has no way to build one.