Implement the default_trait tag

[avl]

#[default_trait="TraitName"] is not implemented yet.

This is a way to be able to provide arbitrary default values of complex types, to fields which did not exist in older versions of protocol.

[avl]

Changed it to be a default_fn instead, that you specify:

'''rust
fn my_default_func() -> String {
"default_value".to_string()
}
//Then, before new struct field:
#[default_fn="my_default_func"]
'''