Macro __mx_result_err_parts 

#[doc(hidden)]
macro_rules! __mx_result_err_parts {
    ($e:expr, $prefix:expr, $marker:ty) => { ... };
}

Internal: the Err(e) probe used by generated wrappers. Not public API.

$prefix is the family class for the serde arms (<crate>_error, computed by the macro from CARGO_CRATE_NAME at expansion time). $marker is the calling crate’s marker type (crate::__MiniextendrInitMarker), under which serde_error_shape! declarations are looked up first, under the serde feature: a declared shape wins over the stages below (the ErrShapeDeclared probe).