Macro __mx_conversion_err_parts 

#[doc(hidden)]
macro_rules! __mx_conversion_err_parts {
    ($e:expr) => { ... };
    ($e:expr, $expected_known:expr) => { ... };
}

Internal: the argument-conversion Err(e) probe used by generated wrappers. Not public API. Yields the error’s own ErrParts; crate::error_value::conversion_condition_value adds the parameter context.

The second argument is expected_known (see ConversionErrBuiltin): true when the message prefix already states what the argument should be. It defaults to false.