Macro __mx_conversion_expectation 

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

Internal: what a failed argument conversion’s error says the value should have been, for a parameter whose Rust type gives the macro no R-facing expectation (#1594). Some("one of \"fast\", \"slow\"") for a match_arg choice error, Some("a data frame") for a class error, the joined expectation of an Either whose arms both refused the kind of value, None otherwise. Not public API.