Function check 

pub fn check(
    target: &dyn ReplayTarget,
    request: &mut CompletionRequest,
) -> Result<(), EncodeError>

Report each option target cannot honour for request under its policy: an error under OnUnsupported::Error; under OnUnsupported::Ignore a warning, and the option is cleared on request, so it is reported once. A set option the wire answers Mapping::Nothing is an error. Completion::prepare calls it on the routed target before every completion encode.

Errors

When an option is refused under OnUnsupported::Error, the wire answers an option wrongly, or the request carries provider options that did not serialize.