This type represents the conditions under which the receiver is considered to "match" a given method candidate. Typically the test is whether the receiver is of a particular type. However, this type is the type of the receiver after accounting for the method's self type (e.g., if the method is an Box<self> method, we have already verified that the receiver is of some type Box<T> and now we must check that the type T is correct). Unfortunately, because traits are not types, this is a pain to do.