Context The inner state of a generator contains so-called "witness" types. Those types influence which auto types a generator implements: a generator may not be Send if its inner state contains a non-Send type. Generators are currently type-checked as follows. The generator has type Generator(parent_substs, resume ty, yield ty, return ty, witness, upvars types). During most of typeck, witness is a type variable. Once most inference is done, the set of captured types is computed on HIR.