Function transform_gen_context 

fn transform_gen_context<'tcx>(body: &mut Body<'tcx>)

Transforms the body of the coroutine applying the following transform:

• Remove the resume argument.

Ideally the async lowering would not add the resume argument.

The async lowering step and the type / lifetime inference / checking are still using the resume argument for the time being. After this transform, the coroutine body doesn’t have the resume argument.