Function create_generic_args 

fn create_generic_args<'tcx>(
    tcx: TyCtxt<'tcx>,
    sig_id: DefId,
    delegation_id: LocalDefId,
    parent_args: &[GenericArg<'tcx>],
    child_args: &[GenericArg<'tcx>],
) -> Vec<GenericArg<'tcx>>

Creates generic arguments for further delegation signature and predicates instantiation. Arguments can be user-specified (in this case they are in parent_args and child_args) or propagated. User can specify either both parent_args and child_args, one of them or none, that is why we firstly create generic arguments from generic params and then adjust them with user-specified args.

The order of produced list is important, it must be of this pattern:

[SELF | maybe self in the beginning] [PARENT | args of delegation parent] [SIG PARENT LIFETIMES] <- lifetimes_end_pos [SIG LIFETIMES] [SELF | maybe self after lifetimes, when we reuse trait fn in free context] [SIG PARENT TYPES/CONSTS] [SIG TYPES/CONSTS]