Function replace_erased_regions_with_placeholders 

fn replace_erased_regions_with_placeholders<'tcx>(
    tcx: TyCtxt<'tcx>,
    inputs_and_output: &[Ty<'tcx>],
) -> Vec<Ty<'tcx>>

This replaces all external regions in the signature of the current item with a unique placeholder to collect its implied bounds. This mirrors the way MIR borrowck replaces all of them with unique NLL vars.