Function erase_free_regions 

pub fn erase_free_regions<'tcx, T>(tcx: TyCtxt<'tcx>, value: T) -> T
where
    T: TypeFoldable<TyCtxt<'tcx>>,

Erase free regions from the given value. Largely copied from tcx.erase_and_anonymize_regions, but also erases bound regions that are bound outside value, so we can call this function inside a Binder.