Function peel_parent_while 

fn peel_parent_while(
    tcx: TyCtxt<'_>,
    did: DefId,
    f: impl FnMut(TyCtxt<'_>, DefId) -> bool,
) -> Option<DefId>

Given a DefId checks if it satisfies f if it does check with it’s parent and continue until it doesn’t satisfies f and return the last DefId checked.

In other word this method return the first DefId that doesn’t satisfies f.