Function expr_use_sites 

pub fn expr_use_sites<'tcx>(
    tcx: TyCtxt<'tcx>,
    typeck: &'tcx TypeckResults<'tcx>,
    ctxt: SyntaxContext,
    e: &'tcx Expr<'tcx>,
) -> impl Iterator<Item = ExprUseSite<'tcx>>

Returns an iterator which walks successive value using parent nodes skipping any node which simply moves a value.