if_chain! {
    if let Expr_::ExprField(ref object, ref field_name) = stmt.node;
    if field_name.node.as_str() == "bar"
    if let Expr_::ExprAddrOf(MutImmutable, ref inner) = object.node;
    if let Expr_::ExprPath(ref path) = inner.node;
    if match_qpath(path, &["a"]);
    then {
        // report your lint here
    }
}