Macro web_attr_allows 

macro_rules! web_attr_allows {
    ($flag:tt; allow) => { ... };
    ($flag:tt; ignore) => { ... };
    ($flag:tt; $other:tt) => { ... };
    ($flag:tt;) => { ... };
}

Maps a feature’s #[web(...)] attribute to whether the feature is allowed on the web backend, rejecting a missing or unrecognized value.