Function may_use_unstable_feature 

pub fn may_use_unstable_feature<'a, I, Infcx>(
    infcx: &'a Infcx,
    param_env: <I as Interner>::ParamEnv,
    symbol: <I as Interner>::Symbol,
) -> bool
where
    I: Interner,
    Infcx: InferCtxtLike<Interner = I>,