Function rustc_ap_rustc_session::parse::feature_err

pub fn feature_err<'a>(
    sess: &'a ParseSess, 
    feature: Symbol, 
    span: impl Into<MultiSpan>, 
    explain: &str
) -> DiagnosticBuilder<'a>

Construct a diagnostic for a language feature error due to the given span. The feature's Symbol is the one you used in active.rs and rustc_span::symbols.