pub fn feature_err(
    sess: &ParseSess,
    feature: Symbol,
    span: impl Into<MultiSpan>,
    explain: impl Into<DiagnosticMessage>
) -> DiagnosticBuilder<'_, ErrorGuaranteed>
Expand description

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.