pub fn report_unstable(
    sess: &Session,
    feature: Symbol,
    reason: Option<Symbol>,
    issue: Option<NonZeroU32>,
    suggestion: Option<(Span, String, String, Applicability)>,
    is_soft: bool,
    span: Span,
    soft_handler: impl FnOnce(&'static Lint, Span, &str)
)