Function warn_on_format_spec 

fn warn_on_format_spec(
    spec: &FormatSpec<'_>,
    warnings: &mut Vec<FormatWarning>,
    input_span: Span,
    is_source_literal: bool,
)

#[rustc_on_unimplemented] and #[diagnostic::...] don’t actually do anything with specifiers, so emit a warning if they are used.