Function expand_unreachable 

pub(crate) fn expand_unreachable<'cx>(
    cx: &'cx mut ExtCtxt<'_>,
    sp: Span,
    tts: TokenStream,
) -> MacroExpanderResult<'cx>

This expands to either

• $crate::panic::unreachable_2015!(...) or
• $crate::panic::unreachable_2021!(...) depending on the edition.