dtolnay/syn 1.0.60

on GitHub

Provide an idiom for testing exhaustiveness of pattern matches on Expr, Type, Pat, Item, ForeignItem, TraitItem, and ImplItem (#694)

match expr {
    Expr::Array(e) => {...}
    Expr::Assign(e) => {...}
    ...
    Expr::Yield(e) => {...}

    #[cfg(test)]
    Expr::__TestExhaustive(_) => unimplemented!(),
    #[cfg(not(test))]
    _ => { /* some sane fallback */ }
}

The above is the only supported idiom for exhaustive matching of those enum. Do not write anything differently as it is not supported.

The conditional compilation on match-arms lets us fail your tests but not break your library when adding a variant. You will be notified by a test failure when a variant is added, so that you can add code to handle it, but your library will continue to compile and work for downstream users in the interim.