Regression in formatting Syn's token macro

[dtolnay]

Our CI hit a surprising formatting change: https://travis-ci.org/dtolnay/syn/jobs/490139618.

 #[cfg(any(feature = "full", feature = "derive"))]
 pub fn fold_abi<V: Fold + ?Sized>(_visitor: &mut V, _i: Abi) -> Abi {
     Abi {
-        extern_token: Token![extern](tokens_helper(_visitor, &_i.extern_token.span)),
+        extern_token: Token ! [ extern ](tokens_helper(_visitor, &_i.extern_token.span)),
         name: (_i.name).map(|it| _visitor.fold_lit_str(it)),
     }
 }

I minimized this to:

echo 'Token ! [ extern ];' | cargo run --bin rustfmt

Before f4f9502:

Token![extern];

Since f4f9502:

Token ! [ extern ];

Mentioning @topecongiro and @scampi who were involved in that PR.

[topecongiro]

It looks like rustfmt fails to format a macro call whose argument is extern. Haven't confirmed, but I think that this is due to the recent change in the rustc parser (rust-lang/rust#57572).