Work around unknown_lints warning on rustc older than 1.64

    warning: unknown lint: `repr_transparent_external_private_fields`
       --> src/lib.rs:255:10
        |
    255 | #![allow(repr_transparent_external_private_fields)]
        |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        |
        = note: `#[warn(unknown_lints)]` on by default

Author: dtolnay

src/lib.rs

@@ -252,7 +252,6 @@
 #![doc(html_root_url = "https://docs.rs/syn/2.0.36")]
 #![cfg_attr(doc_cfg, feature(doc_cfg))]
 #![allow(non_camel_case_types)]
-#![allow(repr_transparent_external_private_fields)] // False positive: https://github.com/rust-lang/rust/issues/78586#issuecomment-1722680482
 #![allow(
     clippy::bool_to_int_with_if,
     clippy::cast_lossless,

src/token.rs

@@ -143,6 +143,7 @@ mod private {
     /// Support writing `token.span` rather than `token.spans[0]` on tokens that
     /// hold a single span.
     #[repr(transparent)]
+    #[allow(unknown_lints, repr_transparent_external_private_fields)] // False positive: https://github.com/rust-lang/rust/issues/78586#issuecomment-1722680482
     pub struct WithSpan {
         pub span: Span,
     }
@@ -365,6 +366,7 @@ macro_rules! define_punctuation_structs {
     ($($token:literal pub struct $name:ident/$len:tt #[doc = $usage:literal])*) => {
         $(
             #[cfg_attr(not(doc), repr(transparent))]
+            #[allow(unknown_lints, repr_transparent_external_private_fields)] // False positive: https://github.com/rust-lang/rust/issues/78586#issuecomment-1722680482
             #[doc = concat!('`', $token, '`')]
             ///
             /// Usage: