Add a warning to Delimiter::None that rustc currently does not respec…

…t it.

It does not provide the behaviour it is indicated to provide when used in
a proc_macro context.

This is a port of rust-lang/rust/124389.

src/lib.rs

@@ -685,6 +685,18 @@ pub enum Delimiter {
     /// operator priorities in cases like `$var * 3` where `$var` is `1 + 2`.
     /// Invisible delimiters may not survive roundtrip of a token stream through
     /// a string.
+    ///
+    /// <div class="warning">
+    ///
+    /// Note: rustc currently can ignore the grouping of tokens delimited by `None` in the output
+    /// of a proc_macro. Only `None`-delimited groups created by a macro_rules macro in the input
+    /// of a proc_macro macro are preserved, and only in very specific circumstances.
+    /// Any `None`-delimited groups (re)created by a proc_macro will therefore not preserve
+    /// operator priorities as indicated above. The other `Delimiter` variants should be used
+    /// instead in this context. This is a rustc bug. For details, see
+    /// [rust-lang/rust#67062](https://github.com/rust-lang/rust/issues/67062).
+    ///
+    /// </div>
     None,
 }