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.

library/proc_macro/src/lib.rs

@@ -815,6 +815,18 @@ pub enum Delimiter {
     /// "macro variable" `$var`. It is important to preserve operator priorities in cases like
     /// `$var * 3` where `$var` is `1 + 2`.
     /// Invisible delimiters might 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>
     #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
     None,
 }