Handle the case where verbatim enters a None group

Fixes #1235. As seen in that issue, a syntax node can actually cross the
border of a None-delimited group, due to such groups being transparent
to the parser in most cases. In the cases that this can occur, the
presence of the group is known to be semantically irrelevant, so we just
explicitly ignore the presence of the group.

It's important that we only ignore None-delimited groups when the
verbatim end requires entering the group, as such groups can have semantic
impact (such as grouping binary operator chains) in other positions.

src/verbatim.rs

@@ -1,5 +1,5 @@
 use crate::parse::{ParseBuffer, ParseStream};
-use proc_macro2::TokenStream;
+use proc_macro2::{Delimiter, TokenStream};
 use std::iter;
 
 pub fn between<'a>(begin: ParseBuffer<'a>, end: ParseStream<'a>) -> TokenStream {
@@ -8,6 +8,22 @@ pub fn between<'a>(begin: ParseBuffer<'a>, end: ParseStream<'a>) -> TokenStream
     let mut tokens = TokenStream::new();
     while cursor != end {
         let (tt, next) = cursor.token_tree().unwrap();
+
+        if next > end {
+            // In some edge cases, a syntax node can actually cross the border
+            // of a None-delimited group, due to such groups being transparent
+            // to the parser in most cases. In the cases that this can occur,
+            // the presence of the group is known to be semantically irrelevant,
+            // so we should just ignore the presence of the group. (Issue #1235)
+            if let Some((inside, _span, after)) = cursor.group(Delimiter::None) {
+                assert!(next == after);
+                cursor = inside;
+                continue;
+            } else {
+                panic!("verbatim end must not be inside a delimited group");
+            }
+        }
+
         tokens.extend(iter::once(tt));
         cursor = next;
     }