Support Rc<T> in ToTokens.

src/to_tokens.rs

@@ -2,6 +2,7 @@ use super::TokenStreamExt;
 
 use std::borrow::Cow;
 use std::iter;
+use std::rc::Rc;
 
 use proc_macro2::{Group, Ident, Literal, Punct, Span, TokenStream, TokenTree};
 
@@ -91,6 +92,12 @@ impl<T: ?Sized + ToTokens> ToTokens for Box<T> {
     }
 }
 
+impl<T: ?Sized + ToTokens> ToTokens for Rc<T> {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        (**self).to_tokens(tokens);
+    }
+}
+
 impl<T: ToTokens> ToTokens for Option<T> {
     fn to_tokens(&self, tokens: &mut TokenStream) {
         if let Some(ref t) = *self {