message: [lang] Add ToTokens impl for FnArg
  This is to make FnArg instances applicable in bail!.
diff: @@ -108,3 +108,13 @@ pub enum FnArg {
  SelfValue(syn::ArgSelf),
  Captured(syn::ArgCaptured),
  }
  +
  +impl quote::ToTokens for FnArg {
  + fn to_tokens(&self, tokens: &mut proc_macro2::TokenStream) {
  + match self {
  + FnArg::SelfRef(arg_self_ref) => arg_self_ref.to_tokens(tokens),
  + FnArg::SelfValue(arg_self_value) => arg_self_value.to_tokens(tokens),
  + FnArg::Captured(arg_captured) => arg_captured.to_tokens(tokens),
  + }
  + }
  +}
  