Avoid configuring interpolated items.

Author: jseyfried

src/libsyntax/config.rs

@@ -14,6 +14,7 @@ use feature_gate::GatedCfgAttr;
 use fold::Folder;
 use {ast, fold, attr};
 use codemap::{Spanned, respan};
+use parse::token;
 use ptr::P;
 
 use util::small_vector::SmallVector;
@@ -247,6 +248,12 @@ impl<T: CfgFolder> fold::Folder for T {
         self.configure(item).map(|item| fold::noop_fold_trait_item(item, self))
                             .unwrap_or(SmallVector::zero())
     }
+
+    fn fold_interpolated(&mut self, nt: token::Nonterminal) -> token::Nonterminal {
+        // Don't configure interpolated AST (c.f. #34171).
+        // Interpolated AST will get configured once the surrounding tokens are parsed.
+        nt
+    }
 }
 
 fn fold_expr<F: CfgFolder>(folder: &mut F, expr: P<ast::Expr>) -> P<ast::Expr> {