Do not panic on interpolated token inside quote macro

src/libsyntax/ext/quote.rs

@@ -669,7 +669,11 @@ fn expr_mk_token(cx: &ExtCtxt, sp: Span, tok: &token::Token) -> P<ast::Expr> {
                                 vec![mk_name(cx, sp, ast::Ident::with_empty_ctxt(ident))]);
         }
 
-        token::Interpolated(_) => panic!("quote! with interpolated token"),
+        token::Interpolated(_) => {
+            cx.span_err(sp, "quote! with interpolated token");
+            // Use dummy name.
+            "Interpolated"
+        }
 
         token::Eq           => "Eq",
         token::Lt           => "Lt",

src/test/compile-fail/quote-with-interpolated.rs

@@ -0,0 +1,19 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(quote)]
+fn main() {
+    macro_rules! foo {
+        ($bar:expr)  => {
+            quote_expr!(cx, $bar) //~ ERROR quote! with interpolated token
+        }
+    }
+    foo!(bar);
+}