Fix ICE on failure to parse token tree

Author: jseyfried

src/libsyntax/parse/parser.rs

@@ -2702,7 +2702,10 @@ impl<'a> Parser<'a> {
                     return Ok(TokenTree::Token(sp, SpecialVarNt(SpecialMacroVar::CrateMacroVar)));
                 } else {
                     sp = mk_sp(sp.lo, self.span.hi);
-                    self.parse_ident()?
+                    self.parse_ident().unwrap_or_else(|mut e| {
+                        e.emit();
+                        keywords::Invalid.ident()
+                    })
                 }
             }
             token::SubstNt(name) => {
@@ -2845,7 +2848,7 @@ impl<'a> Parser<'a> {
                         // and an error emitted then. Thus we don't pop from
                         // self.open_braces here.
                     },
-                    _ => unreachable!(),
+                    _ => {}
                 }
 
                 Ok(TokenTree::Delimited(span, Rc::new(Delimited {

src/test/parse-fail/issue-33569.rs

@@ -0,0 +1,18 @@
+// Copyright 2016 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.
+
+// compile-flags: -Z no-analysis
+
+macro_rules! foo {
+    { $+ } => { //~ ERROR expected identifier, found `+`
+        $(x)(y) //~ ERROR expected `*` or `+`
+                //~^ ERROR no rules expected the token `y`
+    }
+}