libsyntax: better error for lifetimes in patterns

Previously, if you copied a signature from a trait definition such as:

```
fn foo<'a>(&'a Bar) -> bool {}
```

and moved it into an `impl`, there would be an error message:

"unexpected token `'a`"

Adding to the error message that a pattern is expected should help
users to find the actual problem with using a lifetime here.

src/libsyntax/parse/parser.rs

@@ -3196,6 +3196,10 @@ impl<'a> Parser<'a> {
             // Parse &pat / &mut pat
             try!(self.expect_and());
             let mutbl = try!(self.parse_mutability());
+            if let token::Lifetime(ident) = self.token {
+                return Err(self.fatal(&format!("unexpected lifetime `{}` in pattern", ident)));
+            }
+
             let subpat = try!(self.parse_pat_nopanic());
             pat = PatRegion(subpat, mutbl);
           }

src/test/parse-fail/lifetime-in-pattern.rs

@@ -0,0 +1,16 @@
+// Copyright 2015 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.
+
+fn test(&'a str) {
+    //~^ ERROR unexpected lifetime `'a` in pattern
+}
+
+fn main() {
+}