Emit error for pattern arguments in trait methods

The error and check for this already existed, but the parser didn't try to parse trait method arguments as patterns, so the error was never emitted. This surfaces the error, so we get better errors than simple parse errors.

src/libsyntax/lib.rs

@@ -26,6 +26,7 @@
 #![feature(slice_sort_by_cached_key)]
 #![feature(str_escape)]
 #![feature(unicode_internals)]
+#![feature(catch_expr)]
 
 #![recursion_limit="256"]