typeck::check::_match: Better error handling

Previously this was an Option::unwrap() which failed for me.
Unfortunately I've since inadvertently worked around the bug and have
been unable to reproduce it. With this patch hopefully the next person
to encounter this will be in a slightly better position to debug it.

Author: bgamari

src/librustc/middle/typeck/check/_match.rs

@@ -507,14 +507,20 @@ pub fn check_pat(pcx: &pat_ctxt, pat: &ast::Pat, expected: ty::t) {
             ty::ty_struct(cid, ref substs) => {
                 // Verify that the pattern named the right structure.
                 let item_did = tcx.def_map.borrow().get(&pat.id).def_id();
-                let struct_did =
-                    ty::ty_to_def_id(
-                        ty::lookup_item_type(tcx, item_did).ty).unwrap();
-                if struct_did != cid {
-                    span_err!(tcx.sess, pat.span, E0032,
-                        "`{}` does not name the structure `{}`",
-                        pprust::path_to_string(path),
-                        fcx.infcx().ty_to_string(expected));
+                match ty::ty_to_def_id(ty::lookup_item_type(tcx, item_did).ty) {
+                    Some(struct_did) if struct_did != cid => {
+                        span_err!(tcx.sess, path.span, E0032,
+                                  "`{}` does not name the structure `{}`",
+                                  pprust::path_to_string(path),
+                                  fcx.infcx().ty_to_string(expected));
+                    },
+                    Some(_) => {},
+                    None => {
+                        tcx.sess.span_bug(
+                            path.span,
+                            format!("This shouldn't happen: failed to lookup structure. \
+                                item_did = {}", item_did).as_slice())
+                    },
                 }
 
                 check_struct_pat(pcx, pat.id, pat.span, expected, path,