rustc: Add a span_unimpl() for debugging

src/comp/driver/session.rs

@@ -45,6 +45,15 @@ obj session(cfg targ) {
         fail;
     }
 
+    fn span_unimpl(span sp, str msg) {
+        log #fmt("%s:%u:%u:%u:%u: error: unimplemented %s",
+                 sp.filename,
+                 sp.lo.line, sp.lo.col,
+                 sp.hi.line, sp.hi.col,
+                 msg);
+        fail;
+    }
+
     fn unimpl(str msg) {
         log #fmt("error: unimplemented %s", msg);
         fail;

src/comp/middle/typeck.rs

@@ -1402,7 +1402,8 @@ fn demand_expr_full(&@fn_ctxt fcx, @ty.t expected, @ast.expr e,
         }
 
         case (_) {
-            fcx.ccx.sess.unimpl("type unification for expression variant");
+            fcx.ccx.sess.span_unimpl(e.span,
+                "type unification for expression variant");
             fail;
         }
     }