Improve error message for use of a non-pure-fn in a pred

Closes #1883

src/comp/middle/resolve.rs

@@ -681,9 +681,9 @@ fn resolve_constr(e: @env, c: @ast::constr, sc: scopes, _v: vt<scopes>) {
          e.def_map.insert(c.node.id, d);
        }
        _ {
-           e.sess.span_err(c.span,
-                           "Non-predicate in constraint: " +
-                           path_to_str(c.node.path));
+           let s = path_to_str(c.node.path);
+           e.sess.span_err(c.span, #fmt("%s is not declared pure. Try \
+             `pure fn %s` instead of `fn %s`.", s, s, s));
        }
     }
 }

src/test/compile-fail/not-a-pred.rs

@@ -1,5 +1,5 @@
 // -*- rust -*-
-// error-pattern: Non-predicate in constraint: lt
+// error-pattern: lt is not declared pure
 
 fn f(a: int, b: int) : lt(a, b) { }