Negative case of len() -> is_empty()

`s/([^\(\s]+\.)len\(\) [(?:!=)>] 0/!$1is_empty()/g`

src/libsyntax/print/pp.rs

@@ -565,7 +565,7 @@ impl<'a> Printer<'a> {
           Token::End => {
             debug!("print End -> pop End");
             let print_stack = &mut self.print_stack;
-            assert!((print_stack.len() != 0));
+            assert!((!print_stack.is_empty()));
             print_stack.pop().unwrap();
             Ok(())
           }