remove excess string allocation

&format!("...") is the same as "" if we're not doing any interpolation,
and doesn't allocate an intermediate String.

src/libsyntax/parse/parser.rs

@@ -1843,7 +1843,7 @@ impl<'a> Parser<'a> {
                 });
             }
             _ => {
-                return Err(self.fatal(&format!("expected a lifetime name")));
+                return Err(self.fatal("expected a lifetime name"));
             }
         }
     }