Change Scalar to numeric cast

Author: estebank

src/librustc/traits/error_reporting.rs

@@ -425,7 +425,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                     Some(format!("[{}]", self.tcx.type_of(def.did).to_string())),
                 ));
                 if let Some(len) = len.val.try_to_scalar().and_then(|scalar| {
-                    scalar.to_u64().ok()
+                    scalar.to_i64().ok()
                 }) {
                     flags.push((
                         "_Self".to_owned(),