Suggest to take and ignore args while closure args count mismatching

Author: csmoe

src/librustc/traits/error_reporting.rs

@@ -1048,7 +1048,24 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         err.span_label(span, format!( "expected {} that takes {}", kind, expected_str));
 
         if let Some(found_span) = found_span {
-            err.span_label(found_span, format!("takes {}", found_str));
+            // Suggest to take and ignore the arguments with expected_args_length `_`s if
+            // found arguments is empty(Suppose the user just wants to ignore args in this case).
+            // like `|_, _|` for closure with 2 expected args.
+            if found_args.is_empty() && is_closure {
+                let mut underscores = "_".repeat(expected_args.len())
+                                     .split("")
+                                     .filter(|s| !s.is_empty())
+                                     .collect::<Vec<_>>()
+                                     .join(", ");
+                err.span_suggestion(
+                    found_span,
+                    "consider changing this to",
+                    format!("|{}|", underscores),
+                );
+            } else {
+                err.span_label(found_span, format!("takes {}", found_str));
+            }
+
 
             if let &[ArgKind::Tuple(_, ref fields)] = &found_args[..] {
                 if fields.len() == expected_args.len() {