Do not put where clause on a single with multi-lined type

src/items.rs

@@ -656,6 +656,7 @@ pub fn format_impl(
         if !contains_comment(&snippet[open_pos..])
             && items.is_empty()
             && generics.where_clause.predicates.len() == 1
+            && !result.contains('\n')
         {
             option.suppress_comma();
             option.snuggle();