Merge pull request #1301 from bud-adamas/patch-1

eliminate the warnings

Author: marioidival

src/error/multiple_error_types/boxing_errors.md

@@ -13,7 +13,7 @@ use std::error;
 use std::fmt;
 
 // Change the alias to `Box<error::Error>`.
-type Result<T> = std::result::Result<T, Box<error::Error>>;
+type Result<T> = std::result::Result<T, Box<dyn error::Error>>;
 
 #[derive(Debug, Clone)]
 struct EmptyVec;
@@ -29,7 +29,7 @@ impl error::Error for EmptyVec {
         "invalid first item to double"
     }
 
-    fn cause(&self) -> Option<&error::Error> {
+    fn cause(&self) -> Option<&(dyn error::Error)> {
         // Generic error, underlying cause isn't tracked.
         None
     }