Be forward compatible with rust-lang/rust#59928

Author: Centril

src/error.rs

@@ -132,7 +132,7 @@ pub trait ResultExt {
 impl<T> ResultExt for Result<T> {
     type Ok = T;
 
-    fn err_tag<S>(self, msg: S) -> Result<Self::Ok>
+    fn err_tag<S>(self, msg: S) -> Result<T>
     where S: Into<Cow<'static, str>> {
         match self {
             Ok(v) => Ok(v),