Remove unnecessary type casts.

src/libstd/old_io/mod.rs

@@ -353,7 +353,7 @@ pub fn from_errno(errno: i32, detail: bool) -> IoError {
     /// operating system) between the call(s) for which errors are
     /// being checked and the call of this function.
     pub fn last_error() -> IoError {
-        IoError::from_errno(os::errno() as i32, true)
+        IoError::from_errno(os::errno(), true)
     }
 }
 

src/libstd/os.rs

@@ -527,12 +527,12 @@ pub fn errno() -> i32 {
 /// println!("{}", os::error_string(os::errno() as i32));
 /// ```
 pub fn error_string(errnum: i32) -> String {
-    return sys::os::error_string(errnum as i32);
+    return sys::os::error_string(errnum);
 }
 
 /// Get a string representing the platform-dependent last error
 pub fn last_os_error() -> String {
-    error_string(errno() as i32)
+    error_string(errno())
 }
 
 /// Sets the process exit code