more clippy fixes

use is_empty() instead of len comparison (clippy::len_zero)
use if let instead of while let loop that never loops (clippy::never_loop)
remove redundant returns (clippy::needless_return)
remove redundant closures (clippy::redundant_closure)
use if let instead of match and wildcard pattern (clippy::single_match)
don't repeat field names redundantly (clippy::redundant_field_names)

src/libstd/io/stdio.rs

@@ -496,7 +496,7 @@ pub fn stdout() -> Stdout {
         unsafe {
             let ret = Arc::new(ReentrantMutex::new(RefCell::new(LineWriter::new(stdout))));
             ret.init();
-            return ret;
+            ret
         }
     }
 }
@@ -664,7 +664,7 @@ pub fn stderr() -> Stderr {
             *INSTANCE.lock().borrow_mut() = Maybe::Real(stderr);
         }
     });
-    return Stderr { inner: &INSTANCE };
+    Stderr { inner: &INSTANCE }
 }
 
 impl Stderr {