Fix failing repr test

It was comparing the pointers, not the levels themselves.
This also changes transmute to explicitly specify both types to make
this easier to debug in the future.

Author: jyn514

tracing-core/src/metadata.rs

@@ -858,13 +858,13 @@ mod tests {
             (LevelFilter::DEBUG, LevelInner::Debug as usize),
             (LevelFilter::TRACE, LevelInner::Trace as usize),
         ];
-        for &(ref filter, expected) in &mapping {
+        for &(filter, expected) in &mapping {
             let repr = unsafe {
                 // safety: The entire purpose of this test is to assert that the
                 // actual repr matches what we expect it to be --- we're testing
                 // that *other* unsafe code is sound using the transmuted value.
                 // We're not going to do anything with it that might be unsound.
-                mem::transmute::<_, usize>(filter)
+                mem::transmute::<LevelFilter, usize>(filter)
             };
             assert_eq!(expected, repr, "repr changed for {:?}", filter)
         }