feat(metadata): remove manual Send + Sync impls for metadata types (#640)

tonic/src/metadata/map.rs

@@ -1244,9 +1244,6 @@ impl<'a> Iterator for Iter<'a> {
     }
 }
 
-unsafe impl<'a> Sync for Iter<'a> {}
-unsafe impl<'a> Send for Iter<'a> {}
-
 // ===== impl IterMut =====
 
 impl<'a> Iterator for IterMut<'a> {
@@ -1274,9 +1271,6 @@ impl<'a> Iterator for IterMut<'a> {
     }
 }
 
-unsafe impl<'a> Sync for IterMut<'a> {}
-unsafe impl<'a> Send for IterMut<'a> {}
-
 // ===== impl ValueDrain =====
 
 impl<'a, VE: ValueEncoding> Iterator for ValueDrain<'a, VE> {
@@ -1293,9 +1287,6 @@ impl<'a, VE: ValueEncoding> Iterator for ValueDrain<'a, VE> {
     }
 }
 
-unsafe impl<'a, VE: ValueEncoding> Sync for ValueDrain<'a, VE> {}
-unsafe impl<'a, VE: ValueEncoding> Send for ValueDrain<'a, VE> {}
-
 // ===== impl Keys =====
 
 impl<'a> Iterator for Keys<'a> {
@@ -1425,9 +1416,6 @@ where
     }
 }
 
-unsafe impl<'a, VE: ValueEncoding> Sync for ValueIterMut<'a, VE> {}
-unsafe impl<'a, VE: ValueEncoding> Send for ValueIterMut<'a, VE> {}
-
 // ===== impl Entry =====
 
 impl<'a, VE: ValueEncoding> Entry<'a, VE> {
@@ -2725,4 +2713,18 @@ mod tests {
         }
         assert!(found_x_word_bin);
     }
+
+    #[allow(dead_code)]
+    fn value_drain_is_send_sync() {
+        fn is_send_sync<T: Send + Sync>() {}
+
+        is_send_sync::<Iter<'_>>();
+        is_send_sync::<IterMut<'_>>();
+
+        is_send_sync::<ValueDrain<'_, Ascii>>();
+        is_send_sync::<ValueDrain<'_, Binary>>();
+
+        is_send_sync::<ValueIterMut<'_, Ascii>>();
+        is_send_sync::<ValueIterMut<'_, Binary>>();
+    }
 }