Remove unsound use of specialization

See rust-lang/rust#67194 for details

src/map.rs

@@ -202,41 +202,10 @@ impl<K: Clone, V: Clone, S: Clone> Clone for HashMap<K, V, S> {
     }
 
     fn clone_from(&mut self, source: &Self) {
-        // We clone the hash_builder first since this might panic and we don't
-        // want the table to have elements hashed with the wrong hash_builder.
-        let hash_builder = source.hash_builder.clone();
-
-        // For backward-compatibility reasons we can't make the Clone impl
-        // depend on K: Hash + Eq and S: BuildHasher. However we can exploit
-        // this using specialization, which allows us to reuse the existing
-        // storage of the current HashMap to insert the cloned elements into.
-        trait HashClone<S> {
-            fn clone_from(&mut self, source: &Self, hash_builder: &S);
-        }
-        impl<K: Clone, V: Clone, S> HashClone<S> for HashMap<K, V, S> {
-            #[cfg_attr(feature = "inline-more", inline)]
-            default_fn! {
-                fn clone_from(&mut self, source: &Self, _hash_builder: &S) {
-                    self.table.clone_from(&source.table);
-                }
-            }
-        }
-        #[cfg(feature = "nightly")]
-        impl<K: Clone, V: Clone, S> HashClone<S> for HashMap<K, V, S>
-        where
-            K: Eq + Hash,
-            S: BuildHasher,
-        {
-            #[cfg_attr(feature = "inline-more", inline)]
-            fn clone_from(&mut self, source: &Self, hash_builder: &S) {
-                self.table
-                    .clone_from_with_hasher(&source.table, |x| make_hash(hash_builder, &x.0));
-            }
-        }
-        HashClone::clone_from(self, source, &hash_builder);
+        self.table.clone_from(&source.table);
 
         // Update hash_builder only if we successfully cloned all elements.
-        self.hash_builder = hash_builder;
+        self.hash_builder.clone_from(&source.hash_builder);
     }
 }