Recommend Hash#transform_keys & transform_values

These methods are more readable than the more flexible alternatives when
only keys or only values are being transformed, and also execute more
quickly in microbenchmarks

README.adoc

@@ -3707,6 +3707,21 @@ username = data['nickname']
 email, username = data.values_at('email', 'nickname')
 ----
 
+=== `Hash#transform_keys` and `Hash#transform_values` [[hash-transform-methods]]
+
+Prefer `transform_keys` or `transform_values` over `each_with_object` or `map` when transforming just the keys or just the values of a hash.
+
+[source,ruby]
+----
+# bad
+{a: 1, b: 2}.each_with_object({}) { |(k, v), h| h[k] = v * v }
+{a: 1, b: 2}.map { |k, v| [k.to_s, v] }.to_h
+
+# good
+{a: 1, b: 2}.transform_values { |v| v * v }
+{a: 1, b: 2}.transform_keys { |k| k.to_s }
+----
+
 === Ordered Hashes [[ordered-hashes]]
 
 Rely on the fact that as of Ruby 1.9 hashes are ordered.