https://git.ruby-lang.org/ruby.git/commit/?id=8558d5e480

From 8558d5e4801b74b058dc2a534be2be85037cadb5 Mon Sep 17 00:00:00 2001
From: Marc-Andre Lafortune <github@m...>
Date: Tue, 15 Dec 2020 17:05:35 -0500
Subject: Document Hash#transform_keys with hash. Amend NEWS [DOC] [ci skip]


diff --git a/NEWS.md b/NEWS.md
index 817d591..1bce4d8 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -196,8 +196,8 @@ Outstanding ones only. https://github.com/ruby/ruby/blob/trunk/NEWS.md#L196
 
 * Hash
 
-    * Hash#transform_keys now accepts a hash that maps keys to new
-      keys.  [[Feature #16274]]
+    * Hash#transform_keys and transform_keys! now accepts a hash that maps
+      keys to new keys.  [[Feature #16274]]
 
     * Hash#except has been added, which returns a hash excluding the
       given keys and their values.  [[Feature #15822]]
diff --git a/hash.c b/hash.c
index 98405ec..2f53c24 100644
--- a/hash.c
+++ b/hash.c
@@ -3188,17 +3188,28 @@ transform_keys_i(VALUE key, VALUE value, VALUE result) https://github.com/ruby/ruby/blob/trunk/hash.c#L3188
  *  call-seq:
  *    hash.transform_keys {|key| ... } -> new_hash
  *    hash.transform_keys(hash2) -> new_hash
+ *    hash.transform_keys(hash2) {|other_key| ...} -> new_hash
  *    hash.transform_keys -> new_enumerator
  *
  *  Returns a new \Hash object; each entry has:
  *  * A key provided by the block.
  *  * The value from +self+.
  *
+ *  An optional hash argument can be provided to map keys to new keys.
+ *  Any key not given will be mapped using the provided block,
+ *  or remain the same if no block is given.
+ *
  *  Transform keys:
  *      h = {foo: 0, bar: 1, baz: 2}
  *      h1 = h.transform_keys {|key| key.to_s }
  *      h1 # => {"foo"=>0, "bar"=>1, "baz"=>2}
  *
+ *      h.transform_keys(foo: :bar, bar: :foo)
+ *      #=> {bar: 0, foo: 1, baz: 2}
+ *
+ *      h.transform_keys(foo: :hello, &:to_s)
+ *      #=> {:hello=>0, "bar"=>1, "baz"=>2}
+ *
  *  Overwrites values for duplicate keys:
  *    h = {foo: 0, bar: 1, baz: 2}
  *    h1 = h.transform_keys {|key| :bat }
@@ -3243,22 +3254,12 @@ static VALUE rb_hash_flatten(int argc, VALUE *argv, VALUE hash); https://github.com/ruby/ruby/blob/trunk/hash.c#L3254
 /*
  *  call-seq:
  *    hash.transform_keys! {|key| ... } -> self
+ *    hash.transform_keys!(hash2) -> self
+ *    hash.transform_keys!(hash2) {|other_key| ...} -> self
  *    hash.transform_keys! -> new_enumerator
  *
- *  Returns +self+ with new keys provided by the block:
- *    h = {foo: 0, bar: 1, baz: 2}
- *    h.transform_keys! {|key| key.to_s } # => {"foo"=>0, "bar"=>1, "baz"=>2}
- *
- *  Overwrites values for duplicate keys:
- *    h = {foo: 0, bar: 1, baz: 2}
- *    h1 = h.transform_keys! {|key| :bat }
- *    h1 # => {:bat=>2}
- *
- *  Returns a new \Enumerator if no block given:
- *    h = {foo: 0, bar: 1, baz: 2}
- *    e = h.transform_keys! # => #<Enumerator: {"foo"=>0, "bar"=>1, "baz"=>2}:transform_keys!>
- *    h1 = e.each { |key| key.to_s }
- *    h1 # => {"foo"=>0, "bar"=>1, "baz"=>2}
+ *  Same as Hash#transform_keys but modifies the receiver in place
+ *  instead of returning a new hash.
  */
 static VALUE
 rb_hash_transform_keys_bang(int argc, VALUE *argv, VALUE hash)
-- 
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