[ruby-core:119333] [Ruby master Feature#20769] Add `Hash#transform_value`

Issue #20769 has been updated by nobu (Nobuyoshi Nakada).


```ruby
hash.merge(image: nil) {|_, url| download(url)}

hash.merge!(image: nil) {|_, url| download(url)}
```

----------------------------------------
Feature #20769: Add `Hash#transform_value`
https://bugs.ruby-lang.org/issues/20769#change-109945

* Author: seanpdoyle (Sean Doyle)
* Status: Open
----------------------------------------
Add `Hash#transform_value` as a specialized, key-specific version of [Hash#transform_values](https://docs.ruby-lang.org/en/3.3/Hash.html#method-i-transform_values).

```ruby
hash = { image: "https://example.com/image.jpg" }
mutated_hash = hash.transform_value(:image) { |url| download(url) }

hash # => { image: "https://example.com/image.jpg" }
mutated_hash # => { image: File<...> }

hash.transform_value!(:image) { |url| download(url) }
hash # => { image: File<...> }
```

Similar value transformation can be achieved through variable assignment and direct mutation:

```ruby
hash = { image: "https://example.com/image.jpg" }

hash.merge(image: download(hash[:image]))

hash[:image] = download(hash[:image])
```

While simple and currently supported, it requires a local variable (and therefore poses some challenges when chaining other methods) and repeats the Hash key in both the reading and writing portions of the code.



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