Issue #16274 has been reported by sawa (Tsuyoshi Sawada).

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Feature #16274: Transform hash keys by a hash
https://bugs.ruby-lang.org/issues/16274

* Author: sawa (Tsuyoshi Sawada)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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We have `Hash#transform_keys` and its bang version to change the keys of a hash, but that requires passing a block, which assumes that the mapping from old keys to new keys follow some rule. But in reality, we frequently want to change the keys where it is difficult to provide a rule. For example, suppose we have:

```
hash = {created: 2019-10-23 17:54:46 +0900, updated: 2019-10-23 17:59:18 +0900, author: "foo"}
```

and want to achieve:

```
{created_at: 2019-10-23 17:54:46 +0900, update_time: 2019-10-23 17:59:18 +0900, author: "foo"}
```


I request an option to change the keys of a hash not by giving a block, but by passing a hash. I came up with two options.

### 1. Argument for `Hash#transform_keys` and its bang version

Allow `Hash#transform_keys` to optionally take a hash argument instead of a block.

```
hash.transform_values({created: :created_at, updated: :update_time})
# => {created_at: 2019-10-23 17:54:46 +0900, update_time: 2019-10-23 17:59:18 +0900, author: "foo"}
```

### 2. Argument for `Hash#slice` and the counterparts in other classes

Since `Hash#slice` is often the first step of modifying a hash into some other form, it makes sense to let it take an optional hash argument.

```
hash.slice(:created, :author, transform_keys: {created: :created_at})
# => {created_at: 2019-10-23 17:54:46 +0900, author: "foo"}
```




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