[ruby-core:95499] [Ruby master Feature#16274] Transform hash keys by a hash

[duerst@it.aoyama.ac.jp]

Issue #16274 has been updated by duerst (Martin Dürst).


`String#gsub` also can take a block or a hash. Using a hash for `String#gsub` isn't possible in Perl or Python, but can be extremely handy. Maybe the behavior when there's both a block and a hash could be the same as for `String#gsub`?

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Feature #16274: Transform hash keys by a hash
https://bugs.ruby-lang.org/issues/16274#change-82275

* 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 the old keys to the new keys follows 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_keys({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 hash 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"}
```


With option 1, it could make sense to even allow a hash argument and a block simultaneously:

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




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