Issue #16461 has been reported by shugo (Shugo Maeda).

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Feature #16461: Proc#using
https://bugs.ruby-lang.org/issues/16461

* Author: shugo (Shugo Maeda)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 2.8
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## Overview
I propose Proc#using to support block-level refinements.

```ruby
module IntegerDivExt
  refine Integer do
    def /(other)
      quo(other)
    end
  end
end

def instance_eval_with_integer_div_ext(obj, &block)
  block.using(IntegerDivExt) # using IntegerDivExt in the block represented by the Proc object
  obj.instance_eval(&block)
end

# necessary where blocks are defined (not where Proc#using is called)
using Proc::Refinements

p 1 / 2 #=> 0
instance_eval_with_integer_div_ext(1) do
  p self / 2 #=> (1/2)
end
p 1 / 2 #=> 0
```

## PoC implementation
For CRuby: https://github.com/shugo/ruby/pull/2
For JRuby: https://github.com/shugo/jruby/pull/1

## Background
I proposed [Feature #12086: using: option for instance_eval etc.](https://bugs.ruby-lang.org/issues/12086) before, but it has problems:

* Thread safety: The same block can be invoked with different refinements in multiple threads, so it's hard to implement method caching.
* _exec family support: {instance,class,module}_exec cannot be supported.
* Implicit use of refinements: every blocks can be used with refinements, so there was implementation difficulty in JRuby and it has usability issue in headius's opinion.

## Solutions in this proposal

### Thread safety
Proc#using affects the block represented by the Proc object, neither the specific Proc object nor the specific block invocation.
Method calls in a block are resolved with refinements which are used by Proc#using in the block at the time.
Once all possible refinements are used in the block, there is no need to invalidate method cache anymore.

See [these tests](https://github.com/shugo/ruby/pull/2/commits/1c922614ad7d1fb43b73e195348c81da7a4546ef) to understand how it works.
Which refinements are used is depending on the order of Proc#using invocations until all Proc#using calls are finished, but eventually method calls in a block are resolved with the same refinements.

### * _exec family support
[Feature #12086](https://bugs.ruby-lang.org/issues/12086) was an extension of _eval family, so it cannot be used with _exec family, but Proc#using is independent from _eval family, and can be used with _exec family:

```ruby
def instance_exec_with_integer_div_ext(obj, *args, &block)
  block.using(IntegerDivExt)
  obj.instance_exec(*args, &block)
end

using Proc::Refinements

p 1 / 2 #=> 0
instance_exec_with_integer_div_ext(1, 2) do |other|
  p self / other #=> (1/2)
end
p 1 / 2 #=> 0
```

### Implicit use of refinements

Proc#using can be used only if `using Proc::Refinements` is called in the scope of the block represented by the Proc object.
Otherwise, a RuntimeError is raised.

There are two reasons:

* JRuby creates a special CallSite for refinements at compile-time only when `using` is called at the scope.
* When reading programs, it may help understanding behavior.  IMHO, it may be unnecessary if libraries which uses Proc#using are well documented.

`Proc::Refinements` is a dummy module, and has no actual refinements.






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