[ruby-core:120743] [Ruby master Bug#21049] Reconsider handling of the numbered parameters and "it" parameter in `Binding#local_variables`

["mame (Yusuke Endoh) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #21049 has been reported by mame (Yusuke Endoh).

----------------------------------------
Bug #21049: Reconsider handling of the numbered parameters and "it" parameter in `Binding#local_variables`
https://bugs.ruby-lang.org/issues/21049

* Author: mame (Yusuke Endoh)
* Status: Open
* Assignee: matz (Yukihiro Matsumoto)
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
Currently, `Binding#local_variable*` APIs wrongly handles the numbered parameters.

```ruby
# Binding#local_variables includes numbered parameters that should not be visible
"foo".tap do
  _1
  "bar".tap do
    p binding.local_variables #=> expected: [], actual: [:_1]
  end
end

# Binding#local_variable_get can read numbered parameters that should not be readable
"foo".tap do
  _1
  "bar".tap do
    p binding.local_variable_get(:_1) #=> expected: NameError, actual: "foo"
  end
end

# Binding#local_variable_set can update numbered parameter
"foo".tap do
  p _1 #=> "foo"
  binding.local_variable_set(:_1, "bar") # expected: NameError, actual: updates _1
  p _1 #=> "bar"
end
```

My proposal is to stop handling numbered parameters and "it" parameter in `Binding#local_variable*` APIs.

```ruby
"foo".tap do
  _1
  binding.local_variables         #=> proposed: []
  binding.local_variable_get(:_1) #=> proposed: NameError
end
```

Here is a proof-of-concept patch.

https://github.com/ruby/ruby/pull/12601

It would be theoretically possible to fix `Binding#local_variable*` APIs to handle numbered parameters while maintaining compatibility as much as possible.
However, I think the implicit “it” parameter introduced in Ruby 3.4 poses a spec-level problem.
This is because there is no way to distinguish between the implicit “it” parameter and a true local variable named "it".

Also, I don't think numbered parameters are local variables, so I feel uncomfortable handling them by `Binding#local_variable*` APIs.

If it is absolutely necessary to access numbered parameters or "it" parameter via binding, it would be good to introduce dedicated APIs for the purpose, as:

* Binding#numbered_parameters              #=> [:_1, :_2, :_3, ...]
* Binding#numbered_parameter_get(:_1)      #=> obj
* Binding#numbered_parameter_defined?(:_1) #=> true or false
* Binding#it_get       #=> obj
* Binding#it_defined?  #=> true or false

Personally, however, I do not see the need for these APIs currently. I implemented them as proof-of-concept in PR, but I am okay that they are not introduced.




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