[ruby-core:106983] [Ruby master Feature#18461] closures are capturing unused variables

Issue #18461 has been updated by Eregon (Benoit Daloze).


I think Ruby JIT implementers are already aware of this, but these are the current semantics of Ruby.

I think JRuby does some optimization like this (only keeps the outside scope if there is `binding/eval` inside the block, but JRuby does not account for aliases)
That optimization is however unsound and incompatible, although maybe in practice not that big a deal.
```
$ juby -ve 'module Kernel; alias b binding; end; def foo; a = 1; -> { b }; end; p foo.call.local_variable_get :a' 
jruby 9.3.2.0 (2.6.8) 2021-12-01 0b8223f905 OpenJDK 64-Bit Server VM 11.0.10+9 on 11.0.10+9 +jit [linux-x86_64]
-e:1: warning: assigned but unused variable - a
-e:2: warning: Kernel#binding accesses caller method's state and should not be aliased
NameError: local variable `a' not defined for #<Binding:0x79df80a4>
```

As long as Proc#binding exists and it's possible to alias `binding`, I think it's problematic to change semantics for compatibility and soundness.
Proc#binding's purpose AFAIK is to access variables in the outer scope (it cannot variables defined inside the block).

But, maybe we could deprecate Proc#binding and remove it, I think that would be a good start.
What do committers and people think about that?

----------------------------------------
Feature #18461: closures are capturing unused variables
https://bugs.ruby-lang.org/issues/18461#change-95813

* Author: bughit (bug hit)
* Status: Open
* Priority: Normal
----------------------------------------
```rb
def foo
  a = 1
  ->{}
end
p foo.binding.local_variables # [:a]

```

Shouldn't `a` be optimized away? Like v8 does (https://bugs.chromium.org/p/v8/issues/detail?id=3491)



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