Issue #13893 has been updated by Eregon (Benoit Daloze).


From #7 actually the checks would be needed in both cases, because one could save the value of `Thread.current.local` and pass it to another `Thread`.

So either `.local` API seems fine to me, but if it's `Fiber.current.local` then `Fiber.current` should become core.

----------------------------------------
Feature #13893: Add Fiber#[] and Fiber#[]= and restore Thread#[] and Thread#[]= to their original behavior
https://bugs.ruby-lang.org/issues/13893#change-86998

* Author: cremes (Chuck Remes)
* Status: Open
* Priority: Normal
----------------------------------------
Ruby 3 API cleanup suggestion.

The Thread and Fiber classes have a very odd API for setting/getting thread local and fiber local variables. With Ruby 3 coming soon, this is a perfect opportunity to make this API more coherent and return to the Principal of Least Surprise. The concept of Fibers and Threads should be completely separated and we should no longer assume that a Fiber is attached to any particular Thread.

I suggest this:

```
class Fiber
  # Gets a fiber-local variable.
  def [](index)
    ...
  end

  # Sets a fiber-local variable.
  def []=(index, value)
    ...
  end

  # Returns true if the given +key+ exists as a fiber-local variable.
  def key?(key)
    ...
  end

  # Returns an array of fiber-local variable names as symbols.
  def keys
    ...
  end
end

class Thread
  # Gets a thread-local variable.
  def [](index)
    ...
  end

  # Sets a thread-local variable.
  def []=(index, value)
    ...
  end

  # Returns true if the given +key+ exists as a thread-local variable.
  def key?(key)
    ...
  end

  # Returns an array of thread-local variable names as symbols.
  def keys
    ...
  end
end
```

Also, remove ```Thread#thread_variable?```, `Thread#thread_variable_get`, `Thread#variable_set`, and `Thread#thread_variables` since that behavior is already covered by `Thread#key?`, `Thread#keys`, `Thread#[]`, and `Thread#[]=`. The APIs for both Thread and Fiber are more coherent and less surprising with these changes.



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