[ruby-core:95519] [Ruby master Bug#16278] Potential memory leak when an hash is used as a key for another hash

Issue #16278 has been reported by cristiangreco (Cristian Greco).

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Bug #16278: Potential memory leak when an hash is used as a key for another hash
https://bugs.ruby-lang.org/issues/16278

* Author: cristiangreco (Cristian Greco)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.5p114 (2019-10-01 revision 67812) [x86_64-darwin18]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN
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Hi,

I've been hitting what seems to be a memory leak.

When an hash is used as key for another hash, the former object will be retained even after multiple GC runs.

The following code snippet demonstrates how the hash `{:a => 1}` (which is never used outside the scope of `create`) is retained even after 10 GC runs (`find` will look for an object with a given `object_id` on heap).


```ruby
# frozen_string_literal: true

def create
  h = {{:a => 1} => 2}
  h.keys.first.object_id
end

def find(object_id)
  ObjectSpace.each_object(Hash).any?{|h| h.object_id == object_id} ? 1 : 0
end


leaked = create

10.times do
  GC.start(full_mark: true, immediate_sweep: true)
end

exit find(leaked)
```

This code snippet is expected to exit with `0` while it exits with `1` in my tests. I've tested this on multiple recent ruby versions and OSs, either locally (OSX with homebrew) or in different CIs (e.g. [here](https://github.com/cristiangreco/ruby-hash-leak/commit/285e586b7193104989f59b92579fe8f25770141e/checks?check_suite_id=278711566)).

Can you please help understand what's going on here? Thanks!



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