[ruby-core:106135] [Ruby master Feature#16035] Allow non-finalizable objects such as Integer, static Symbol etc in ObjectSpace::WeakMap

Issue #16035 has been updated by headius (Charles Nutter).


I am behind the times here, but it is worth noting that implementations which cannot guarantee idempotency of fixnum and flonum-ranged Integers and Floats will have trouble implementing the spirit of this change. On JRuby, it is not possible to treat two fixnums created separately as the same object, since the WeakMap implementation needs to compare by object identity and JRuby represents all fixnums and flonums as full objectswith their own identities.

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Feature #16035: Allow non-finalizable objects such as Integer, static Symbol etc in ObjectSpace::WeakMap
https://bugs.ruby-lang.org/issues/16035#change-94732

* Author: byroot (Jean Boussier)
* Status: Closed
* Priority: Normal
* Assignee: nobu (Nobuyoshi Nakada)
----------------------------------------
This goes one step farther than what @nobu did in https://bugs.ruby-lang.org/issues/13498

With this patch, special objects such as static symbols, integers, etc can be used as either key or values inside WeakMap. They simply don't have a finalizer defined on them.

This is useful if you need to deduplicate value objects, e.g. some minimal use case:

```ruby
class Money
  REGISTRY = ObjectSpace::WeakMap.new
  private_constant :REGISTRY

  def self.new(amount)
    REGISTRY[amount] ||= super.freeze
  end

  def initialize(amount)
    @amount = amount
  end
end

if Money.new(42).eql?(Money.new(42))
  puts "Same instance"
else
  puts "Different instances"
end
```

This is a very simple example, but more complex examples can create use a dynamically created symbol as deduplication key, etc.

It also removes one weirdness introduced in the mentioned patch:

```ruby
wmap = ObjectSpace::WeakMap.new
wmap["foo".to_sym] = Object.new # works fine with dynamic symbols
wmap[:bar] = Object.new # cannot define finalizer for Symbol (ArgumentError)
```

Proposed patch: https://github.com/ruby/ruby/pull/2313



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