[ruby-core:106294] [Ruby master Feature#16038] Provide a public WeakMap that compares by equality rather than by identity

Issue #16038 has been updated by Eregon (Benoit Daloze).


For the deduplication use-case as in the description, `WeakValuesMap` is possible too:
```ruby
REGISTRY = WeakValuesMap.new

key = [x, y, z]
if instance = REGISTRY[key]
  instance
else
  REGISTRY[key] = instance
end
```
Then that mapping stays in the map as long as the instance is referenced somewhere.
We can't use the instance itself as the key, otherwise no mapping would ever GC, since the key is referenced strongly by the map.

Which to use between the two is based on what you want to refer to keep the mapping alive:
If one wants the mapping to exist as long as the key is referenced, then it's WeakKeysMap.
If one wants the mapping to exist as long as the value is referenced, then it's WeakValuesMap.

As examples from TruffleRuby, the internal map for Symbols or for frozen literal/interned strings use a WeakValuesMap. They need to guarantee uniqueness, so as long as the value is alive it makes sure to to avoid any duplicate value.
WeakKeysMap is used to keep some extra data about an object outside of it, as long as the object is alive (e.g., the `eval` line offset for a given Source file object).
One can also build a weak set based on WeakKeysMap, by just using e.g. `true` as the value. This is used e.g. to track subclasses of a class in a weakly manner.

----------------------------------------
Feature #16038: Provide a public WeakMap that compares by equality rather than by identity 
https://bugs.ruby-lang.org/issues/16038#change-94919

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
----------------------------------------
I know `ObjectSpace::WeakMap` isn't really supposed to be used, and that the blessed interface is `WeakRef`. However, I'd like to make a case for a better public WeakMap.

### Usage

As described in [Feature #16035], `WeakMap` is useful for deduplicating "value objects". A typical use case is as follows:

```ruby
class Position
  REGISTRY = {}
  private_constant :REGISTRY

  class << self
    def new(*)
      instance = super
      REGISTRY[instance] ||= instance
    end
  end

  attr_reader :x, :y, :z

  def initialize(x, y, z)
    @x = x
    @y = y
    @z = z
    freeze
  end

  def hash
    self.class.hash ^
      x.hash >> 1 ^
      y.hash >> 2 ^
      y.hash >> 3
  end

  def ==(other)
    other.is_a?(Position) &&
      other.x == x &&
      other.y == y &&
      other.z == z
  end
  alias_method :eql?, :==
end

p Position.new(1, 2, 3).equal?(Position.new(1, 2, 3))
```

That's pretty much the pattern [I used in Rails to deduplicate database metadata and save lots of memory](https://github.com/rails/rails/blob/f3c68c59ed57302ca54f4dfad0e91dbff426962d/activerecord/lib/active_record/connection_adapters/deduplicable.rb).

The big downside here is that these value objects can't be GCed anymore, so this pattern is not viable in many case.

### Why not use WeakRef

A couple of reasons. First, when using this pattern, the goal is to reduce memory usage, so having one extra `WeakRef` for every single value object is a bit counter productive. 

Then it's a bit annoying to work with, as you have to constantly check wether the reference is still alive, and/or rescue `WeakRef::RefError`.

Often, these two complications make the tradeoff not worth it.

### Ruby 2.7

Since [Feature #13498] `WeakMap` is a bit more usable as you can now use an interned string as the unique key, e.g.

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

  class << self
    def new(*)
      instance = super
      REGISTRY[instance.unique_id] ||= instance
    end
  end

  attr_reader :x, :y, :z, :unique_id

  def initialize(x, y, z)
    @x = x
    @y = y
    @z = z
    @unique_id = -"#{self.class}-#{x},#{y},#{z}"
    freeze
  end

  def hash
    self.class.hash ^
      x.hash >> 1 ^
      y.hash >> 2 ^
      y.hash >> 3
  end

  def ==(other)
    other.is_a?(Position) &&
      other.x == x &&
      other.y == y &&
      other.z == z
  end
  alias_method :eql?, :==
end

p Position.new(1, 2, 3).equal?(Position.new(1, 2, 3))
```

That makes the pattern much easier to work with than dealing with `WeakRef`, but there is still that an extra instance.

### Proposal

What would be ideal would be a `WeakMap` that works by equality, so that the first snippet could simply replace `{}` by `WeakMap.new`. 

Changing `ObjectSpace::WeakMap`'s behavior would cause issues, and I see two possibilities:

  - The best IMO would be to have a new top level `::WeakMap` be the equality based map, and have `ObjectSpace::WeakMap` remain as a semi-private interface for backing up `WeakRef`.
  - Or alternatively, `ObjectSpace::WeakMap` could have a `compare_by_equality` method (inverse of `Hash#compare_by_identity`) to change its behavior post instantiation.

I personally prefer the first one.




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