[ruby-core:107305] [Ruby master Feature#18552] Expose `VALUE rb_singleton_class_get(VALUE)` to extensions

Issue #18552 has been updated by alanwu (Alan Wu).


> `!SPECIAL_CONST(obj) && FL_TEST(RBASIC(obj)->klass, FL_SINGLETON);`
> Which does seem very clean.

Beyond cleanliness, it uses `FL_SINGLETON`, which is [marked] `@internal`. I
agree with @shyouhei that this flag should not be visible in the first place.
Also, it's tempting to think the predicate being true implies
`RBASIC(obj)->klass` gives the singleton class of `obj` while that is not
necessarily true and has lead to bugs in the past. [^1][^2] These subtleties
are why it's generally a bad idea for extensions to take advantage of
implementation details.

I can emphasize with trying to avoid allocations, but I want to be cautious
about expanding the C extension API surface. In this case, it's exposing
_yet another_ operation unavailable to plain Ruby code. It might make some
optimizations in JITs more complex to implement or worse, introduce bugs down
the line in the interpreter because C APIs are generally less frequently
exercised and their extra capabilities easy to be forgotten.

We should review how much `rb_singleton_class_get()` is exposing and not exposing, and
whether it's feasible to implement in other runtimes such as TruffleRuby.

[marked]: https://github.com/ruby/ruby/blob/v3_1_0/include/ruby/internal/fl_type.h#L424-L430
[^1]: https://bugs.ruby-lang.org/issues/17321
[^2]: https://github.com/ruby/ruby/commit/0cd3b97e027332236625835578329580be12023c


----------------------------------------
Feature #18552: Expose `VALUE rb_singleton_class_get(VALUE)` to extensions
https://bugs.ruby-lang.org/issues/18552#change-96203

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
----------------------------------------
Right now the only way to check wether an object has a singleton class is to do something akin to:

```c
!SPECIAL_CONST(obj) && FL_TEST(RBASIC(obj)->klass, FL_SINGLETON);
```

Which does seem very clean.

### Use case

This came up in `msgpack`. The library have a registry of serializers on a per class basis, and wish to support singleton classes too.
So it is calling `rb_singleton_class()` which cause lots of useless singleton classes to be created: https://github.com/msgpack/msgpack-ruby/pull/245


### Proposed patch

https://github.com/ruby/ruby/pull/5499



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