[ruby-core:107308] [Ruby master Feature#18552] Expose `VALUE rb_singleton_class_get(VALUE)` to extensions

Issue #18552 has been updated by byroot (Jean Boussier).


> The name rb_singleton_class_get() doesn't seem great though.

Yeah, it could definitely be expose under another name.

> bool has_singleton_class = rb_class_of(obj) != rb_obj_class(obj);

I don't follow, how could this expression ever be false?

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Feature #18552: Expose `VALUE rb_singleton_class_get(VALUE)` to extensions
https://bugs.ruby-lang.org/issues/18552#change-96207

* 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 doesn't 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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