[ruby-core:116760] [Ruby master Feature#20265] Deprecate and remove rb_newobj and rb_newobj_of

["peterzhu2118 (Peter Zhu) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #20265 has been reported by peterzhu2118 (Peter Zhu).

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Feature #20265: Deprecate and remove rb_newobj and rb_newobj_of
https://bugs.ruby-lang.org/issues/20265

* Author: peterzhu2118 (Peter Zhu)
* Status: Open
* Priority: Normal
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GitHub PR: https://github.com/ruby/ruby/pull/9964

I’m proposing deprecating and removing the rb_newobj and rb_newobj_of APIs because they are difficult to use, fragile to use, and requires knowledge of the internal implementation of data types in Ruby.

The rb_newobj function creates a T_NONE object. T_NONE objects are tricky to deal with since T_NONE objects cannot be marked, T_NONE objects are not reclaimed by the GC, and changing the object to other types require internal knowledge about the data type.

T_NONE objects are not allowed to be marked, so it cannot be GC managed. Since T_NONE objects are skipped during sweeping, it will leak Ruby heap memory if the developer never changes the object to another type.

Changing a T_NONE object to another type is tricky. For example, T_STRING objects have many flags for embedded, shared, shared root, encoding, coderange, etc. Many of these flags are not public, preventing direct use by developers. Developers must understand these flags to convert a T_NONE object into a T_STRING object.

While the rb_newobj_of function is easier to use than the rb_newobj function, it still requires developers to understand flags, meaning some issues of rb_newobj also apply to rb_newobj_of.

Below is the usage of RB_NEWOBJ, rb_newobj, rb_newobj_of, RB_NEWOBJ_OF with vendored Ruby and comments removed. You can see that there are very few gems using these APIs and all are from over a decade ago (the most recent one is from 2011).

```
2009-11-18 /srv/gems/bleak_house-7.2/ruby/ruby-1.8.7.patch:@@ -438,10 +438,8 @@ rb_newobj()
2023-07-01 /srv/gems/daqing_rucaptcha-3.2.2/ext/rucaptcha/target/release/build/rb-sys-6bdd5b2895b9570a/out/bindings-0.9.78-arm64-darwin22-3.2.2.rs:    pub fn rb_newobj() -> VALUE;
2023-07-01 /srv/gems/daqing_rucaptcha-3.2.2/ext/rucaptcha/target/release/build/rb-sys-6bdd5b2895b9570a/out/bindings-0.9.78-arm64-darwin22-3.2.2.rs:    pub fn rb_newobj_of(klass: VALUE, flags: VALUE) -> VALUE;
2010-08-06 /srv/gems/langscan-1.2/ext/langscan/ruby/compat/ripper/ripper.c:    NODE *n = (NODE*)rb_newobj();
2011-02-03 /srv/gems/memprof-0.3.10/ext/memprof.c:  VALUE ret = rb_newobj();
2011-02-03 /srv/gems/memprof-0.3.10/ext/tracers/objects.c:  last_obj = orig_rb_newobj();
2006-02-28 /srv/gems/refe-0.8.0.3/data/refe/function_source/rb/newobj:rb_newobj()
2006-02-28 /srv/gems/refe-0.8.0.3/data/refe/function_source/rb/node/newnode:    NODE *n = (NODE*)rb_newobj();
2011-08-31 /srv/gems/ruby-rpm-1.3.1/ext/rpm/dependency.c:       prov = rb_newobj();
2011-08-31 /srv/gems/ruby-rpm-1.3.1/ext/rpm/dependency.c:       req = rb_newobj();
2011-08-31 /srv/gems/ruby-rpm-1.3.1/ext/rpm/dependency.c:       conf = rb_newobj();
2011-08-31 /srv/gems/ruby-rpm-1.3.1/ext/rpm/dependency.c:       obso = rb_newobj();
2011-08-31 /srv/gems/ruby-rpm-1.3.1/ext/rpm/file.c:     file = rb_newobj();
2011-08-31 /srv/gems/ruby-rpm-1.3.1/ext/rpm/source.c:   src = rb_newobj();
2011-08-31 /srv/gems/ruby-rpm-1.3.1/ext/rpm/source.c:   src = rb_newobj();
2011-08-31 /srv/gems/ruby-rpm-1.3.1/ext/rpm/source.c:   src = rb_newobj();
2011-08-31 /srv/gems/ruby-rpm-1.3.1/ext/rpm/version.c:  ver = rb_newobj();
2011-08-31 /srv/gems/ruby-rpm-1.3.1/ext/rpm/version.c:  ver = rb_newobj();
2011-08-31 /srv/gems/ruby-rpm-1.3.1/ext/rpm/version.c:  ver = rb_newobj();
2006-11-30 /srv/gems/sydparse-1.2.0/sydparse.c:    syd_scope = (struct SCOPE*)rb_newobj();
2006-11-30 /srv/gems/sydparse-1.2.0/sydparse.y:    syd_scope = (struct SCOPE*)rb_newobj();
```



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