[ruby-core:112318] [Ruby master Bug#11230] Should rb_struct_s_members() be public API?

Issue #11230 has been updated by mame (Yusuke Endoh).


Discussed at the dev meeting.

When we considered removing rb_struct_s_members two years ago, we did not find its use in a major gem. Therefore, we agreed to remove it once.

However, two years have passed since then, and now we have found that oj gem is using it.

```
$ gem-codesearch rb_struct_s_members | sort
...
2023-02-01 /srv/gems/oj-3.14.1/ext/oj/custom.c:        ma   = rb_struct_s_members(clas);
2023-02-01 /srv/gems/oj-3.14.1/ext/oj/dump_object.c:        VALUE       ma = rb_struct_s_members(clas);
2023-02-01 /srv/gems/oj-3.14.1/ext/oj/rails.c:    ma = rb_struct_s_members(rb_obj_class(obj));
```

It's possible to fix oj gem and have a deprecation warning period. However, it would be easier to just keep this C API, unless there is a strong reason why we really have to remove it.

----------------------------------------
Bug #11230: Should rb_struct_s_members() be public API?
https://bugs.ruby-lang.org/issues/11230#change-101759

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* ruby -v: ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-linux]
* Backport: 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN
----------------------------------------
rb_struct_s_members() is declared in include/ruby/intern.h.

However it seems this is a fairly internal API as it returns an hidden Array.

For instance, there was a try to specify the behavior in
https://github.com/rubinius/rubinius/commit/2642a30c57973987d2a7b7e85b3ce1e78100a31b
but that produces only erratic behavior on MRI because that usage is not intended.

A valid usage might be:

    static VALUE get_struct_member(VALUE self, VALUE obj, VALUE i) {
        return RARRAY_AREF(rb_struct_s_members(obj), NUM2INT(i));
    }

    p get_struct_member(Struct.new(:a, :bb), 1)
    

But do we need such access and is it worth exposing a potentially dangerous function like this?



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