[ruby-core:116242] [Ruby master Feature#19742] Introduce `Module#anonymous?`

Issue #19742 has been updated by Eregon (Benoit Daloze).


Ah BTW one line below that link we see something interesting which can be repro'd on CRuby like:
```ruby
irb(main):006:0> m = Module.new
=> #<Module:0x00007fd24624d540>
irb(main):007:0> m::C = Class.new
=> #<Module:0x00007fd24624d540>::C
irb(main):008:0> c=m::C.new
=> #<#<Module:0x00007fd24624d540>::C:0x00007fd246515f20>
irb(main):009:0> Marshal.dump c
(irb):9:in `dump': can't dump anonymous class #<Module:0x00007fd24624d540>::C (TypeError)
```
So Ruby already calls a module/class with a temporary name as "anonymous" (yet `m::C.name` returns a String).

----------------------------------------
Feature #19742: Introduce `Module#anonymous?`
https://bugs.ruby-lang.org/issues/19742#change-106263

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
----------------------------------------
As a follow-on <from https://bugs.ruby-lang.org/issues/19521>, I'd like propose we introduce `Module#anonymous?`.

In some situations, like logging/formatting, serialisation/deserialization, debugging or meta-programming, we might like to know if a class is a proper constant or not.

However, this brings about some other issues which might need to be discussed.

After assigning a constant, then removing it, the internal state of Ruby still believes that the class name is permanent, even thought it's no longer true.

e.g.

```
m = Module.new
m.anonymous? # true

M = m
m.anonyomous # false

Object.send(:remove_const, :M)
M # uninitialized constant M (NameError)

m.anonymous? # false
```

Because RCLASS data structure is not updated after the constant is removed, internally the state still has a "permanent class name".

I want to use this proposal to discuss this issue and whether there is anything we should do about such behaviour (or even if it's desirable).

Proposed PR: https://github.com/ruby/ruby/pull/7966

cc @fxn



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