[ruby-core:112880] [Ruby master Feature#19520] Support for `Module.new(name)` and `Class.new(superclass, name)`.

Issue #19520 has been updated by Eregon (Benoit Daloze).


I had a call with @ioquatix, trying to explain the importance of the name that Ruby shows us, for example:
`undefined method 'zzz' for #<Foo::Bar:0x00007efc38711fc0> (NoMethodError)`
people will of course expect that `Foo::Bar` refers to the class of the object.
For instance maybe they want to create a new instance of Foo::Bar, so writing Foo::Bar in the code must refer to that class.
Yes, it's possible to break it with remove_const/const_set, but in practice this almost never happens, and if it does I would consider it a very serious bug of whatever uses remove_const/const_set to break it.

And the same for Zeitwerk, while I guess it's possible to break the constant path<->module mapping there with reloading e.g. maybe by storing an old Class in a global variable and doing that only once per process (e.g. only if the global is unset), it's just extremely uncommon.
I don't think Rails users get this problem often at all (e.g. we'd see more issues on rails or zeitwerk if they did).

One thing that could help is for this name to be visually different than a regular constant path, so e.g. it cannot start with an uppercase letter (as said in https://bugs.ruby-lang.org/issues/19450#note-17), and probably start with some symbol to make it even more obvious.
(Actually C code can define lowercase constants, so just not uppercase first letter is not enough, e.g. `IO::generic_writable` seen from `StringIO.ancestors`).

> One you assign a permanent name, it replaces any fake/temporary name.

That wasn't clear to me and the description doesn't seem to mention it.
It means there is yet another state of naming for modules in addition to the existing fully-anonymous (`#<Module:0x0123>`), knows its own name but not its lexical parent/nesting (`#<Module:0x0123>::A`) and fully-named (`A::B`).
So that's some extra complexity both for the user and for implementations.
I thought the "fake name" would be considered fully-named. IIRC naming of constants under a module is only done if a module is fully-named, and I think that's the feature you want here.

What I didn't have time to discuss on the call is what's the use-case for this besides tests which want to label anonymous modules/classes to make it easier to debug them.
@ioquatix mentioned his own reloading logic, but I'm not sure how that works, e.g., how do you refer to another model if not all models have a fully-named constant path?

----------------------------------------
Feature #19520: Support for `Module.new(name)` and `Class.new(superclass, name)`.
https://bugs.ruby-lang.org/issues/19520#change-102396

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
----------------------------------------
See <https://bugs.ruby-lang.org/issues/19450> for previous discussion and motivation.

[This proposal](https://github.com/ruby/ruby/pull/7376) introduces the `name` parameter to `Class.new` and `Module.new`:

```ruby
Class.new(superclass, name)
Module.new(name)
```

As a slight change, we could use keyword arguments instead.

## Example usage

The current Ruby test suite has code which shows the usefulness of this new method:

```ruby
  def labeled_module(name, &block)
    Module.new do
      singleton_class.class_eval {
        define_method(:to_s) {name}
        alias inspect to_s
        alias name to_s
      }
      class_eval(&block) if block
    end
  end
  module_function :labeled_module

  def labeled_class(name, superclass = Object, &block)
    Class.new(superclass) do
      singleton_class.class_eval {
        define_method(:to_s) {name}
        alias inspect to_s
        alias name to_s
      }
      class_eval(&block) if block
    end
  end
  module_function :labeled_class
```

The updated code would look like this:

```ruby
  def labeled_module(name, &block)
    Module.new(name, &block)
  end

  def labeled_class(name, superclass = Object, &block)
    Class.new(superclass, name, &block)
  end
  module_function :labeled_class
```



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