[ruby-core:113947] [Ruby master Feature#19521] Support for `Module#name=` and `Class#name=`.

Issue #19521 has been updated by Eregon (Benoit Daloze).


Dan0042 (Daniel DeLorme) wrote in #note-32:
> Ah interesting, I didn't realize it was already used that way. Since the ruby main repo already demonstrates how to "lie" about the class name in a sensible and useful way, I believe this is perfectly fine.

I think there is a misunderstanding, currently there is no lying in exception messages showing the module name of such a labeled module:
```ruby
# labeled_class from the description
c = labeled_class("Parent")
c.new.foo # => undefined method `foo' for #<#<Class:0x00007f1c4c1f5878>:0x00007f1c4c1f50a8> (NoMethodError)
```
If `set_temporary_name` is unrestricted, it would be (for `labeled_class("Parent")`):
```ruby
c.new.foo # => undefined method `foo' for #<Parent:0x00007f1c4c1f50a8> (NoMethodError)
```
which would naturally make you look for a class called `Parent` except either there isn't one or it's not the same class, which is rather confusing.

> However, this will prevent labeled_class/labeled_module from using set_temporary_name, at least until those labels are updated to be "not a constant name" or we remove this restriction. 

This seems very easy to address, labeled_module/labeled_class could wrap the name in `<...>` or `#<...>` or `#<Class:...>` or the callers could start with a lowercase letter, etc. All these make it clear those are not the constant paths (and never have been) of these anonymous modules, which is what I believe is most important here.

Every Rubyist expects the name of a module if it looks like a constant path to be a way to reach it, let's preserve that. Yes it's not a guarantee and yes it's possible to break it, but production code doing that would just be broken code. And this unrestricted `set_temporary_name` would make super easy to break it, without even realizing that, that I believe would make it a very confusing and dangerous feature, because it breaks something that every Rubyist relies on every day, not on purpose.

It's easy to add the restriction that `set_temporary_name` can only be used with a name which is not a valid constant path now. It's impossible to add it later. I am sure many Rubyists will regret it if we don't add this restriction which preserves the design of Ruby module naming.

----------------------------------------
Feature #19521: Support for `Module#name=` and `Class#name=`.
https://bugs.ruby-lang.org/issues/19521#change-103600

* 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/7483) introduces `Module#name=` (and thus also `Class#name=`) to set the temporary class name. The name assignment has no effect if the module/class already has a permanent name.

```ruby
c = Class.new do
  self.name = "fake name"
end

c = Class.new
c.name = "fake name"
```

Alternatively, we could use `set_name`:

```ruby
Class.new do
  set_name "fake_name"
end
```

Setting the name of a class changes its current name, irrespective of whether it's been assigned a permanent name, or has nested modules/classes which have cached a previous name. We might like to limit the cases where a name is set, e.g. only once, only if the name is nil, or only if it's not already permanent. There is no real harm in any of those options, just inconsistency.

## 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 do
      self.name = name
      class_eval(&block) if block
    end
  end

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

Because the name cannot be set as part of `.new`, we have to have a separate block to set the name, before calling `class_eval`. I think the ergonomics and performance of this are slightly worse than the [counter proposal](https://bugs.ruby-lang.org/issues/19520).



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