Change context-mode's default to new mode 4.

[marcandre]

This new mode uses a copy of the TOPLEVEL_BINDING. This is compatible with refinements (contrary to mode 3), while keeping nested IRB sessions separate.

Current default makes it impossible to call using SomeModule from the top level.
Other modes work:

irb --single-irb # => works (shared context for nested sessions)
irb --context 0 # => works
irb --context 1 # => works
irb --context 2 # => works
irb --context 3 # (current default) => does not work
irb --context 4 # (proposed default) => works

I believe the default mode can not be made to work as it is by design that one can not call using from inside a method.

This PR proposes a 4th mode that seems the most versatile and least error prone: TOPLEVEL_BINDING.dup.

I am surprised that it isn't used already, I was expecting this to be the default. Either there is some limitation that I am not aware of, or else maybe it is for historical reason that is not used?

Fixes redmine bug #9580

[aycabta]

It's just for historical reason. Great, thank you!

[marcandre]

Super, thanks!

[marcandre]

Will you take care of copying this commit to main ruby?

[aycabta]

@marcandre

Will you take care of copying this commit to main ruby?

Done!

[marcandre]

Great, thanks! 🎉

[eregon]

I think this change might cause https://bugs.ruby-lang.org/issues/17623#note-3

To be more precise, TOPLEVEL_BINDING captures the local variables of the main script (here bin/irb).

[marcandre]

@eregon indeed...

I only get TOPLEVEL_BINDING.local_variables # => [:str, :version] though...
Oh, right, there's a conditional.

We could fix this by modifying slightly bin/irb to not create any local variable... I'll open a PR

[eregon]

I think bin/irb in general might be generated, e.g., when doing gem install irb, so if we change it it should probably be done in RubyGems (the whole logic could be in a -> { ... }.call to avoid local vars).
I guess in general it could be useful if Ruby exposed (a frozen) EMPTY_BINDING or so, which could be a copy of TOPLEVEL_BINDING but before loading the main script (e.g., libraries required through -rmylib see TOPLEVEL_BINDING as empty).

[marcandre]

I see the comment about them being generated, but I also notice that bin/racc has a different layout than the others so I'm no sure they are actually generated.

[marcandre]

I created ruby/ruby#4176