[ruby-core:122003] [Ruby Feature#21311] Namespace on read (revised)

11 May 2025

      Issue #21311 has been updated by byroot (Jean Boussier).

I submitted a fix for the crash here: https://github.com/tagomoris/ruby/pull/5

But now it's crashing in `require`

```
/Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflector/inflections.rb:61: [BUG] Segmentation fault at 0x0000000000000339
ruby 3.5.0dev (2025-05-11T00:30:16Z namespace-on-read-.. 6629795fff) +PRISM [arm64-darwin24]

-- Crash Report log information --------------------------------------------
   See Crash Report log file in one of the following locations:             
     * ~/Library/Logs/DiagnosticReports                                     
     * /Library/Logs/DiagnosticReports                                      
   for more details.                                                        
Don't forget to include the above Crash Report log file in bug reports.     

-- Control frame information -----------------------------------------------
c:0030 p:0017 s:0140 e:000136 METHOD /Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflector/inflections.rb:61
c:0029 p:0005 s:0132 e:000131 BLOCK  /Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflector/inflections.rb:51 [FINISH]
c:0028 p:---- s:0128 e:000127 CFUNC  :map
c:0027 p:0025 s:0124 e:000122 METHOD /Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflector/inflections.rb:51
c:0026 p:0007 s:0118 e:000117 METHOD /Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflector/inflections.rb:209
c:0025 p:0492 s:0113 e:000112 BLOCK  /Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflections.rb:70
c:0024 p:0017 s:0109 e:000108 METHOD /Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflector/inflections.rb:267
c:0023 p:0007 s:0104 e:000103 CLASS  /Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflections.rb:12
c:0022 p:0013 s:0101 e:000100 TOP    /Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflections.rb:11 [FINISH]
c:0021 p:---- s:0098 e:000097 CFUNC  :require
c:0020 p:0023 s:0093 e:000092 METHOD <internal:/opt/rubies/head-namespaces/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136 [FINISH]
c:0019 p:---- s:0087 e:000086 CFUNC  :require
c:0018 p:0005 s:0082 e:000081 TOP    /Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflector/methods.rb:3 [FINISH]
c:0017 p:---- s:0079 e:000078 CFUNC  :require
c:0016 p:0023 s:0074 e:000073 METHOD <internal:/opt/rubies/head-namespaces/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136 [FINISH]
c:0015 p:---- s:0068 e:000067 CFUNC  :require
c:0014 p:0005 s:0063 e:000062 TOP    /Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/dependencies/autoload.rb:3 [FINISH]
c:0013 p:---- s:0060 e:000059 CFUNC  :require
c:0012 p:0023 s:0055 e:000054 METHOD <internal:/opt/rubies/head-namespaces/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136 [FINISH]
c:0011 p:---- s:0049 e:000048 CFUNC  :require
c:0010 p:0011 s:0044 e:000043 TOP    /Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support.rb:27 [FINISH]
c:0009 p:---- s:0041 e:000040 CFUNC  :require
c:0008 p:0023 s:0036 e:000035 METHOD <internal:/opt/rubies/head-namespaces/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136 [FINISH]
c:0007 p:---- s:0030 e:000029 CFUNC  :require
c:0006 p:0005 s:0025 e:000024 TOP    /Users/byroot/.gem/rubies/head-namespaces/gems/activerecord-7.2.2.1/lib/active_record.rb:26 [FINISH]
c:0005 p:---- s:0022 e:000021 CFUNC  :require
c:0004 p:0023 s:0017 e:000016 METHOD <internal:/opt/rubies/head-namespaces/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136 [FINISH]
c:0003 p:---- s:0011 e:000010 CFUNC  :require
c:0002 p:0023 s:0006 e:000005 EVAL   benchmark.rb:6 [FINISH]
c:0001 p:0000 s:0003 E:000d50 DUMMY  [FINISH]

-- Ruby level backtrace information ----------------------------------------
benchmark.rb:6:in '<main>'
benchmark.rb:6:in 'require'
<internal:/opt/rubies/head-namespaces/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136:in 'require'
<internal:/opt/rubies/head-namespaces/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136:in 'require'
/Users/byroot/.gem/rubies/head-namespaces/gems/activerecord-7.2.2.1/lib/active_record.rb:26:in '<top (required)>'
/Users/byroot/.gem/rubies/head-namespaces/gems/activerecord-7.2.2.1/lib/active_record.rb:26:in 'require'
<internal:/opt/rubies/head-namespaces/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136:in 'require'
<internal:/opt/rubies/head-namespaces/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136:in 'require'
/Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support.rb:27:in '<top (required)>'
/Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support.rb:27:in 'require'
<internal:/opt/rubies/head-namespaces/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136:in 'require'
<internal:/opt/rubies/head-namespaces/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136:in 'require'
/Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/dependencies/autoload.rb:3:in '<top (required)>'
/Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/dependencies/autoload.rb:3:in 'require'
<internal:/opt/rubies/head-namespaces/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136:in 'require'
<internal:/opt/rubies/head-namespaces/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136:in 'require'
/Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflector/methods.rb:3:in '<top (required)>'
/Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflector/methods.rb:3:in 'require'
<internal:/opt/rubies/head-namespaces/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136:in 'require'
<internal:/opt/rubies/head-namespaces/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136:in 'require'
/Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflections.rb:11:in '<top (required)>'
/Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflections.rb:12:in '<module:ActiveSupport>'
/Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflector/inflections.rb:267:in 'inflections'
/Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflections.rb:70:in 'block in <module:ActiveSupport>'
/Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflector/inflections.rb:209:in 'uncountable'
/Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflector/inflections.rb:51:in 'add'
/Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflector/inflections.rb:51:in 'map'
/Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflector/inflections.rb:51:in 'block in add'
/Users/byroot/.gem/rubies/head-namespaces/gems/activesupport-7.2.2.1/lib/active_support/inflector/inflections.rb:61:in 'to_regex'

-- Threading information ---------------------------------------------------
Total ractor count: 1
Ruby thread count for this ractor: 1

-- Machine register context ------------------------------------------------
  x0: 0x0000000000000331  x1: 0x0000000000000000  x2: 0x0000000000000f9f
  x3: 0x000000016b4544b8  x4: 0x000000016b454664  x5: 0x0000000000000000
  x6: 0x0000000000000001  x7: 0x0000000000000028 x18: 0x0000000000000000
 x19: 0x0000000000000331 x20: 0x000000015600fc00 x21: 0x000000011e59eec0
 x22: 0x0000000000000000 x23: 0x0000000000000f9f x24: 0x000060000098f660
 x25: 0x0000000000000001 x26: 0x000000016b454448 x27: 0x0000000000000001
 x28: 0x00000001049d16dc  lr: 0x00000001049d16fc  fp: 0x000000016b454390
  sp: 0x000000016b454340

-- C level backtrace information -------------------------------------------
/opt/rubies/head-namespaces/bin/ruby(rb_vm_bugreport+0xb6c) [0x104c2d5f8]
/opt/rubies/head-namespaces/bin/ruby(rb_bug_for_fatal_signal+0x100) [0x104a5af40]
/opt/rubies/head-namespaces/bin/ruby(sigsegv+0x84) [0x104b857e8]
/usr/lib/system/libsystem_platform.dylib(_sigtramp+0x38) [0x18de56de4]
/opt/rubies/head-namespaces/bin/ruby(class_classext_foreach_i+0x20) [0x1049d16fc]
/opt/rubies/head-namespaces/bin/ruby(class_classext_foreach_i+0x20) [0x1049d16fc]
/opt/rubies/head-namespaces/bin/ruby(rb_st_foreach+0xa4) [0x104b8d0c0]
/opt/rubies/head-namespaces/bin/ruby(rb_class_classext_foreach+0x4c) [0x1049d1698]
/opt/rubies/head-namespaces/bin/ruby(rb_gc_mark_children+0x1bc) [0x104a7ad90]
/opt/rubies/head-namespaces/bin/ruby(gc_mark_stacked_objects_incremental+0xac) [0x104a84710]
/opt/rubies/head-namespaces/bin/ruby(gc_continue+0xd0) [0x104a82b88]
/opt/rubies/head-namespaces/bin/ruby(newobj_cache_miss+0x180) [0x104a827b0]
/opt/rubies/head-namespaces/bin/ruby(newobj_of+0x170) [0x104a77548]
/opt/rubies/head-namespaces/bin/ruby(str_duplicate+0xb4) [0x104b9d924]
/opt/rubies/head-namespaces/bin/ruby(rb_reg_initialize_str+0xc4) [0x104b4d994]
/opt/rubies/head-namespaces/bin/ruby(rb_reg_new_ary+0x1e8) [0x104b4dc1c]
/opt/rubies/head-namespaces/bin/ruby(vm_exec_core+0x5df4) [0x104c05b90]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_exec+0x184) [0x104bfe838]
/opt/rubies/head-namespaces/bin/ruby(invoke_block_from_c_bh+0x368) [0x104c27274]
/opt/rubies/head-namespaces/bin/ruby(rb_yield+0xa8) [0x104c0e2fc]
/opt/rubies/head-namespaces/bin/ruby(rb_ary_collect+0xd4) [0x1049adeb4]
/opt/rubies/head-namespaces/bin/ruby(vm_call_cfunc_with_frame_+0xf0) [0x104c1e8cc]
/opt/rubies/head-namespaces/bin/ruby(vm_exec_core+0x1d60) [0x104c01afc]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_exec+0x184) [0x104bfe838]
/opt/rubies/head-namespaces/bin/ruby(rb_ensure+0xb8) [0x104a66ee4]
/opt/rubies/head-namespaces/bin/ruby(load_iseq_eval+0x280) [0x104acb488]
/opt/rubies/head-namespaces/bin/ruby(require_internal+0x99c) [0x104ac90ac]
/opt/rubies/head-namespaces/bin/ruby(rb_require_string_internal+0x58) [0x104ac8320]
/opt/rubies/head-namespaces/bin/ruby(rb_f_require+0x44) [0x104ac81f0]
/opt/rubies/head-namespaces/bin/ruby(vm_call_cfunc_with_frame_+0xf0) [0x104c1e8cc]
/opt/rubies/head-namespaces/bin/ruby(vm_call_alias+0x70) [0x104c19b38]
/opt/rubies/head-namespaces/bin/ruby(vm_exec_core+0x2228) [0x104c01fc4]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_exec+0x184) [0x104bfe838]
/opt/rubies/head-namespaces/bin/ruby(vm_call0_body+0x470) [0x104c2482c]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_call_kw+0xd8) [0x104c0c660]
/opt/rubies/head-namespaces/bin/ruby(rb_call_super_kw+0x1bc) [0x104c0cbf0]
/opt/rubies/head-namespaces/bin/ruby(rb_ensure+0xb8) [0x104a66ee4]
/opt/rubies/head-namespaces/bin/ruby(rb_namespace_user_loading_func+0x80) [0x104ad7b0c]
/opt/rubies/head-namespaces/bin/ruby(vm_call_cfunc_with_frame_+0xf0) [0x104c1e8cc]
/opt/rubies/head-namespaces/bin/ruby(vm_exec_core+0x2228) [0x104c01fc4]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_exec+0x184) [0x104bfe838]
/opt/rubies/head-namespaces/bin/ruby(rb_ensure+0xb8) [0x104a66ee4]
/opt/rubies/head-namespaces/bin/ruby(load_iseq_eval+0x280) [0x104acb488]
/opt/rubies/head-namespaces/bin/ruby(require_internal+0x99c) [0x104ac90ac]
/opt/rubies/head-namespaces/bin/ruby(rb_require_string_internal+0x58) [0x104ac8320]
/opt/rubies/head-namespaces/bin/ruby(rb_f_require+0x44) [0x104ac81f0]
/opt/rubies/head-namespaces/bin/ruby(vm_call_cfunc_with_frame_+0xf0) [0x104c1e8cc]
/opt/rubies/head-namespaces/bin/ruby(vm_call_alias+0x70) [0x104c19b38]
/opt/rubies/head-namespaces/bin/ruby(vm_exec_core+0x2228) [0x104c01fc4]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_exec+0x184) [0x104bfe838]
/opt/rubies/head-namespaces/bin/ruby(vm_call0_body+0x470) [0x104c2482c]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_call_kw+0xd8) [0x104c0c660]
/opt/rubies/head-namespaces/bin/ruby(rb_call_super_kw+0x1bc) [0x104c0cbf0]
/opt/rubies/head-namespaces/bin/ruby(rb_ensure+0xb8) [0x104a66ee4]
/opt/rubies/head-namespaces/bin/ruby(rb_namespace_user_loading_func+0x80) [0x104ad7b0c]
/opt/rubies/head-namespaces/bin/ruby(vm_call_cfunc_with_frame_+0xf0) [0x104c1e8cc]
/opt/rubies/head-namespaces/bin/ruby(vm_exec_core+0x2228) [0x104c01fc4]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_exec+0x184) [0x104bfe838]
/opt/rubies/head-namespaces/bin/ruby(rb_ensure+0xb8) [0x104a66ee4]
/opt/rubies/head-namespaces/bin/ruby(load_iseq_eval+0x280) [0x104acb488]
/opt/rubies/head-namespaces/bin/ruby(require_internal+0x99c) [0x104ac90ac]
/opt/rubies/head-namespaces/bin/ruby(rb_require_string_internal+0x58) [0x104ac8320]
/opt/rubies/head-namespaces/bin/ruby(rb_f_require+0x44) [0x104ac81f0]
/opt/rubies/head-namespaces/bin/ruby(vm_call_cfunc_with_frame_+0xf0) [0x104c1e8cc]
/opt/rubies/head-namespaces/bin/ruby(vm_call_alias+0x70) [0x104c19b38]
/opt/rubies/head-namespaces/bin/ruby(vm_exec_core+0x2228) [0x104c01fc4]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_exec+0x184) [0x104bfe838]
/opt/rubies/head-namespaces/bin/ruby(vm_call0_body+0x470) [0x104c2482c]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_call_kw+0xd8) [0x104c0c660]
/opt/rubies/head-namespaces/bin/ruby(rb_call_super_kw+0x1bc) [0x104c0cbf0]
/opt/rubies/head-namespaces/bin/ruby(rb_ensure+0xb8) [0x104a66ee4]
/opt/rubies/head-namespaces/bin/ruby(rb_namespace_user_loading_func+0x80) [0x104ad7b0c]
/opt/rubies/head-namespaces/bin/ruby(vm_call_cfunc_with_frame_+0xf0) [0x104c1e8cc]
/opt/rubies/head-namespaces/bin/ruby(vm_exec_core+0x2228) [0x104c01fc4]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_exec+0x184) [0x104bfe838]
/opt/rubies/head-namespaces/bin/ruby(rb_ensure+0xb8) [0x104a66ee4]
/opt/rubies/head-namespaces/bin/ruby(load_iseq_eval+0x280) [0x104acb488]
/opt/rubies/head-namespaces/bin/ruby(require_internal+0x99c) [0x104ac90ac]
/opt/rubies/head-namespaces/bin/ruby(rb_require_string_internal+0x58) [0x104ac8320]
/opt/rubies/head-namespaces/bin/ruby(rb_f_require+0x44) [0x104ac81f0]
/opt/rubies/head-namespaces/bin/ruby(vm_call_cfunc_with_frame_+0xf0) [0x104c1e8cc]
/opt/rubies/head-namespaces/bin/ruby(vm_call_alias+0x70) [0x104c19b38]
/opt/rubies/head-namespaces/bin/ruby(vm_exec_core+0x2228) [0x104c01fc4]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_exec+0x184) [0x104bfe838]
/opt/rubies/head-namespaces/bin/ruby(vm_call0_body+0x470) [0x104c2482c]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_call_kw+0xd8) [0x104c0c660]
/opt/rubies/head-namespaces/bin/ruby(rb_call_super_kw+0x1bc) [0x104c0cbf0]
/opt/rubies/head-namespaces/bin/ruby(rb_ensure+0xb8) [0x104a66ee4]
/opt/rubies/head-namespaces/bin/ruby(rb_namespace_user_loading_func+0x80) [0x104ad7b0c]
/opt/rubies/head-namespaces/bin/ruby(vm_call_cfunc_with_frame_+0xf0) [0x104c1e8cc]
/opt/rubies/head-namespaces/bin/ruby(vm_exec_core+0x2228) [0x104c01fc4]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_exec+0x184) [0x104bfe838]
/opt/rubies/head-namespaces/bin/ruby(rb_ensure+0xb8) [0x104a66ee4]
/opt/rubies/head-namespaces/bin/ruby(load_iseq_eval+0x280) [0x104acb488]
/opt/rubies/head-namespaces/bin/ruby(require_internal+0x99c) [0x104ac90ac]
/opt/rubies/head-namespaces/bin/ruby(rb_require_string_internal+0x58) [0x104ac8320]
/opt/rubies/head-namespaces/bin/ruby(rb_f_require+0x44) [0x104ac81f0]
/opt/rubies/head-namespaces/bin/ruby(vm_call_cfunc_with_frame_+0xf0) [0x104c1e8cc]
/opt/rubies/head-namespaces/bin/ruby(vm_call_alias+0x70) [0x104c19b38]
/opt/rubies/head-namespaces/bin/ruby(vm_exec_core+0x2228) [0x104c01fc4]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_exec+0x184) [0x104bfe838]
/opt/rubies/head-namespaces/bin/ruby(vm_call0_body+0x470) [0x104c2482c]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_call_kw+0xd8) [0x104c0c660]
/opt/rubies/head-namespaces/bin/ruby(rb_call_super_kw+0x1bc) [0x104c0cbf0]
/opt/rubies/head-namespaces/bin/ruby(rb_ensure+0xb8) [0x104a66ee4]
/opt/rubies/head-namespaces/bin/ruby(rb_namespace_user_loading_func+0x80) [0x104ad7b0c]
/opt/rubies/head-namespaces/bin/ruby(vm_call_cfunc_with_frame_+0xf0) [0x104c1e8cc]
/opt/rubies/head-namespaces/bin/ruby(vm_exec_core+0x2228) [0x104c01fc4]
/opt/rubies/head-namespaces/bin/ruby(rb_vm_exec+0x184) [0x104bfe838]
/opt/rubies/head-namespaces/bin/ruby(rb_ec_exec_node+0x98) [0x104a6607c]
/opt/rubies/head-namespaces/bin/ruby(ruby_run_node+0x44) [0x104a65f9c]
/opt/rubies/head-namespaces/bin/ruby(main+0x68) [0x1049a4cc4]
```

----------------------------------------
Feature #21311: Namespace on read (revised)
https://bugs.ruby-lang.org/issues/21311#change-113121

* Author: tagomoris (Satoshi Tagomori)
* Status: Open
----------------------------------------
This replaces #19744 

## Concept

This proposes a new feature to define virtual top-level namespaces in Ruby. Those namespaces can require/load libraries (either .rb or native extension) separately from other namespaces. Dependencies of required/loaded libraries are also required/loaded in the namespace.

This feature will be disabled by default at first, and will be enabled by an env variable `RUBY_NAMESPACE=1` as an experimental feature.
(It could be enabled by default in the future possibly.)

### "on read" approach

The "on write" approach here is the design to define namespaces on the loaded side. For example, Java packages are defined in the .java files and it is required to separate namespaces from each other. It can be implemented very easily, but it requires all libraries to be updated with the package declaration. (In my opinion, it's almost impossible in the Ruby ecosystem.)

The "on read" approach is to create namespaces and then require/load applications and libraries in them. Programmers can control namespace separation at the "read" time. So, we can introduce the namespace separation incrementally.

## Motivation

The "namespace on read" can solve the 2 problems below, and can make a path to solve another problem:

* Avoiding name conflicts between libraries
  * Applications can require two different libraries safely which use the same module name.
* Avoiding unexpected globally shared modules/objects
  * Applications can make an independent/unshared module instance.
* Multiple versions of gems can be required
  * Application developers will have fewer version conflicts between gem dependencies if rubygems/bundler will support the namespace on read. (Support from RubyGems/Bundler and/or other packaging systems will be needed)

For the motivation details, see [Feature #19744].

## How we can use Namespace

```ruby
# app1.rb
PORT = 2048
class App
  def self.port = ::PORT
  def val = PORT.to_s
end

p App.port # 2048

# app2.rb
class Number
  def double = self * 2
end

PORT = 2048.double
class App
  def self.port = ::PORT
  def val = PORT.double.to_s
end

p App.port # 4096

# main.rb - executed as `ruby main.rb`
ns1 = Namespace.new
ns1.require('./app1') # 2048
ns2 = Namespace.new
ns2.require('./app2') # 4096

PORT = 8080
class App
  def self.port = ::PORT
  def val = PORT.to_s
end

p App.port # 8080
p App.new.val # "8080"

p ns1::App.port # 2048
p ns1::App.new.val # "2048"

p ns2::App.port # 4096
p ns2::App.new.val # "8192"

1.double # NoMethodError
```

## Namespace specification

### Types of namespaces

There are two namespace types, "root" and "user" namespace. "Root" namespace exists solely in a Ruby process, and "user" namespaces can be created as many as Ruby programmers want.

### Root namespace

Root namespace is a unique namespace to be defined when a Ruby process starts. It only contains built-in classes/modules/constants, which are available without any `require` calls, including RubyGems itself (when `--disable-gems` is not specified).

At here, "builtin" classes/modules are classes/modules accessible when users' script evaluation starts, without any require/load calls.

### User namespace

User namespace is a namespace to run users' Ruby scripts. The "main" namespace is the namespace to run the user's `.rb` script specified by the `ruby` command-line argument. Other user namespaces ("optional" namespaces) can be created by `Namespace.new` call.

In user namespace (both main and optional namespaces), built-in class/module definitions are copied from the root namespace, and other new classes/modules are defined in the namespace, separately from other (root/user) namespaces.
The newly defined classes/modules are top-level classes/modules in the main namespace like `App`, but in optional namespaces, classes/modules are defined under the namespace (subclass of Module), like `ns::App`.

In that namespace `ns`, `ns::App` is accessible as `App` (or `::App`). There is no way to access `App` in the main namespace from the code in the different namespace `ns`.

### Constants, class variables and global variables

Constants, Class variables of built-in classes and global variables are also separated by namespace. Values set to class/global variables in a namespace are invisible in other namespaces.

### Methods and procs

Methods defined in a namespace run with the defined namespace, even when called from other namespaces.
Procs created in a namespace run with the defined namespace too.

### Dynamic link libraries

Dynamic link libraries (typically .so files) are also loaded in namespaces as well as .rb files.

### Open class (Changes on built-in classes)

In user namespaces, built-in class definitions can be modified. But those operations are processed as copy-on-write of class definition from the root namespace, and the changed definitions are visible only in the (user) namespace.

Definitions in the root namespace are not modifiable from other namespaces. Methods defined in the root namespace run only with root-namespace definitions.

## Enabling Namespace

Specify `RUBY_NAMESPACE=1` environment variable when starting Ruby processes. `1` is the only valid value here.

Namespace feature can be enabled only when Ruby processes start. Setting `RUBY_NAMESPACE=1` after starting Ruby scripts performs nothing.

## Pull-request

https://github.com/ruby/ruby/pull/13226

-- 
https://bugs.ruby-lang.org/