September 2025 - ruby-core - ml.ruby-lang.org

[ruby-core:118784] [Ruby master Feature#20664] Add `before` and `until` options to Enumerator.produce
by knu (Akinori MUSHA) 08 Feb '26

08 Feb '26

Issue #20664 has been reported by knu (Akinori MUSHA). ---------------------------------------- Feature #20664: Add `before` and `until` options to Enumerator.produce https://bugs.ruby-lang.org/issues/20664 * Author: knu (Akinori MUSHA) * Status: Open ---------------------------------------- Enumerator.produce provides a nice way to generate an infinite sequence but is a bit awkward to define how to end a sequence. It lacks a simple and easy way to produce typical finite sequences in an intuitive syntax. This proposal attempts to solve the problem by adding these two options to the method: - `before`: when provided, it is used as a predicate to determine if an iteration should end before a generated value gets yielded. - `until`: when provided, it is used as a predicate to determine if an iteration should end until after a generated value gets yielded. Any value that responds to `to_proc` and returns a `Proc` object is accepted in these options. A typical use case for the `before` option is traversing a tree structure to iterate over the ancestors or following/preceding siblings of a node. The `until` option can be used when there is a clear definition of the "last" value to yield. ```ruby enum = Enumerator.produce(File, before: :nil?, &:superclass) enum.to_a #=> [File, IO, Object, BasicObject] enum = Enumerator.produce(3, until: :zero?, &:pred) enum_to_a #=> [3, 2, 1, 0] ``` ---Files-------------------------------- 0001-Add-before-and-until-options-to-Enumerator.produce.patch (10.7 KB) -- https://bugs.ruby-lang.org/

6 12

[ruby-core:122842] [Ruby Feature#21518] Statistical helpers to `Enumerable`
by Amitleshed (Amit Leshed) 07 Feb '26

07 Feb '26

Issue #21518 has been reported by Amitleshed (Amit Leshed). ---------------------------------------- Feature #21518: Statistical helpers to `Enumerable` https://bugs.ruby-lang.org/issues/21518 * Author: Amitleshed (Amit Leshed) * Status: Open ---------------------------------------- **Summary** I'd like to add two statistical helpers to `Enumerable`: - `Enumerable#average` (arithmetic mean) - `Enumerable#median` Both are small, well-defined operations that many Rubyists re-implement in apps and gems. Providing them in core avoids repeated, ad-hoc code and aligns with `Enumerable#sum`, which Ruby already ships. **Motivation** - These are among the most common “roll-your-own” helpers for arrays/ranges of numbers. - They are conceptually simple, universally useful beyond web/Rails. - Similar to `sum`, they’re primitives for quick data analysis, ETL scripts, CLI tooling, etc. - Including them encourages consistent semantics (what to do with empty sets, mixed numerics, etc.). ## Proposed API & Semantics ```ruby Enumerable#average -> Float or nil Enumerable#median -> Numeric or nil ``` ```ruby [1, 2, 3, 4].average # => 2.5 (1..4).average # => 2.5 [].average # => nil [1, 3, 2].median # => 2 [1, 2, 3, 10].median # => 2.5 (1..6).median # => 3.5 [].median # => nil ``` Ruby implementation ```ruby module Enumerable def average count = 0 total = 0.0 each do |x| raise TypeError, "non-numeric value for average" unless x.is_a?(Numeric) total += x count += 1 end count.zero? ? nil : total / count end def median arr = to_a return nil if arr.empty? arr.each { |x| raise TypeError, "non-numeric value for median" unless x.is_a?(Numeric) } arr.sort! mid = arr.length / 2 arr.length.odd? ? arr[mid] : (arr[mid - 1] + arr[mid]) / 2.0 end end ``` **Upon approval I'm more than willing to implement spec and code in C.** -- https://bugs.ruby-lang.org/

9 12

[ruby-core:121982] [Ruby Bug#21323] irb fails to start with Namespace
by Eregon (Benoit Daloze) 07 Feb '26

07 Feb '26

Issue #21323 has been reported by Eregon (Benoit Daloze). ---------------------------------------- Bug #21323: irb fails to start with Namespace https://bugs.ruby-lang.org/issues/21323 * Author: Eregon (Benoit Daloze) * Status: Open * ruby -v: ruby 3.5.0dev (2025-05-10T07:50:29Z namespace-on-read-.. bd4f57f96b) +PRISM [x86_64-linux] * Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- ``` $ irb irb(main):001> 6 * 7 => 42 $ RUBY_NAMESPACE=1 irb /home/eregon/prefix/ruby-master/bin/ruby: warning: Namespace is experimental, and the behavior may change in the future! See doc/namespace.md for know issues, etc. /home/eregon/prefix/ruby-master/lib/ruby/3.5.0+0/did_you_mean/core_ext/name_error.rb:11: warning: irb is not part of the default gems since Ruby 3.5.0. Install irb from RubyGems. /home/eregon/prefix/ruby-master/lib/ruby/3.5.0+0/did_you_mean/core_ext/name_error.rb:11: warning: irb is not part of the default gems since Ruby 3.5.0. Install irb from RubyGems. <internal:/home/eregon/prefix/ruby-master/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:141:in 'Kernel#require': cannot load such file -- irb (LoadError) Did you mean? erb from <internal:/home/eregon/prefix/ruby-master/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:141:in 'Kernel#require' from /home/eregon/prefix/ruby-master/lib/ruby/gems/3.5.0+0/gems/irb-1.15.2/exe/irb:7:in 'Namespace::Loader#require' from /home/eregon/prefix/ruby-master/lib/ruby/gems/3.5.0+0/gems/irb-1.15.2/exe/irb:7:in '<top (required)>' from /home/eregon/.rubies/ruby-master/bin/irb:25:in 'Kernel#load' from /home/eregon/.rubies/ruby-master/bin/irb:25:in 'Namespace::Loader#load' from /home/eregon/.rubies/ruby-master/bin/irb:25:in '<main>' <internal:/home/eregon/prefix/ruby-master/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136:in 'Kernel#require': cannot load such file -- irb (LoadError) Did you mean? erb from <internal:/home/eregon/prefix/ruby-master/lib/ruby/3.5.0+0/rubygems/core_ext/kernel_require.rb>:136:in 'Kernel#require' from /home/eregon/prefix/ruby-master/lib/ruby/gems/3.5.0+0/gems/irb-1.15.2/exe/irb:7:in 'Namespace::Loader#require' from /home/eregon/prefix/ruby-master/lib/ruby/gems/3.5.0+0/gems/irb-1.15.2/exe/irb:7:in '<top (required)>' from /home/eregon/.rubies/ruby-master/bin/irb:25:in 'Kernel#load' from /home/eregon/.rubies/ruby-master/bin/irb:25:in 'Namespace::Loader#load' from /home/eregon/.rubies/ruby-master/bin/irb:25:in '<main>' zsh: exit 1 RUBY_NAMESPACE=1 irb ``` -- https://bugs.ruby-lang.org/

3 5

[ruby-core:116028] [Ruby master Feature#20152] mkmf / extconf: Add a proper way to not compile the extension
by byroot (Jean Boussier) 06 Feb '26

06 Feb '26

Issue #20152 has been reported by byroot (Jean Boussier). ---------------------------------------- Feature #20152: mkmf / extconf: Add a proper way to not compile the extension https://bugs.ruby-lang.org/issues/20152 * Author: byroot (Jean Boussier) * Status: Open * Priority: Normal ---------------------------------------- ### Context There are various gems that ship with a native extension as a way to speedup part of the gem, but also ship with a pure Ruby version of these methods as a fallback. So they only want to compile the extension if the platform supports it, and if not, just fallback to the slightly slower Ruby version. Right now users rely on one of two hacks to do this. Either they create an empty Makefile, but then still depend on `make` being available, or publish platform specific packages without any extension in them. Examples: - [`bootsnap` skip compilation if not on MRI or TruffleRuby](https://github.com/Shopify/bootsnap/blob/070151f1305f23102365d… - [`erb` has an extension for MRI but then need to publish a `java` version of the gem that doesn't actually contain Java code, just to skip compilation on JRuby](https://github.com/ruby/erb/issues/52) - [`hiredis-client` skips the compilation for Windows and non-MRI rubies](https://github.com/redis-rb/redis-client/blob/1ab081c1d0e47df5d55e0… ### Feature It would be very useful to have some proper first class API to skip compiling the extension. Something like: ```ruby require "mkmf" if RUBY_ENGINE != "ruby" || RUBY_PLATFORM.match?(/mswin/) skip_compilation else # ... end ``` cc @k0kubun @headius -- https://bugs.ruby-lang.org/

10 21

[ruby-core:121993] [Ruby Bug#21326] Instruction generation differences between parse.y and prism for `def a(x, ...); b(...); end`
by zenspider (Ryan Davis) 25 Jan '26

25 Jan '26

Issue #21326 has been reported by zenspider (Ryan Davis). ---------------------------------------- Bug #21326: Instruction generation differences between parse.y and prism for `def a(x, ...); b(...); end` https://bugs.ruby-lang.org/issues/21326 * Author: zenspider (Ryan Davis) * Status: Open * ruby -v: ruby 3.5.0dev * Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN ---------------------------------------- Diff instructions using: ```sh DUMP="puts RubyVM::InstructionSequence.compile(ARGV.first).disasm.gsub(/^\d+ /, '')" RUBY="def a(x, ...); b(...); end" env -u RUBYOPT \ diff -U99 \ <(./build/ruby -v --parser=parse.y -e "$DUMP" "$RUBY" 2>/dev/null) \ <(./build/ruby -v --parser=prism -e "$DUMP" "$RUBY" 2>/dev/null) ``` shows: ```diff --- /dev/fd/63 2025-05-10 14:46:37 +++ /dev/fd/62 2025-05-10 14:46:37 @@ -1,17 +1,18 @@ -ruby 3.5.0dev (2025-05-10T17:32:21Z master 98667f82d2) +GC [arm64-darwin24] +ruby 3.5.0dev (2025-05-10T17:32:21Z master 98667f82d2) +PRISM +GC [arm64-darwin24] == disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(1,26)> definemethod :a, a ( 1)[Li] putobject :a leave == disasm: #<ISeq:a@<compiled>:1 (1,0)-(1,26)> local table (size: 5, argc: 1 [opts: 0, rest: 1, post: 0, block: 3, kw: -1@-1, kwrest: 2]) [ 5] x@0<Arg> [ 4] "*"@1<AnonRest>[ 3] "**"@2<AnonKwrest>[ 2] "&"@3<Block>[ 1] "..."@4 putself ( 1)[LiCa] getlocal_WC_0 "*"@1 -splatarray false +splatarray true getlocal_WC_0 "**"@2 getblockparamproxy "&"@3, 0 splatkw -send <calldata!mid:b, argc:2, ARGS_SPLAT|ARGS_BLOCKARG|FCALL|KW_SPLAT>, nil +splatkw +send <calldata!mid:b, argc:2, ARGS_SPLAT|ARGS_SPLAT_MUT|ARGS_BLOCKARG|FCALL|KW_SPLAT>, nil leave [Re] ``` PR here: https://github.com/ruby/ruby/pull/13299 goes back to at least 3.4. -- https://bugs.ruby-lang.org/

3 2

[ruby-core:116935] [Ruby master Bug#18743] Enumerator#next / peek re-use each others stacktraces
by nagachika (Tomoyuki Chikanaga) 22 Jan '26

22 Jan '26

Issue #18743 has been updated by nagachika (Tomoyuki Chikanaga). Backport changed from 3.0: REQUIRED, 3.1: REQUIRED, 3.2: REQUIRED to 3.0: REQUIRED, 3.1: REQUIRED, 3.2: DONE ruby_3_2 5e4606423da96c2e26b00981c22bddfce0d970bf merged revision(s) f15123c34ce80f3928612befe2a9aaf4c9d27fda. ---------------------------------------- Bug #18743: Enumerator#next / peek re-use each others stacktraces https://bugs.ruby-lang.org/issues/18743#change-106970 * Author: sos4nt (Stefan Schüßler) * Status: Closed * Assignee: ko1 (Koichi Sasada) * Backport: 3.0: REQUIRED, 3.1: REQUIRED, 3.2: DONE ---------------------------------------- I encountered an odd behavior. If I rescue the `StopIteration` exception from `peek` and call `next` afterwards: (or vice-versa) ```ruby # enum.rb # 1 # 2 enum = [].each # 3 enum.peek rescue nil # 4 enum.next # 5 ``` it will show the stacktrace from the rescued `peek` call: ``` $ ruby enum.rb enum.rb:4:in `peek': iteration reached an end (StopIteration) from enum.rb:4:in `<main>' ``` Whereas the error should refer to `next` on line number 5. The same happens when calling `peek` after `next` or when having muliple `peek` / `next` calls: ```ruby # enum.rb # 1 # 2 enum = [].each # 3 enum.peek rescue nil # 4 enum.next rescue nil # 5 enum.peek rescue nil # 6 puts "line #{__LINE__}" # 7 enum.next # 8 ``` The stacktrace from the first (rescued) `peek` or `next` call will be shown which doesn't reflect the actual error location: ``` $ ruby enum.rb line 7 enum.rb:4:in `peek': iteration reached an end (StopIteration) from enum.rb:4:in `<main>' ``` This is very confusing when debugging code. ---Files-------------------------------- 01-Recreate-stacktrace-enumerator.patch (1.29 KB) -- https://bugs.ruby-lang.org/

2 1

[ruby-core:112045] [Ruby master Bug#19378] Windows: Use less syscalls for faster require of big gems
by aidog (Andi Idogawa) 12 Jan '26

12 Jan '26

Issue #19378 has been reported by aidog (Andi Idogawa). ---------------------------------------- Bug #19378: Windows: Use less syscalls for faster require of big gems https://bugs.ruby-lang.org/issues/19378 * Author: aidog (Andi Idogawa) * Status: Open * Priority: Normal * ruby -v: 3.2.0 * Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN ---------------------------------------- Hello 🙂 ## Problem require is slow on windows for big gems. (example: require 'gtk3'=> 3 seconds+). This is a problem for people who want to make cross platform GUI apps with ruby. ## Possible Reason As touched on in [#15797](https://bugs.ruby-lang.org/issues/15797) it seems like require uses realpath, which is emulated on windows. It checks every parent directory. The same syscalls run many times. ## Testfile C:\tmp\speedtest\testrequire.rb: ``` ruby require __dir__ + "/helloworld1.rb" require __dir__ + "/helloworld2.rb" ``` ``` shell ruby --disable-gems C:\tmp\speedtest\testrequire.rb ``` ### Syscalls per File/Directory: 1. CreateFile 2. QueryInformationVolume 3. QueryIdInformation 4. QueryAllInformationFile 5. QueryNameInformationFile 6. QueryNameInformationFile 7. QueryNormalizedNameInformationFile 8. CloseFile ### Files/Directories checked 1. C:\tmp 2. C:\tmp\speedtest 3. C:\tmp\speedtest\helloworld1.rb 4. C:\tmp 5. C:\tmp\speedtest 6. C:\tmp\speedtest\helloworld2.rb For two required files Ruby had to do 8*6 = **48** syscalls. The syscalls orginate from rb_w32_reparse_symlink_p / lstat Rubygems live in subfolders with 9+ parts: "C:\Ruby32-x64\lib\ruby\gems\3.2.0\gems\glib2-4.0.8\lib\glib2\variant.rb" Each file takes 8 * 9 = **72**+ calls. For variant.rb it is **80** calls. The result for the syscalls don't change in such a short time, so it should be possible to cache it. With require_relative it's twice as many calls. ## Other testcases Same result: ``` ruby File.realpath __dir__ + "/helloworld1.rb" File.realpath __dir__ + "/helloworld2.rb" ``` ``` ruby File.stat __dir__ + "/helloworld1.rb" File.stat __dir__ + "/helloworld2.rb" ``` It does not happen in $LOAD_PATH.resolve_feature_path(__dir__ + "/helloworld1.rb") ## Request Would it be possible to cache the stat calls when using require? I tried to implement a cache inside the ruby source code, but failed. If not, is there now a way to combine ruby files into one? I previously talked about require here: [YJIT: Windows support lacking.](https://bugs.ruby-lang.org/issues/19325#note-11) ## How to reproduce Ruby versions: At least 3.0+, most likely older ones too. Tested using Ruby Installer 3.1 and 3.2. [Procmon Software by Sysinternals](https://learn.microsoft.com/en-us/sysinternals/downloads/proc… -- https://bugs.ruby-lang.org/

6 7

[ruby-core:122552] [Ruby Feature#21442] Make tsort to bundled gems
by hsbt (Hiroshi SHIBATA) 08 Jan '26

08 Jan '26

Issue #21442 has been reported by hsbt (Hiroshi SHIBATA). ---------------------------------------- Feature #21442: Make tsort to bundled gems https://bugs.ruby-lang.org/issues/21442 * Author: hsbt (Hiroshi SHIBATA) * Status: Open ---------------------------------------- In last year, I gave up to make `tsort` to bundled gems because `set` depends `tsort`. https://github.com/ruby/set/blob/29a253d2259e8d6d9b313dec8418ec5ed0caab08/l… Fortunately, `set` is now core class. So, there is no library depends on `tsort` in ruby repository. I propose to make `tsort` to bundled gems at the next release(not Ruby 3.5/4.0, for 4.0(?)/4.1) -- https://bugs.ruby-lang.org/

1 3

[ruby-core:121699] [Ruby Feature#21277] Windows ARM support
by hsbt (Hiroshi SHIBATA) 08 Jan '26

08 Jan '26

Issue #21277 has been reported by hsbt (Hiroshi SHIBATA). ---------------------------------------- Feature #21277: Windows ARM support https://bugs.ruby-lang.org/issues/21277 * Author: hsbt (Hiroshi SHIBATA) * Status: Open * Assignee: hsbt (Hiroshi SHIBATA) ---------------------------------------- I added the new CI job for Windows ARM support. * https://github.blog/changelog/2025-04-14-windows-arm64-hosted-runners-now-a… * https://github.com/ruby/ruby/pull/13118 We could build Ruby with Windows ARM platform. But `nmake check` is not working yet. I'm tracking that issues on this ticket. ## Can't generate `revision.h` `windows-11-arm` runner couldn't generate `revision.h` with `ruby tool/file2lastrev.rb --revision.h --output=revision.h`. I tried that with git provided by `scoop`. But that is also not working. I confirmed to work `git rev-parse HEAD` directly with `pwsh` of GitHub Actions. But `tool/file2lastrev.rb` returns the followings: ``` Run ruby tool/file2lastrev.rb --revision.h --output=revision.h file2lastrev.rb: git rev-parse failed fatal: unable to access 'NUL': Invalid argument ``` ## `nmake check` is not working https://github.com/ruby/ruby/actions/runs/14564201209/job/40851166866 ``` running file: C:/a/ruby/ruby/src/test/rubygems/test_gem_resolver_index_set.rb A test worker crashed. It might be an interpreter bug or a bug in test/unit/parallel.rb. Try again without the -j option. ``` https://github.com/ruby/ruby/actions/runs/14564667517/job/40852193517 ``` NMAKE : fatal error U1077: '@ .\ruby.exe -I../src/lib -I".ext/arm64-mswin64_140" -I. "../src/tool/runruby.rb" --extout=".ext" -- --disable-gems -r./arm64-mswin64_140-fake -r../src/tool/lib/_tmpdir ../src/spec/mspec/bin/mspec run -B ../src/spec/default.mspec ' : return code '0xc0000005' Stop. ``` They may be caused by Visual Studio 2022 issue same as #21255. -- https://bugs.ruby-lang.org/

1 1

[ruby-core:120749] [Ruby master Feature#21084] Declare objects have weak references
by peterzhu2118 (Peter Zhu) 30 Dec '25

30 Dec '25

Issue #21084 has been reported by peterzhu2118 (Peter Zhu). ---------------------------------------- Feature #21084: Declare objects have weak references https://bugs.ruby-lang.org/issues/21084 * Author: peterzhu2118 (Peter Zhu) * Status: Open ---------------------------------------- # Summary The current way of marking weak references uses `rb_gc_mark_weak(VALUE *ptr)`. This presents challenges because Ruby's GC is incremental, meaning that if the `ptr` changes (e.g. realloc'd or free'd), then we could have an invalid memory access. This also overwrites `*ptr = Qundef` if `*ptr` is dead, which prevents any cleanup to be run (e.g. freeing memory or deleting entries from hash tables). This ticket proposes `rb_gc_declare_weak_references` which declares that an object has weak references and calls a cleanup function after marking, allowing the object to clean up any memory for dead objects. # Introduction In [[Feature #19783]](https://bugs.ruby-lang.org/issues/19783), I introduced an API allowing objects to mark weak references, the function signature looks like this: ```c void rb_gc_mark_weak(VALUE *ptr); ``` `rb_gc_mark_weak` is called during the marking phase of the GC to specify that the memory at `ptr` holds a pointer to a Ruby object that is weakly referenced. `rb_gc_mark_weak` appends this pointer to a list that is processed after the marking phase of the GC. If the object at `*ptr` is no longer alive, then it overwrites the object reference with a special value (`*ptr = Qundef`). However, this API resulted in two challenges: 1. Ruby's default GC is incremental, which means that the GC is not ran in one phase, but rather split into chunks of work that interleaves with Ruby execution. The `ptr` passed into `rb_gc_mark_weak` could be on the malloc heap, and that memory could be realloc'd or even free'd. We had to use workarounds such as `rb_gc_remove_weak` to ensure that there were no illegal memory accesses. This made `rb_gc_mark_weak` difficult to use, impacted runtime performance, and increased memory usage. 2. When an object dies, `rb_gc_mark_weak` only overwites the reference with `Qundef`. This means that if we want to do any cleanup (e.g. free a piece of memory or delete a hash table entry), we could not do that and had to defer this process elsewhere (e.g. during marking or runtime). # Declarative weak references In this ticket, I'm proposing a new API for weak references. Instead of an object marking its weak references during the marking phase, the object declares that it has weak references using the `rb_gc_declare_weak_references` function. This declaration occurs during runtime (e.g. after the object has been created) rather than during GC. After an object declares that it has weak references, it will have its callback function called after marking as long as that object is alive. This callback function can then call a special function `rb_gc_handle_weak_references_alive_p` to determine whether its references are alive. This will allow the callback function to do whatever it wants on the object, allowing it to perform any cleanup work it needs. This significantly simplifies the code for `ObjectSpace::WeakMap` and `ObjectSpace::WeakKeyMap` because it no longer needs to have the workarounds for the limitations of `rb_gc_mark_weak`. # Performance The performance results below demonstrate that `ObjectSpace::WeakMap#[]=` is now about 60% faster because the implementation has been simplified and the number of allocations has been reduced. We can see that there is not a significant impact on the performance of `ObjectSpace::WeakMap#[]`. Base: ``` ObjectSpace::WeakMap#[]= 4.620M (± 6.4%) i/s (216.44 ns/i) - 23.342M in 5.072149s ObjectSpace::WeakMap#[] 30.967M (± 1.9%) i/s (32.29 ns/i) - 154.998M in 5.007157s ``` Branch: ``` ObjectSpace::WeakMap#[]= 7.336M (± 2.8%) i/s (136.31 ns/i) - 36.755M in 5.013983s ObjectSpace::WeakMap#[] 30.902M (± 5.4%) i/s (32.36 ns/i) - 155.901M in 5.064060s ``` Code: ``` require "bundler/inline" gemfile do source "https://rubygems.org" gem "benchmark-ips" end wmap = ObjectSpace::WeakMap.new key = Object.new val = Object.new wmap[key] = val Benchmark.ips do |x| x.report("ObjectSpace::WeakMap#[]=") do |times| i = 0 while i < times wmap[Object.new] = Object.new i += 1 end end x.report("ObjectSpace::WeakMap#[]") do |times| i = 0 while i < times wmap[key] wmap[val] # does not exist i += 1 end end end ``` # Alternative designs Currently, `rb_gc_declare_weak_references` is designed to be an internal-only API. This allows us to assume the object types that call `rb_gc_declare_weak_references`. In the future, if we want to open up this API to third parties, we may want to change this function to something like: ```c void rb_gc_add_cleaner(VALUE obj, void (*callback)(VALUE obj)); ``` This will allow the third party to implement a custom `callback` that gets called after the marking phase of GC to clean up any dead references. I chose not to implement this design because it is less efficient as we would need to store a mapping from `obj` to `callback`, which requires extra memory. -- https://bugs.ruby-lang.org/

4 6