Issue #18487 has been updated by alanwu (Alan Wu). @ttilberg That's right. The backtrace has `bind_call` as the caller of `binding`, and `bind_call` is a C method, hence the exception. ---------------------------------------- Bug #18487: Kernel#binding behaves differently depending on implementation language of items on the stack https://bugs.ruby-lang.org/issues/18487#change-114523 * Author: alanwu (Alan Wu) * Status: Closed * ruby -v: ruby 3.1.0p0 (2021-12-25 revision fb4df44d16) * Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN ---------------------------------------- Recently I [discovered] that one could use `Kernel#binding` to capture the environment of a frame that is not directly below the stack frame for `Kernel#binding`. I've known that C extensions have this [privilege] for a while, but didn't realize that it was also possible using only the core library. This is a powerful primitive that allows for some funky programs: ```ruby def lookup(hash, key) hash[key] hash end p lookup( Hash.new(&( Kernel.instance_method(:send).method(:bind_call).to_proc >> ->(binding) { binding.local_variable_set(:hash, :action_at_a_distance!) } ) ), :binding ) # => :action_at_a_distance! ``` There might be ways to compose core library procs such that it's less contrived and more useful, but I couldn't figure out a way to do it. Maybe there is a way to make a "local variable set" proc that takes only a name-value pair and no block? ### What's the big deal? This behavior makes the implementation language of methods part of the API surface for `Kernel#binding`. In other words, merely changing a Ruby method to be a C method can be a breaking change for the purposes of `Kernel#binding`, even if the method behaves the same in all other observable ways. I feel that whether a method is native or not should be an implementation detail and should not impact `Kernel#binding`. This is a problem for Ruby implementations that want to implement many core methods in Ruby, because they risk breaking compatibility with CRuby. TruffleRuby has this [problem][privilege] as I alluded to earlier, and CRuby risks making unintentional breaking changes as more methods change to become Ruby methods in the core library. ### Leaking less details I think a straight forward way to fix this issue is by making it so that `Kernel#binding` only ever looks at the stack frame directly below it. If the frame below is a not a Ruby frame, it can return an empty binding. I haven't done the leg work of figuring out how hard this would be to implement in CRuby, though. This new behavior allows observing the identity of native frames, which is new. ### Does the more restrictive behavior help YJIT? Maybe. It's hard to tell without building out more optimizations that are related to local variables. YJIT currently doesn't do much in that area. If I had to guess I wouuld say the more restrictive semantics at least open up the possibility of some deoptimization strategies that are more memory efficient. ### What do you think? This is not a huge issue, but it might be nice to start thinking about for the next release. If a lot of people actually rely on the current behavior we can provide a migration plan. Since it might take years to land, I would like to solicit feedback now. [discovered]: https://github.com/ruby/ruby/commit/54c91042ed61a869d4a66fc089b21f56d165265f [privilege]: https://github.com/oracle/truffleruby/issues/2171 -- https://bugs.ruby-lang.org/