⚡️ RUBY 4.0 IN BRIEF:

• ZJIT is a new prototype JIT compiler pitched as the 'next generation' of YJIT. It aims to offer extra performance in the long term but isn't considered production ready yet, so it's behind a --zjit flag if you want to test it out.

• Ruby Box is an experimental isolation feature for separating definitions (monkey patches, globals, class definitions). It's akin to what you might call namespaces elsewhere (and stems largely from a proposal to include namespaces into Ruby) but differs enough to warrant the term 'box'.

• ⚠️ Net::HTTP no longer auto-sets Content-Type. You must now explicitly set it for POST/PUT requests. This is quite a significant, yet subtle, breaking change with potentially negative outcomes, particularly when using API clients that aren't updated for Ruby 4.0.

• Kernel#inspect's output can now be customized with which instance variables you want to display – potentially useful for hiding sensitive data like tokens.

• Logical operators can now start lines (see above) and will continue the expression from the previous line (akin to method chaining with .).

• Backtraces are cleaner, with internal frames hidden meaning C-implemented methods now show the Ruby source location.

• Ractors have seen some improvements, but remain experimental for now (though the aim is for the experimental status to be removed later in 2026).

• Set is now a core class and no longer needs to be autoloaded on use.

• Array#rfind is a new, more efficient alternative to arr.reverse_each.find.

• A myriad of performance boosts, like a faster Class#new and faster GC.

• Ruby 4.0 is now available in the official Ruby Docker images and is how I tested out some of the things above, as I usually wait for the x.0.1 release for my day-to-day work machine... ;-)

🎄 OK, this really is the final issue of 2025. Our next scheduled issue is on Thursday, January 8 where we'll have even more Ruby 4.0 goodies.