Issue #19315 has been updated by byroot (Jean Boussier). `SHARABLE_MIDDLE_SUBSTRING` was introduced circa 2014 in https://github.com/ruby/ruby/commit/a707ab4bc8a by @nobu. @nobu maybe you have some insights to share on this topic? Is there a reason you remember why this flag was never enabled by default? I assume compatibility issues with C extensions but there might be more. ---------------------------------------- Feature #19315: Lazy substrings in CRuby https://bugs.ruby-lang.org/issues/19315#change-101076 * Author: Eregon (Benoit Daloze) * Status: Open * Priority: Normal ---------------------------------------- CRuby should implement lazy substrings, i.e., "abcdef"[1..3] must not copy bytes. Currently CRuby only reuse the char* if the substring is until the end of the buffer. But it should also work wherever the substring starts and ends. Yes, it means RSTRING_PTR() might need to allocate to \0-terminate, so be it, it's worth it. There is already code for this (`SHARABLE_MIDDLE_SUBSTRING`), but it's disabled by default and `RSTRING_PTR()` needs to be changed to deal with this. It seems a good idea to introduce a variant of `RSTRING_PTR` which doesn't guarantee \0-termination, so such callers can then use the existing bytes always without copy. There are countless workarounds for this missing optimization, all not worth and all less readable: * https://bugs.ruby-lang.org/issues/19314 * https://github.com/ruby/net-protocol/pull/14 * Manual lazy substrings which track string + index + length * More but I don't remember all now, feel free to comment or link more urls/tickets. -- https://bugs.ruby-lang.org/