[ruby-core:111847] [Ruby master Feature#19315] Lazy substrings in CRuby

Issue #19315 has been updated by mame (Yusuke Endoh).


I heard that Java stopped the shared substring technique 10 years ago (https://www.infoq.com/news/2013/12/Oracle-Tunes-Java-String/) because of the potential for memory leaks

I don't disagree this proposal, but it would be nice if we could evaluate the effectiveness of this optimization.

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Feature #19315: Lazy substrings in CRuby
https://bugs.ruby-lang.org/issues/19315#change-101258

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
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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 it with lazy substring and all less readable:
* https://bugs.ruby-lang.org/issues/19314
* https://bugs.ruby-lang.org/issues/18598#note-3
* 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.



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