From: sam.saffron@...
Date: 2018-02-15T09:43:22+00:00
Subject: [ruby-core:85581] [Ruby trunk Feature#14478] String #uminus should de-dupe unconditionally

Issue #14478 has been updated by sam.saffron (Sam Saffron).


> how about we do heap sharing 

I am very against this, I just don't see the compatibility risk I think myself and maybe only another like 5 people know about the uminus change in 2.5 :) I am willing to bet a very large sum of money nobody is depending on object_id stability when calling uminus on a frozen string. This would be people who

1. Do not depend on object_id stability when calling uminus on non frozen strings
2. Somehow for some crazy reason depend on object_id stability when called on frozen strings

I also can not even think of an actual use case where you would depend on this inconsistent behavior

If anything I think we are not going far enough, I think uminus should unconditionally untaint, but that is a topic for another issue. Also I wish we would just remove taint and trust. 

----------------------------------------
Feature #14478: String #uminus should de-dupe unconditionally
https://bugs.ruby-lang.org/issues/14478#change-70377

* Author: sam.saffron (Sam Saffron)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
continuing: https://bugs.ruby-lang.org/issues/14475

Current documentation for String uminus says: "If the string is frozen, then return the string itself."

Trouble is that there is no simple way to de-duplicate unconditionally without inefficiency:

Say `x` is an arbitrary string (either frozen or unfrozen) :

```
x = -x # may return a non fstring

x = -+x # will return fstring, but makes an unneeded copy

x = -x.dup # fstring again, uneeded copy 

x = x.frozen? ? -+x : -x # too verbose, uneeded copy
```

Instead why not change it so `-` is deduped unconditionally? 

I would argue this is worth backporting, cause if we are making fstring optimisations now, we are going to be stuck with legacy inefficient code going forward.

An alternative may be a c-extension gem that adds #fstring to String but that just feel wrong. 

I think the documentation should say:

String uminus says: "If the string is de-duplicated, then return the string itself."

Happy to make the change it is quite simple:`(FL_TEST(str, RSTRING_FSTR)` 






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