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


The contract says "possibly pre-existing copy of it." ... where possibly means "if the string is not frozen" 

So the contract now is "return a de-duplicated string if the string is not frozen" which is a bit of a crazy contract if you ask me, instead the contract should simply be "return a de-duplicated string always" 

What I am asking for here is change str_uminus from: 

"sometimes return a de-duplicated string" 

To 

"always return a de-duplicated string" 

I feel the change is not risky at all and will not break anyone in practice, I also think it should be backported to 2.5 cause `-+x` is an ugly and horrible pattern for us to be promoting. 



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Feature #14478: String #uminus should de-dupe unconditionally
https://bugs.ruby-lang.org/issues/14478#change-70367

* 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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