From: sam.saffron@...
Date: 2018-02-15T02:04:40+00:00
Subject: [ruby-core:85571] [Ruby trunk Feature#14478] String #uminus should de-dupe unconditionally

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


> -@ should be one such exposure 

Oh this is already the case, see the source:

```
/*
 * call-seq:
 *   -str  -> str (frozen)
 *
 * If the string is frozen, then return the string itself.
 *
 * If the string is not frozen, return a frozen, possibly pre-existing
 * copy of it.
 */
static VALUE
str_uminus(VALUE str)
{
    if (OBJ_FROZEN(str)) {
	return str;
    }
    else {
	return rb_fstring(str);
    }
}

```

I am proposing to change this to:

```
/*
 * call-seq:
 *   -str  -> str (frozen)
 *
 * If the string is de-duplicated, then return the string itself.
 *
 * If the string is not return a frozen, possibly pre-existing
 * copy of it.
 */
static VALUE
str_uminus(VALUE str)
{
    if (FL_TEST(str, RSTRING_FSTR)) {
	return str;
    }
    else {
	return rb_fstring(str);
    }
}
```

----------------------------------------
Feature #14478: String #uminus should de-dupe unconditionally
https://bugs.ruby-lang.org/issues/14478#change-70365

* 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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