Issue #16150 has been updated by Eregon (Benoit Daloze).


I also tried this version which always returns the same String for a given Symbol:

```diff
diff --git a/string.c b/string.c
index 05ce0ed8d6..f306c905bc 100644
--- a/string.c
+++ b/string.c
@@ -10866,7 +10866,7 @@ sym_inspect(VALUE sym)
 VALUE
 rb_sym_to_s(VALUE sym)
 {
-    return str_new_shared(rb_cString, rb_sym2str(sym));
+    return rb_sym2str(sym);
 }
 
 
```

And that passes both test-spec and test-all, except this test which probably needs to be adapted:
```
[5/8] Test_StringCapacity#test_capacity_shared = 0.00 s            
  1) Failure:
Test_StringCapacity#test_capacity_shared [/home/eregon/code/ruby/test/-ext-/string/test_capacity.rb:17]:
<0> expected but was
<26>.
```

I'll make a PR.

----------------------------------------
Feature #16150: Add a way to request a frozen string from to_s
https://bugs.ruby-lang.org/issues/16150#change-81460

* Author: headius (Charles Nutter)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
Much of the time when a user calls to_s, they are just looking for a simple string representation to display or to interpolate into another string. In my brief exploration, the result of to_s is rarely mutated directly.

It seems that we could save a lot of objects by providing a way to explicitly request a *frozen* string.

For purposes of discussion I will call this to_frozen_string, which is a terrible name.

This would reduce string allocations dramatically when applied to many common to_s calls:

* Symbol#to_frozen_string could always return the same cached String representation. This method is *heavily* used by almost all Ruby code that intermingles Symbols and Strings.
* nil, true, false, and any other singleton values in the system could similarly cache and return the same String object.
* The strings coming from core types could also be in the fstring cache and deduplicated as a result.
* User-provided to_s implementations could opt-in to caching and returning the same frozen String object when the author knows that the result will always be the same.

A few ideas for what to call this:

* `to_fstring` or `fstring` reflects internal the "fstring" cache but is perhaps not obvious for most users.
* `to_s(frozen: true)` is clean but there will be many cases when the kwargs hash doesn't get eliminated, making matters worse.
* `def to_s(frozen = false)` would be mostly free but may not be compatible with existing to_s params (like `Integer#to_s(radix)`

This idea was inspired by @schneems's talk at RubyConf Thailand, where he showed significant overhead in ActiveRecord from Symbol#to_s allocation.



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