[ruby-core:82705] [Ruby trunk Bug#13857] frozen string literal: can freeze same string into two unique frozen strings

Issue #13857 has been updated by kernigh (George Koehler).


I expect String#freeze to return the receiver. For example,

```ruby
str = "Content-Length".downcase
str.freeze        # freezes and returns str
str.concat(": ")  # raises RuntimeError: can't modify frozen String
```

I know that str.freeze returns str, so I ignore the return value, and continue using str. Tiago Cardoso proposes that str.freeze would return a deduplicated string, perhaps not str. But my program would ignore the deduplicated string and use str, so the deduplication would be useless.

----------------------------------------
Bug #13857: frozen string literal: can freeze same string into two unique frozen strings
https://bugs.ruby-lang.org/issues/13857#change-66541

* Author: chucke (Tiago Cardoso)
* Status: Rejected
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 2.3.4, 2.4.1
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
----------------------------------------
Running an interpreter with `--enable-frozen-string-literal` on, I get the following:

```ruby
> "bang".object_id #=> 70303434940940  GOOD!
> "bang".object_id #=> 70303434940940  GOOD!
> "bang".object_id #=> 70303434940940  GOOD!
> c = "bang" 
> c.object_id #=> 70303434940940 GOOD!
> c.downcase #=> "bang"
> c.downcase.object_id #=> 70303430619560  SO SO!
> c.downcase.freeze.object_id #=> 70303430601780  BAD!
```

This ticket concerns the last two examples. In the case of performing an operation on the string, it makes sense to return a new string, even if the result is the same. However, I think that the last one could be done differently, in that the frozen result of the downcased value should be the original literal. 

I didn't see yet how the frozen string literals are implemented, so this might be dependent on it, but I think that this misses a few optimization use cases. One notable example is keeping a headers hash from an http library. `net/http` keeps a version of the headers hash with the keys downcased, only to capitalize them on send. Something like this:

```ruby
request["Content-Type"] = "text/html" #=> key stored in request will be "content-type"
```

will create more allocations than expected. 



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