[ruby-core:82621] [Ruby trunk Bug#13857] frozen string literal: can freeze same string into two unique frozen strings

Issue #13857 has been updated by chucke (Tiago Cardoso).


Please don't get me wrong, I'm not arguing that the spec for the feature is vague.

I understood that the introduction of the feature was to reduce memory consumption in template generation (like erb templates), and to avoid those `CONTENT_LENGTH = "Content-Length".freeze` assignments seen a bit everywhere from ruby web servers to rack. In most of these libs (here's [rack's example](https://github.com/rack/rack/blob/911c4fe15e3e57d44ac891c0cbabbf44bdf71201/lib/rack/utils.rb#L445)), there's an header hash abstraction which applies downcase operation to the keys, and then (optionally) freezes. 

Point being, at any given time, we might have two strings in memory (ex: `"Content-Length"`), both frozen, one of them a literal. 

----------------------------------------
Bug #13857: frozen string literal: can freeze same string into two unique frozen strings
https://bugs.ruby-lang.org/issues/13857#change-66447

* 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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