Issue #16150 has been updated by byroot (Jean Boussier).


> Do you want to do the PR for nil/true/false as well then?

I tried but I'm quite unsure how to do it, so I don't mind if you handle it.

On a side note I'm currently running our test suite with the following monkey patch:

```ruby
module FreezeName
  def name
    name = super
    name = -name if name
    name
  end
end
Module.singleton_class.prepend(FreezeName)

module FreezeToS
  def to_s
    -super
  end
end

Symbol.prepend(FreezeToS)
NilClass.prepend(FreezeToS)
TrueClass.prepend(FreezeToS)
FalseClass.prepend(FreezeToS)
```

This way I should be able to identify some breakage across a large amount of gems.

----------------------------------------
Feature #16150: Add a way to request a frozen string from to_s
https://bugs.ruby-lang.org/issues/16150#change-81738

* Author: headius (Charles Nutter)
* Status: Assigned
* Priority: Normal
* Assignee: Eregon (Benoit Daloze)
* 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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