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


The plan is Rails 6 users can update to Rails 6.0.1 and Rails 5.2 users can update to Rails 5.2.4 (assuming 5.2.4 will be released before 2.7, we need to confirm with Rails devs).
That should be easy and recommended for security and other bug fixes anyway.

I agree it's not ideal, but I also feel it's really not the biggest problem for Ruby 2.7 in production, and we already have a good plan.
The fact that people trying Ruby 2.7 will get many keyword argument warnings (which also affects performance) seems far more problematic to me (https://bugs.ruby-lang.org/issues/16289#note-5).
And keyword arguments warnings fixes are currently not backported (I guess because they require so many changes).

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Feature #16150: Add a way to request a frozen string from to_s
https://bugs.ruby-lang.org/issues/16150#change-82442

* Author: headius (Charles Nutter)
* Status: Closed
* Priority: Normal
* Assignee: Eregon (Benoit Daloze)
* Target version: 
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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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