Issue #16150 has been updated by Dan0042 (Daniel DeLorme).


> For example, String#to_s returns itself and mutating sth.to_s is therefore not safe if sth can be a String.

`String#to_s` is of course a special case. But that means if a string is currently not frozen, `to_frozen_string` would have to do `dup.freeze`... doesn't that ruin the entire point of this proposal, which is about efficiency? It would mean "a #{str}" creates a duplicate of str because interpolation uses the supposedly efficient `to_frozen_string`. So maybe this proposal really should be called `to_efficient_string`, which doesn't guarantee a frozen string but rather attempts to minimize allocations. In that context, `to_s!` makes even more sense because you don't know what kind of string you'll get.

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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-81456

* Author: headius (Charles Nutter)
* Status: Open
* Priority: Normal
* Assignee: 
* 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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