[ruby-core:93727] [Ruby master Feature#15940] Coerce symbols internal fstrings in UTF8 rather than ASCII to better share memory with string literals

Issue #15940 has been updated by byroot (Jean Boussier).


Just to extend your example:

```ruby
p "foo".encoding     # => "euc-jp"
p :foo.to_s.encoding # => "us-ascii"
p :"注意書".to_s.encoding # => "euc-jp"

#new behavior
p "foo".encoding     # => "euc-jp"
p :foo.to_s.encoding # => "utf-8"
p :"注意書".to_s.encoding # => "euc-jp"
```

If the symbol is not pure ASCII, the current behavior doesn't change.

----------------------------------------
Feature #15940: Coerce symbols internal fstrings in UTF8 rather than ASCII to better share memory with string literals
https://bugs.ruby-lang.org/issues/15940#change-79359

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
Patch: https://github.com/ruby/ruby/pull/2242

It's not uncommon for symbols to have literal string counterparts, e.g.

```ruby
class User
  attr_accessor :name

  def as_json
    { 'name' => name }
  end
end
```

Since the default source encoding is UTF-8, and that symbols coerce their internal fstring to ASCII when possible, the above snippet will actually keep two instances of `"name"` in the fstring registry. One in ASCII, the other in UTF-8.

Considering that UTF-8 is a strict superset of ASCII, storing the symbols fstrings as UTF-8 instead makes no significant difference, but allows in most cases to reuse the equivalent string literals.

The only notable behavioral change is `Symbol#to_s`.

Previously `:name.to_s.encoding` would be `#<Encoding:US-ASCII>`.
After this patch it's `#<Encoding:UTF-8>`. I can't foresee any significant compatibility impact of this change on existing code.

However, there are several ruby specs asserting this behavior, but I don't know if they can be changed or not: https://github.com/ruby/spec/commit/a73a1c11f13590dccb975ba4348a04423c009453

If this specification is impossible to change, then we could consider changing the encoding of the String returned by `Symbol#to_s`, e.g in ruby pseudo code:

```ruby
def to_s
  str = fstr.dup
  str.force_encoding(Encoding::ASCII) if str.ascii_only?
  str
end
```





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