[ruby-core:119571] [Ruby master Feature#20792] String#forcible_encoding?

Issue #20792 has been updated by Eregon (Benoit Daloze).


Right.
But if it is valid in that encoding, wouldn't you always or almost always then want the String (or a copy of it) in that encoding?
If you do, `.with_encoding` would be more efficient as it keeps the computed coderange.
If you don't then indeed just a predicate would avoid the String instance allocation (Strings are copy-on-write, so it's just one object allocation, string bytes are not copied).

Do you have an example where you wouldn't want a String in that encoding?
The [linked example](https://github.com/ruby/prism/blob/d6e9b8de36b4d18debfe36e4545116539964ceeb/lib/prism/parse_result.rb#L15-L30) needs a UTF-8 String on line 19. So `with_encoding` seems a perfect fit there and more efficient than the predicate (1 vs 2 coderange scans).
The code also has to explicitly workaround `force_encoding` being inplace (a common inconvenience with `force_encoding`) on line 27, which `with_encoding` addresses.

----------------------------------------
Feature #20792: String#forcible_encoding?
https://bugs.ruby-lang.org/issues/20792#change-110185

* Author: kddnewton (Kevin Newton)
* Status: Open
----------------------------------------
I would like to add a method to String called `forcible_encoding?(encoding)`. This would return true or false depending on whether the receiver can be forced into the given encoding without breaking the string. It would effectively be an alias for:

```ruby
def forcible_encoding?(enc)
  original = encoding
  result = force_encoding(enc).valid_encoding?
  force_encoding(original)
  result
end
```

I would like this method because there are extremely rare but possible circumstances where source files are marked as binary but contain UTF-8-encoded characters. In that case I would like to check if it's possible to cleanly force UTF-8 before actually doing it. The code I'm trying to replace is here: https://github.com/ruby/prism/blob/d6e9b8de36b4d18debfe36e4545116539964ceeb/lib/prism/parse_result.rb#L15-L30.

The pull request for the code is here: https://github.com/ruby/ruby/pull/11851.



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