[ruby-core:95276] [Ruby master Feature#16029] Expose fstring related APIs to C-extensions

[jean.boussier@gmail.com]

Issue #16029 has been updated by byroot (Jean Boussier).


@sam.saffron Unless I'm missing something, that's exactly what `rb_fstring_new / rb_fstring_cstr` does.

```
VALUE rb_fstring_new(const char *ptr, long len);
VALUE rb_fstring_cstr(const char *str);
```

AFAICT It does allocate an RVALUE to lookup the table, but it does it on the stack, so I think it's fine GC wise.

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Feature #16029: Expose fstring related APIs to C-extensions
https://bugs.ruby-lang.org/issues/16029#change-81952

* Author: byroot (Jean Boussier)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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As discussed with @tenderlove here: https://github.com/ruby/ruby/pull/2287#issuecomment-513865160

We'd like to update various data format parsers (JSON, MessagePack, etc) to add the possibility to deduplicate strings while parsing.

But unfortunately the `rb_fstring_*` family of functions isn't available to C-extensions, so the only available fallback is `rb_funcall(str, rb_intern("-@"))` which most parsers will likely consider too slow. So the various `rb_fstring_*` functions would need to be public.

Proposed patch: https://github.com/ruby/ruby/pull/2299



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