This guard was removed in https://github.com/ruby/ruby/pull/13497
on the justification that some GC may need to be notified even for
immediate.

But the two currently available GCs don't, and there are plenty
of assumtions GCs don't everywhere, notably in YJIT and ZJIT.

This optimization is also not so micro (but not huge either).
I routinely see 1-2% wasted there on micro-benchmarks.

So perhaps if in the future we actually need this, it might make
sense to introduce a way for GCs to declare that as an option,
but in the meantime it's extra overhead with little gain.

Followup changes in https://github.com/ruby/ruby/pull/14470 /
03c86b053197f3cd6bece1925e634c1d74d196d0

Similar to f3206cc79bec2fd852e81ec56de59f0a67ab32b7 but for TypedData.

It's quite common for TypedData objects to have a mix of reference in
their struct and some ivars.
Since we do happen to have 8B free in the RtypedData struct, we could
use it to keep a direct reference to the IMEMO/fields saving having
to synchronize the VM and lookup the `gen_fields_tbl` on every ivar
access.

For old school Data classes however, we don't have free space, but
this API is soft-deprecated and no longer very common.

Some GC implementations want to always know when an object is written to,
even if the written value is a special constant. Checking special constants
in rb_obj_written was a micro-optimization that made assumptions about
the GC implementation.

`struct RTypedData` was changed significantly in https://github.com/ruby/ruby/pull/13190
which breaks many extensions.

Bumping the ABI version might save some people from needlessly
investigating crashes.

* Use FL_USER0 for ELTS_SHARED

This makes space in RString for two bits for chilled strings.

* Mark strings returned by `Symbol#to_s` as chilled

[Feature #20350]

`STR_CHILLED` now spans on two user flags. If one bit is set it
marks a chilled string literal, if it's the other it marks a
`Symbol#to_s` chilled string.

Since it's not possible, and doesn't make much sense to include
debug info when `--debug-frozen-string-literal` is set, we can't
include allocation source, but we can safely include the symbol
name in the warning message, making it much easier to find the source
of the issue.

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>

---------

Co-authored-by: Étienne Barrié <etienne.barrie@gmail.com>
Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

It is for dynamically loading, useless for statically linked objects.

A few extension libraries, to hide all symbols except for necessary to
load, hardcode the symbols to be exported in symbol list files for
linker without even checking by `have_func`.  As a workaround for such
libraries, retain `ruby_abi_version` symbol always even in released
versions for now.