Add best effort macOS reproducibility flags

[keith]

Without -ffile-compilation-dir=. (or a similar flag), debug info in
.o files contains the absolute path to the compile time execroot.
Without -Wl,-oso_prefix,. final binaries contain N_OSO stabs that
contain absolute paths to the files link time execroot.

This still leaves 1 known hole that is the absolute path to Xcode is
embedded in debug info, but that doesn't break debuggability like these
2 issues do, it just makes the binaries non hermetic. Fixing that one
requires a compiler wrapper.

All of these issues are fixed in apple_support, but for folks not using
that toolchain these are pretty low risk to add here. The only issue is
if people have very old versions of Xcode / the Xcode command line tools
these flags won't exist.

More info about these paths in https://www.smileykeith.com/2025/09/21/understanding-apple-debug-info/

[keith]

• oso_prefix was added in Xcode 11.3.1 in january 2020 (using . as the value wasn't supported until 13.0, but it would still accept it, it would just not work)
• ffile-compilation-dir was added in Xcode 13.3 in march 2022

i think we're past those enough to not worry about that anymore

[armandomontanez]

Nice! Would be nice to do similar with Linux with the caveat that toolchains are much more diverse in that world.

[armandomontanez]

Might be worth discussing toolchain configuration compatibility expectations at the next sync. Added to the agenda.

[keith]

that is actually somewhat solved today by this:

rules_cc/cc/private/link/finalize_link_action.bzl

Lines 363 to 366 in 1a5d8a3

	if "requires_darwin" not in execution_info:
	# This prevents gcc from writing the unpredictable (and often irrelevant)
	# value of getcwd() into the debug info.
	env = env | {"PWD": "/proc/self/cwd"}

[keith]

as discussed in the meeting today I updated this to dynamically check if this feature is supported

[armandomontanez]

Thanks!