bazel: Don't propagate flags to exec config

This will improve cache hit rates, and also allow us to use
@bazel_tool//tools/cpp:link_extra_libs for pw_assert and pw_log
implementation targets (without these flags this does not work because
you invariably end up with circular dependencies).

Requires Bazel 8.

Bug: 234877642
Bug: 315871648
Change-Id: I503c501329638b666c54b8aedd0a294b8563b907
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/220812
Reviewed-by: Armando Montanez <[email protected]>
Commit-Queue: Ted Pudlik <[email protected]>
Lint: Lint 🤖 <[email protected]>

.bazelrc

@@ -37,6 +37,12 @@ common --incompatible_default_to_explicit_init_py
 # Required for new toolchain resolution API.
 build --incompatible_enable_cc_toolchain_resolution
 
+# Don't propagate flags or defines to the exec config. This will become the
+# default one day (https://github.com/bazelbuild/bazel/issues/22457) and will
+# improve cache hit rates between builds targeting different platforms.
+common --experimental_exclude_defines_from_exec_config
+common --experimental_exclude_starlark_flags_from_exec_config
+
 # Skip building or testing C++20 targets by default
 # TODO: b/340568834 Remove when a proper solution is available.
 build --build_tag_filters=-requires_cxx_20

docs/build_system.rst

@@ -706,8 +706,10 @@ This can be added in a couple ways:
  <https://bazel.build/reference/be/c-cpp#cc_binary.link_extra_lib>`_. Set
  the ``@bazel_tools//tools/cpp:link_extra_libs`` label flag to point to
  ``@pigweed//pw_build:default_link_extra_lib``, probably using `bazelrc
- <https://bazel.build/run/bazelrc>`_. This is only supported in Bazel 7.0.0
- or newer.
+ <https://bazel.build/run/bazelrc>`_. This is only likely to work if you
+ specify the ``--experimental_exclude_starlark_flags_from_exec_config``
+ flag, available in Bazel 8 and newer; see `bazelbuild/bazel#22457
+ <https://github.com/bazelbuild/bazel/issues/22457>`__.
 
  The con is that these libraries are linked into *all* C++ binaries that are
  part of your project's build, including ones that have no dependencies on