Toolchain is incompatible with `--experimental_sibling_repository_layout` flag

[pikinier20]

I've prepared a small repro for the issue:

https://github.com/pikinier20/hermetic_cc_toolchain_repro

Basically, the issue is described properly by the title. Trying to build the target with the flag enabled leads to error:

ERROR: /Volumes/LMP_2TB/work/bazel_hermetic_cc_toolchain_test/BUILD.bazel:1:10: Compiling hello_world.cc failed: absolute path inclusion(s) found in rule '//:hello_world':
the source file 'hello_world.cc' includes the following non-builtin files with absolute paths (if these are builtin files, make sure these paths are in your toolchain):
  '/private/var/tmp/_bazel_fzybala/096f4fcdb4de284ed2c5a33ed1889d5a/sandbox/darwin-sandbox/4/execroot/hermetic_cc_toolchain++toolchains+zig_sdk/lib/libcxx/include/iostream'
  '/private/var/tmp/_bazel_fzybala/096f4fcdb4de284ed2c5a33ed1889d5a/sandbox/darwin-sandbox/4/execroot/hermetic_cc_toolchain++toolchains+zig_sdk/lib/libcxx/include/__assert'
  '/private/var/tmp/_bazel_fzybala/096f4fcdb4de284ed2c5a33ed1889d5a/sandbox/darwin-sandbox/4/execroot/hermetic_cc_toolchain++toolchains+zig_sdk/lib/libcxx/include/__config'
  '/private/var/tmp/_bazel_fzybala/096f4fcdb4de284ed2c5a33ed1889d5a/sandbox/darwin-sandbox/4/execroot/hermetic_cc_toolchain++toolchains+zig_sdk/lib/libcxx/include/__verbose_abort'
  '/private/var/tmp/_bazel_fzybala/096f4fcdb4de284ed2c5a33ed1889d5a/sandbox/darwin-sandbox/4/execroot/hermetic_cc_toolchain++toolchains+zig_sdk/lib/libcxx/include/__availability'
  '/private/var/tmp/_bazel_fzybala/096f4fcdb4de284ed2c5a33ed1889d5a/sandbox/darwin-sandbox/4/execroot/hermetic_cc_toolchain++toolchains+zig_sdk/lib/libcxx/include/version'
  '/private/var/tmp/_bazel_fzybala/096f4fcdb4de284ed2c5a33ed1889d5a/sandbox/darwin-sandbox/4/execroot/hermetic_cc_toolchain++toolchains+zig_sdk/lib/libcxx/include/ios'
...

removing the flag fixes the problem

[Silic0nS0ldier]

This incompatibility occurs when sandboxing is enabled (the default).

The following is successful with the repro.

 bazel build //:hello_world --spawn_strategy=local

[Silic0nS0ldier]

Further investigation shows that while --spawn_strategy=local allows builds to complete, the .d files still contain absolute paths (just not rooted in the sandbox).

I spent some time trying to fix this without luck, as under --experimental_sibling_repository_layout an upward path traversal is needed (e.g. ../hermetic_cc_toolchain++toolchains+zig_config) which triggers this line in zig.
https://github.com/ziglang/zig/blob/738d2be9d6b6ef3ff3559130c05159ef53336224/src/introspect.zig#L146

Leading to the relative path being resolved relative to cwd.
https://github.com/ziglang/zig/blob/738d2be9d6b6ef3ff3559130c05159ef53336224/src/introspect.zig#L163

Despite this, the problem can be worked around within zig-wrapper.zig. In case it is useful to someone, here is the extremely rough (and brittle) prototype I cobbled together.

0001-sibling-repo-layout.patch