[cfe-users] Fail to build C++ module with Bazel

[Wynand Pieterse via cfe-users]

Hi all, trust all is well!

I'm trying to build C++ modules using CLang and Bazel. I've created a
special rule to compile modules, and inside the rule I've created two
actions for the seperate steps of compiling the module to a precompiled
file, and the second action builds the LLVM bitcode from the pre-compiled
file.

This is where I'm having trouble. The precompiled file builds correctly,
but the second step, building the bitcode from the precompiled header fails
with the following error:

fatal error: malformed or corrupted AST file: 'could not find file
'/home/wpieterse/.cache/bazel/_bazel_wpieterse/097a98d4b8ea7d3c9ae2315ed87ce0f0/sandbox/linux-sandbox/1/execroot/bazel_llvm
_stuff/cpp/add.cppm' referenced by AST file
'bazel-out/k8-fastbuild/bin/cpp/add.pcm''

I presume this has something to do with Bazel creating a sandbox for each
action. This is why the file cannot be found.

Also, these are the parameters sent to CLang 9.0 when compiling:

*Compiling the precompiled header:*
./clang -std=c++2a -fmodules-ts --precompile -o add.pcm add.cppm

*Compiling the bitcode:*
./clang -std=c++2a -fmodules-ts -S -emit-llvm -c add.pcm -o add.bc

I've tried searching for a way to use relative paths, but to no avail. The
file is there, inside the Bazel execution root, but not in the sandbox.

Is there a switch on CLang that can be used to use relative paths? Am I
using the correct command line switches.
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