Oh, https://reviews.llvm.org/D91359 adds an include of another internal header to refstring.h, so if we were to revert we'd have to copy that too. That doesn't seem great.

So maybe using #include "src/include/refstring.h" and adding the include search path in the build system is the least bad alternative? But the cyclic dep feels yucky too.

See also LIBCXXABI_LIBCXX_INCLUDES for a cmake toggle for the public includes.

Libc++ and libc++abi have required being built in a monorepo-like layout for a while now.

We have a strong desire to share more code between libc++ and libc++abi (in particular some CMake code), so I don't see us going back to allowing libc++/abi being built from arbitrary locations. Can you tell me a bit more about your setup?

If you're currently storing libc++ and libc++abi in t_p/libcxx/src and t_p/libcxxabi/src, one option would be to instead have something like t_p/llvm/src/{libcxx,libcxxabi}?

Libc++ and libc++abi have required being built in a monorepo-like layout for
a while now.

This is the only thing requiring this at the source level.

LIBCXXABI_STANDALONE_BUILD still exists, too.

We have a strong desire to share more code between libc++ and libc++abi (in
particular some CMake code), so I don't see us going back to allowing
libc++/abi being built from arbitrary locations. Can you tell me a bit more
about your setup?

Sure! We don't want to deps in all of llvm into our build, but only the runtime bits (actually, only libcxx, libcxxabi, and libunwind). So we have git mirrors of those subtrees. We then deps in each of these subtrees at t_p/depname/src and have a build file for each at t_p/depname/buildfile.

If you're currently storing libc++ and libc++abi in t_p/libcxx/src and
t_p/libcxxabi/src, one option would be to instead have something like
t_p/llvm/src/{libcxx,libcxxabi}?

That'd mean we can't independently roll libcxx, libcxxabi, and libunwind.

...would something like the following work for you? That keeps things basically the same upstream and makes things easier for us:

$ git diff
diff --git a/libcxxabi/src/CMakeLists.txt b/libcxxabi/src/CMakeLists.txt
index ff1e45705dca..8eca5185e0c0 100644
--- a/libcxxabi/src/CMakeLists.txt
+++ b/libcxxabi/src/CMakeLists.txt
@@ -57,6 +57,9 @@ endif()

include_directories("${LIBCXXABI_LIBCXX_INCLUDES}")

+# stdlib_stdexcept.cpp depends on libc++ internals.
+include_directories("${LIBCXXABI_LIBCXX_PATH}/src/include")
+
if (LIBCXXABI_HAS_CXA_THREAD_ATEXIT_IMPL)
add_definitions(-DHAVE___CXA_THREAD_ATEXIT_IMPL)
endif()
diff --git a/libcxxabi/src/stdlib_stdexcept.cpp b/libcxxabi/src/stdlib_stdexcept.cpp
index 4a464e48893d..66e6f14561ec 100644
--- a/libcxxabi/src/stdlib_stdexcept.cpp
+++ b/libcxxabi/src/stdlib_stdexcept.cpp
@@ -6,7 +6,6 @@
//
//===----------------------------------------------------------------------===//

-#include "../../libcxx/src/include/refstring.h"
#include "stdexcept"
#include "new"
#include
@@ -14,6 +13,9 @@
#include
#include

+// This is a file form libc++.
+#include "refstring.h"
+
static_assert(sizeof(std::__libcpp_refstring) == sizeof(const char *), "");

namespace std // purposefully not using versioning namespace

Uploaded basically that diff to https://reviews.llvm.org/D97379