Mis-use of -isystem in rust 1.91.0's LLVM with siphash header?

Summary

I'm cross-building the rust compiler with the bundled LLVM, and part of that includes using the target's cross-compiler to compile LLVM. I am observing the same symptom as observed in #143508.

Command used

With lots of environment tweaking, in this case also for cross compilation:

python3.12 ./x.py dist -j 40

Expected behaviour

I expected the build to succeed.

Actual behaviour

/usr/pkgsrc/wip/rust191/work/rustc-1.91.0-src/src/llvm-project/llvm/lib/Support/SipHash.cpp:15:10: fatal error: siphash/SipHash.h: No such file or directory
 #include "siphash/SipHash.h"
          ^~~~~~~~~~~~~~~~~~~
compilation terminated.

Bootstrap configuration (bootstrap.toml)

profile = 'dist'
[llvm]
release-debuginfo = false
static-libstdcpp = false
ninja = false
targets = 'ARM;X86'
[gcc]
[build]
host = ['armv7-unknown-netbsd-eabihf']
target = ['armv7-unknown-netbsd-eabihf']
cargo = '/usr/pkgsrc/wip/rust191/work/rust-bootstrap/bin/cargo'
rustc = '/usr/pkgsrc/wip/rust191/work/rust-bootstrap/bin/rustc'
docs = false
compiler-docs = false
python = '/usr/pkg/bin/python3.12'
vendor = true
extended = true
cargo-native-static = true
configure-args = ['--prefix=/usr/pkg', '--mandir=/usr/pkg/man', '--sysconfdir=/usr/pkg/etc', '--python=/usr/pkg/bin/python3.12', '--release-channel=stable', '--local-rust-root=/usr/pkgsrc/wip/rust191/work/rust-bootstrap', '--enable-extended', '--enable-rpath', '--disable-codegen-tests', '--disable-compiler-docs', '--disable-llvm-static-stdcpp', '--disable-ninja', '--dist-compression-formats=xz', '--set', 'dist.vendor=false', '--host=armv7-unknown-netbsd-eabihf', '--target=armv7-unknown-netbsd-eabihf', '--set=target.armv7-unknown-netbsd-eabihf.cc=/usr/pkgsrc/wip/rust191/work/scripts/gcc-wrap', '--set=target.armv7-unknown-netbsd-eabihf.cxx=/usr/pkgsrc/wip/rust191/work/scripts/c++-wrap', '--set=target.armv7-unknown-netbsd-eabihf.linker=/usr/pkgsrc/wip/rust191/work/scripts/gcc-wrap', '--set=target.armv7-unknown-netbsd-eabihf.ar=/u/evbarm-armv7hf/tools/bin/armv7--netbsdelf-eabihf-ar', '--enable-vendor', '--disable-debug', '--disable-debug-assertions', '--disable-llvm-release-debuginfo', '--debuginfo-level=0', '--debuginfo-level-rustc=0', '--debuginfo-level-std=0', '--debuginfo-level-tools=0', '--debuginfo-level-tests=0', '--set', 'llvm.targets=ARM;X86', '--enable-cargo-native-static', '--disable-docs']
[install]
prefix = '/usr/pkg'
sysconfdir = '/usr/pkg/etc'
mandir = '/usr/pkg/man'
[rust]
debug = false
debug-assertions = false
debuginfo-level = 0
debuginfo-level-rustc = 0
debuginfo-level-std = 0
debuginfo-level-tools = 0
debuginfo-level-tests = 0
channel = 'stable'
rpath = true
codegen-tests = false
[dist]
compression-formats = ['xz']
vendor = false
[target.x86_64-unknown-netbsd]
[target.armv7-unknown-netbsd-eabihf]
cc = '/usr/pkgsrc/wip/rust191/work/scripts/gcc-wrap'
cxx = '/usr/pkgsrc/wip/rust191/work/scripts/c++-wrap'
ar = '/u/evbarm-armv7hf/tools/bin/armv7--netbsdelf-eabihf-ar'
linker = '/usr/pkgsrc/wip/rust191/work/scripts/gcc-wrap'

Operating system

Host: NetBSD/amd64 10.1

HEAD

$ echo `cat work/rustc-1.91.0-src/git-commit-hash`
f8297e351a40c1439a467bbbb6879088047f50b3

Additional context

This parituclar problem is an interaction between the new LLVM in rust 1.91.0 and the cross compilation rig I use to cross-build for the various NetBSD targets (e.g. armv7hf shown above).

In this setup, I need to use --sysroot=/... to point to the targets "system-supplied headers". Likewise, -isystem /some/path/ is rewritten to -isystem =/some/path. This means that when the LLVM build uses -isystem to point to a location in the source distribution, the intended header file is not found. I think that this is mis-use of the -isystem compiler directive, and that simply -I should be used instead. To that end I have added the following gross hack to the gcc-wrap script I'm using to transform the compiler arguments as required for cross compilation, ref.:

castor: {37} diff -u ../rust190/files/gcc-wrap files/gcc-wrap
--- ../rust190/files/gcc-wrap   2025-09-20 07:18:41.413625088 +0000
+++ files/gcc-wrap      2025-11-03 13:54:59.472143568 +0000
@@ -90,7 +90,15 @@
 # (I thought this wasn't reqired, but apparently it is...)
                -isystem)
                        shift
-                       args="$args -isystem =$1"
+                       case "$1" in
+                       # Apparent mis-use of -isystem:
+                       */siphash/*)
+                               args="$args -I $1"
+                               ;;
+                       *)
+                               args="$args -isystem =$1"
+                               ;;
+                       esac
                        ;;
 # Also doctor -I directives of known paths and
 # redirect them to the --sysroot.
castor: {38} 

The failing compiler invocation ended up as

/usr/pkgsrc/wip/rust191/work/.gcc/bin/g++ -fstack-protector-strong -D_FORTIFY_SOURCE=2 -Wl,-zrelro -fPIC -DEXPERIMENTAL_KEY_INSTRUCTIONS -DGTEST_HAS_RTTI=0 -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/usr/pkgsrc/wip/rust191/work/rustc-1.91.0-src/build/x86_64-unknown-netbsd/llvm/build/lib/Support -I/usr/pkgsrc/wip/rust191/work/rustc-1.91.0-src/src/llvm-project/llvm/lib/Support -I/usr/pkgsrc/wip/rust191/work/rustc-1.91.0-src/build/x86_64-unknown-netbsd/llvm/build/include -I/usr/pkgsrc/wip/rust191/work/rustc-1.91.0-src/src/llvm-project/llvm/include -isystem /usr/pkgsrc/wip/rust191/work/rustc-1.91.0-src/src/llvm-project/llvm/../third-party/siphash/include -O2 -ffunction-sections -fdata-sections -fPIC -m64 -O2 -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=c++17 -fno-exceptions -funwind-tables -fno-rtti -MD -MT lib/Support/CMakeFiles/LLVMSupport.dir/SipHash.cpp.o -MF CMakeFiles/LLVMSupport.dir/SipHash.cpp.o.d -o CMakeFiles/LLVMSupport.dir/SipHash.cpp.o -c /usr/pkgsrc/wip/rust191/work/rustc-1.91.0-src/src/llvm-project/llvm/lib/Support/SipHash.cpp -I/usr/pkgsrc/wip/rust191/work/.buildlink/include

With the above modification, the cross-build of rust on NetBSD/amd64 for NetBSD/earmv7hf succeeds.

Build Log

Well, I think the essence of the problem is captured above.

If really needed to narrow down the problem, I can paste the complete build log here later.

Start with the failing invocation and src/llvm-project/llvm/lib/Support/SipHash.cpp, then inspect how the bundled LLVM build supplies the path src/llvm-project/llvm/../third-party/siphash/include through -isystem. Reproduce with python3.12 ./x.py dist using the shown cross-compilation configuration and verify that the NetBSD/armv7hf build succeeds without the gcc-wrap workaround.