nix: dev-shell fixes

[rkoser-nv]

Add the packages required for prebuilt LLVM to LD_LIBRARY_PATH.

Also, switch from stdenvNoCC to a proper clang/llvm stdenv.

[CLAassistant]

All committers have signed the CLA.

[samestep]

Thanks for these fixes! I haven't tested them yet, but I did have one request in the meantime.

[rkoser-nv]

The clang/clangd interaction is upsetting and subtle. I broke it before, but it should be fixed with the latest commit.

[samestep]

Wow, this does indeed look subtle... thank you for figuring this all out!

[samestep]

@rkoser-nv since you mentioned that this PR is meant to add support for prebuilt LLVM, do you have a particular (set of) build command(s) I can run in the Nix dev shell before and after this PR to see what specifically didn't work before but does now?

For instance, just running this command doesn't seem to work:

cmake --preset default -DSLANG_SLANG_LLVM_FLAVOR=USE_SYSTEM_LLVM

That is, on both commit 671b391 and commit 5674b1b, I get the same error:

CMake Error at cmake/LLVM.cmake:41 (find_package):
  Could not find a package configuration file provided by "LLVM" (requested
  version 21.1) with any of the following names:

    LLVMConfig.cmake
    llvm-config.cmake

  Add the installation prefix of "LLVM" to CMAKE_PREFIX_PATH or set
  "LLVM_DIR" to a directory containing one of the above files.  If "LLVM"
  provides a separate development package or SDK, be sure it has been
  installed.
Call Stack (most recent call first):
  CMakeLists.txt:494 (fetch_or_build_slang_llvm)

[rkoser-nv]

This fixes the downloaded LLVM binaries used by slang-test. Without the LD_LIBRARY_PATH changes, slang-test fails to load libslang-llvm.so so all the tests that rely on FileCheck are ignored:

slang/tools/slang-test/slang-test-main.cpp

Lines 798 to 805 in 671b391

	IFileCheck* fc = context.getFileCheck();
	if (!fc)
	{
	// Ignore if FileCheck is not available.
	// We could report an error, but our ARM64 CI doesn't have FileCheck yet.
	testReporter.message(TestMessageType::Info, "FileCheck is not available");
	return TestResult::Ignored;
	}

[samestep]

@rkoser-nv right yeah, I understand that: that was why I opened #8028 and #8031 in August, at which point in time I was having to follow the Building with a Local Build of slang-llvm instructions from CONTRIBUTING.md; so I know that slang-test already successfully runs the FileCheck tests if I use those build commands (unless something's regressed, of course):

external/build-llvm.sh --source-dir build/slang-llvm_src --install-prefix build/slang-llvm_install
cmake --preset default --fresh -DSLANG_SLANG_LLVM_FLAVOR=USE_SYSTEM_LLVM -DLLVM_DIR=build/slang-llvm_install/lib/cmake/llvm -DClang_DIR=build/slang-llvm_install/lib/cmake/clang
cmake --build --preset release

What I'm asking is: does this PR now enable me to successfully run the FileCheck tests with a different set of commands? If so, which ones? (Since the one I tried in my message above didn't work.)

[rkoser-nv]

Yes, the following should work:

cmake -S . -B build/  # I imagine this is equivalent to cmake --preset default but haven't confirmed that
cmake --build -j$(nproc) ./build

the resulting slang-test should be able to use filecheck in a nix dev-shell after this PR.

[samestep]

Got it, thanks! In the future it would be cool if one could just use -DSLANG_SLANG_LLVM_FLAVOR=USE_SYSTEM_LLVM with LLVM from Nix instead of having to download a prebuilt slang-llvm binary, but obviously that's beyond the scope of this PR.

Just now I ran these commands to see the difference produced by this PR:

cmake --preset default
cmake --build --preset release
build/Release/bin/slang-test

On commit dd4ba6e (the base of this PR branch), I got this output at the end:

===
99% of tests passed (1828/1829), 4396 tests ignored
===

1 failing tests:
---
tests/hlsl-intrinsic/scalar-int64.slang (cpu)
---

And on commit 5674b1b (the tip of this PR branch), I got this output at the end instead:

===
100% of tests passed (4264/4264), 2641 tests ignored
===

I'm guessing those 2436 extra passing tests / 1755 fewer ignored tests mean that slang-llvm is working properly using this PR.

[expipiplus1]

Got it, thanks! In the future it would be cool if one could just use -DSLANG_SLANG_LLVM_FLAVOR=USE_SYSTEM_LLVM with LLVM from Nix instead of having to download a prebuilt slang-llvm binary, but obviously that's beyond the scope of this PR

this is what I've been doing for a while now! I should really integrate my version...

[samestep]

@expipiplus1 that would be amazing! It would definitely have saved me and @rkoser-nv from working on these inferior solutions 😅