Rustc depending on libstdc++ makes build fail on nixos

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1

i have successfully built rustc with ./x build --stage 1, but it fails dynamic linking.

$ ldd build/x86_64-unknown-linux-gnu/stage1/bin/rustc
	linux-vdso.so.1 (0x00007ffd38ba5000)
	librustc_driver-463fc6f98380ceb9.so => /home/binarycat/src/rs/rust/build/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustc_driver-463fc6f98380ceb9.so (0x00007fd313a00000)
	libstd-d9ec9c88a6240622.so => /home/binarycat/src/rs/rust/build/x86_64-unknown-linux-gnu/stage1/bin/../lib/libstd-d9ec9c88a6240622.so (0x00007fd31389a000)
	libc.so.6 => /nix/store/p9ysh5rk109gyjj3cn6jr54znvvlahfl-glibc-2.38-66/lib/libc.so.6 (0x00007fd3136b1000)
	libLLVM.so.18.1-rust-1.81.0-nightly => /home/binarycat/src/rs/rust/build/x86_64-unknown-linux-gnu/stage1/bin/../lib/../lib/libLLVM.so.18.1-rust-1.81.0-nightly (0x00007fd30b600000)
	libstdc++.so.6 => not found
	libgcc_s.so.1 => /nix/store/00r99fxv6l92wk0k6wd60klhs7fkmmqj-xgcc-12.3.0-libgcc/lib/libgcc_s.so.1 (0x00007fd318813000)
	libm.so.6 => /nix/store/p9ysh5rk109gyjj3cn6jr54znvvlahfl-glibc-2.38-66/lib/libm.so.6 (0x00007fd3135d1000)
	/nix/store/p9ysh5rk109gyjj3cn6jr54znvvlahfl-glibc-2.38-66/lib/ld-linux-x86-64.so.2 => /nix/store/p9ysh5rk109gyjj3cn6jr54znvvlahfl-glibc-2.38-66/lib64/ld-linux-x86-64.so.2 (0x00007fd31883c000)
	librt.so.1 => /nix/store/p9ysh5rk109gyjj3cn6jr54znvvlahfl-glibc-2.38-66/lib/librt.so.1 (0x00007fd31880e000)
	libdl.so.2 => /nix/store/p9ysh5rk109gyjj3cn6jr54znvvlahfl-glibc-2.38-66/lib/libdl.so.2 (0x00007fd318809000)
	libpthread.so.0 => /nix/store/p9ysh5rk109gyjj3cn6jr54znvvlahfl-glibc-2.38-66/lib/libpthread.so.0 (0x00007fd318802000)
	libz.so.1 => /nix/store/drnjs8wypqqc0la2i4xdy38rs0c3fq2a-rust-stage0-dependencies/lib/libz.so.1 (0x00007fd3187e4000)

this seems like a simple issue of missing dependencies in a nix derivation somewhere, but i can't find any mention of the dependancies that are needed.

there is some mention of "the libstdc++ shipped with llvm", but that doesn't make sense to me, since libLLVM doesn't link to libstdc++ according to ldd

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also, every source i can find says that rustc doesn't directly depend on libstdc++, so this honestly feels like a bug in the build system

3

related bug report

4

LLVM certainly does use libstdc++, but the default Rust configuration will link that statically -- you can decide whether that makes sense for you. There are further C++ wrappers for LLVM on the Rust side, but I think those are supposed to respect the static setting too.

Edit: actually the example config says that the default is [llvm] static-libstdcpp = false -- but if you use download-ci-llvm then it will use static libstdc++. I suspect you're getting a mix that way, static from the download and dynamic from Rust's local C++ bindings.

5

any idea how to fix it?

6

Probably either add libstdc++.so.6 to your nix config, or try setting [llvm] static-libstdcpp = true.

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The best chance for a fix would be to link + repost this topic over at https://discourse.nixos.org/ to find someone experienced with how they set up the derivation, since it would fix the problem for everyone

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i am not building rust via a nix derivation. i am building rust via x.py. this worked fine until recently, as rustc did not link to libstdc++ directly, but it does now.

getting libstdc++ from nix is trivial, the hard part is convincing the rust build system to patch rustc to use this library. stage0 automatically gets patched from .nix-deps, but stage1 does not. stage1 expects all its dependancies to be in stage1/lib, but libstdc++ does not get put there.

also:

setting static_libstdcpp is incompatible with download-ci-llvm.

1 Like
9

setting static_libstdcpp is incompatible with download-ci-llvm.

Hmm, that's guarded here:

but I wonder if that's missing the effect that it also has on compiler/rustc_llvm bindings. That is, you probably do want to match whatever CI has, but I don't see anything trying to figure that out.

10

yeah, that looks to be the problem. commenting out that line seems to do the trick.

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