rustc: fix building with llvm

pkgs/development/compilers/rust/rustc.nix

@@ -1,6 +1,6 @@
 { lib, stdenv, removeReferencesTo, pkgsBuildBuild, pkgsBuildHost, pkgsBuildTarget, targetPackages
 , llvmShared, llvmSharedForBuild, llvmSharedForHost, llvmSharedForTarget, llvmPackages
-, fetchurl, file, python3
+, runCommandLocal, fetchurl, file, python3
 , darwin, cargo, cmake, rustc, rustfmt
 , pkg-config, openssl, xz
 , libiconv
@@ -58,7 +58,7 @@ in stdenv.mkDerivation (finalAttrs: {
 
  NIX_LDFLAGS = toString (
  # when linking stage1 libstd: cc: undefined reference to `__cxa_begin_catch'
- optional (stdenv.isLinux && !withBundledLLVM) "--push-state --as-needed -lstdc++ --pop-state"
+ optional (stdenv.isLinux && !withBundledLLVM) "--push-state --as-needed -L${llvmPackages.libcxx}/lib -lc++ --pop-state"
  ++ optional (stdenv.isDarwin && !withBundledLLVM) "-lc++ -lc++abi"
  ++ optional stdenv.isDarwin "-rpath ${llvmSharedForHost}/lib");
 
@@ -158,6 +158,9 @@ in stdenv.mkDerivation (finalAttrs: {
  ] ++ optionals (stdenv.isDarwin && stdenv.isx86_64) [
  # https://github.com/rust-lang/rust/issues/92173
  "--set rust.jemalloc"
+ ] ++ optionals (stdenv.targetPlatform.useLLVM or false) [
+ # https://github.com/NixOS/nixpkgs/issues/311930
+ "--llvm-libunwind=${if withBundledLLVM then "in-tree" else "system"}"
  ];
 
  # if we already have a rust compiler for build just compile the target std
@@ -182,7 +185,7 @@ in stdenv.mkDerivation (finalAttrs: {
  python ./x.py --keep-stage=0 --stage=1 install library/std
  mkdir -v $out/bin $doc $man
  ln -s ${rustc.unwrapped}/bin/{rustc,rustdoc} $out/bin
- ln -s ${rustc.unwrapped}/lib/rustlib/{manifest-rust-std-,}${stdenv.hostPlatform.rust.rustcTargetSpec} $out/lib/rustlib/
+ ln -sf ${rustc.unwrapped}/lib/rustlib/{manifest-rust-std-,}${stdenv.hostPlatform.rust.rustcTargetSpec} $out/lib/rustlib/
  echo rust-std-${stdenv.hostPlatform.rust.rustcTargetSpec} >> $out/lib/rustlib/components
  lndir ${rustc.doc} $doc
  lndir ${rustc.man} $man
@@ -236,7 +239,17 @@ in stdenv.mkDerivation (finalAttrs: {
 
  buildInputs = [ openssl ]
  ++ optionals stdenv.isDarwin [ libiconv Security ]
- ++ optional (!withBundledLLVM) llvmShared;
+ ++ optional (!withBundledLLVM) llvmShared
+ ++ optional (stdenv.targetPlatform.useLLVM or false && !withBundledLLVM) [
+ llvmPackages.libunwind
+ llvmPackages.libcxx
+ # Hack which is used upstream https://github.com/gentoo/gentoo/blob/master/dev-lang/rust/rust-1.78.0.ebuild#L284
+ (runCommandLocal "libunwind-libgcc" {} ''
+ mkdir -p $out/lib
+ ln -s ${llvmPackages.libunwind}/lib/libunwind.so $out/lib/libgcc_s.so
+ ln -s ${llvmPackages.libunwind}/lib/libunwind.so $out/lib/libgcc_s.so.1
+ '')
+ ];
 
  outputs = [ "out" "man" "doc" ];
  setOutputFlags = false;