rustc: be extremely principled about cross builds

There were a lot of collapsed assumptions about the cross build process
in this derivation, especially with the overloaded meanings of useLLVM.
This diff splits useLLVM into a value for each of the build, host, and
target platforms. We also specify more target-specific configure flags,
including passing through the llvm-config-native binary which is built
in some cross configurations. This also allows us to clean up some hacky
NIX_LDFLAGS code.

This commit was tested with the following build configurations:

* x86_64-freebsd -> x86_64-freebsd -> x86_64-freebsd
* x86_64-freebsd -> x86_64-freebsd -> x86_64-linux
* x86_64-freebsd -> x86_64-linux -> x86_64-linux
* x86_64-freebsd -> x86_64-freebsd -> aarch64-linux
* x86_64-freebsd -> aarch64-linux -> aarch64-linux
* x86_64-linux -> x86_64-linux -> x86_64-linux
* x86_64-linux -> x86_64-linux -> aarch64-linux
* x86_64-linux -> aarch64-linux -> aarch64-linux
* x86_64-linux -> x86_64-linux -> x86_64-freebsd
* x86_64-linux -> x86_64-freebsd -> x86_64-freebsd

pkgs/development/compilers/rust/1_82.nix

@@ -37,7 +37,7 @@ let
       {
         enableSharedLibraries = true;
       }
-      // lib.optionalAttrs (stdenv.targetPlatform.useLLVM or false) {
+      // lib.optionalAttrs (pkgSet.stdenv.targetPlatform.useLLVM or false) {
         # Force LLVM to compile using clang + LLVM libs when targeting pkgsLLVM
         stdenv = pkgSet.stdenv.override {
           allowedRequisites = null;

pkgs/development/compilers/rust/rustc.nix

@@ -25,7 +25,9 @@
 let
   inherit (lib) optionals optional optionalString concatStringsSep;
   inherit (darwin.apple_sdk.frameworks) Security;
-  useLLVM = stdenv.targetPlatform.useLLVM or false;
+  useLLVMTarget = stdenv.targetPlatform.useLLVM or false;
+  useLLVMHost =   stdenv.hostPlatform.useLLVM or false;
+  useLLVMBuild =  llvmSharedForBuild.stdenv.cc.libcxx.isLLVM or false;
 in stdenv.mkDerivation (finalAttrs: {
   pname = "${targetPackages.stdenv.cc.targetPrefix}rustc";
   inherit version;
@@ -65,16 +67,8 @@ in stdenv.mkDerivation (finalAttrs: {
     "${pkgsBuildHost.stdenv.cc.targetPrefix}pkg-config";
 
   NIX_LDFLAGS = toString (
-       # when linking stage1 libstd: cc: undefined reference to `__cxa_begin_catch'
-       # This doesn't apply to cross-building for FreeBSD because the host
-       # uses libstdc++, but the target (used for building std) uses libc++
-      optional (stdenv.hostPlatform.isLinux && !withBundledLLVM && !stdenv.targetPlatform.isFreeBSD && !useLLVM)
-        "--push-state --as-needed -lstdc++ --pop-state"
-    ++ optional (stdenv.hostPlatform.isLinux && !withBundledLLVM && !stdenv.targetPlatform.isFreeBSD && useLLVM)
-        "--push-state --as-needed -L${llvmPackages.libcxx}/lib -lc++ -lc++abi -lLLVM-${lib.versions.major llvmPackages.llvm.version} --pop-state"
-    ++ optional (stdenv.hostPlatform.isDarwin && !withBundledLLVM) "-lc++ -lc++abi"
-    ++ optional stdenv.hostPlatform.isFreeBSD "-rpath ${llvmPackages.libunwind}/lib"
-    ++ optional stdenv.hostPlatform.isDarwin "-rpath ${llvmSharedForHost.lib}/lib");
+       optional (stdenv.buildPlatform.isDarwin && !withBundledLLVM) "-lc++ -lc++abi"
+    ++ optional stdenv.buildPlatform.isDarwin "-rpath ${llvmSharedForHost.lib}/lib");
 
   # Increase codegen units to introduce parallelism within the compiler.
   RUSTFLAGS = "-Ccodegen-units=10";
@@ -153,13 +147,15 @@ in stdenv.mkDerivation (finalAttrs: {
   ] ++ optionals (!withBundledLLVM) [
     "--enable-llvm-link-shared"
     "${setBuild}.llvm-config=${llvmSharedForBuild.dev}/bin/llvm-config"
-    "${setHost}.llvm-config=${llvmSharedForHost.dev}/bin/llvm-config"
-    "${setTarget}.llvm-config=${llvmSharedForTarget.dev}/bin/llvm-config"
-  ] ++ optionals fastCross [
+  ] ++ (if (llvmShared.stdenv.hostPlatform == llvmShared.stdenv.buildPlatform) then [
+    "${setHost}.llvm-config=${llvmShared.dev}/bin/llvm-config"
+  ] else [
+    "${setHost}.llvm-config=${llvmShared.dev}/bin/llvm-config-native"
+  ]) ++ optionals fastCross [
     # Since fastCross only builds std, it doesn't make sense (and
     # doesn't work) to build a linker.
     "--disable-llvm-bitcode-linker"
-  ] ++ optionals (stdenv.targetPlatform.isLinux && !(stdenv.targetPlatform.useLLVM or false)) [
+  ] ++ optionals (stdenv.targetPlatform.isLinux && !useLLVMTarget) [
     "--enable-profiler" # build libprofiler_builtins
   ] ++ optionals stdenv.buildPlatform.isMusl [
     "${setBuild}.musl-root=${pkgsBuildBuild.targetPackages.stdenv.cc.libc}"
@@ -172,9 +168,12 @@ in stdenv.mkDerivation (finalAttrs: {
   ] ++ optionals (stdenv.hostPlatform.isDarwin && stdenv.hostPlatform.isx86_64) [
     # https://github.com/rust-lang/rust/issues/92173
     "--set rust.jemalloc"
-  ] ++ optionals (useLLVM && !stdenv.targetPlatform.isFreeBSD) [
+  ] ++ [
     # https://github.com/NixOS/nixpkgs/issues/311930
-    "--llvm-libunwind=${if withBundledLLVM then "in-tree" else "system"}"
+    "${setBuild}.llvm-libunwind=${if (!useLLVMBuild || stdenv.buildPlatform.isFreeBSD) then "no" else if withBundledLLVM then "in-tree" else "system"}"
+    "${setHost}.llvm-libunwind=${if (!useLLVMHost || stdenv.hostPlatform.isFreeBSD) then "no" else if withBundledLLVM then "in-tree" else "system"}"
+    "${setTarget}.llvm-libunwind=${if (!useLLVMTarget || stdenv.targetPlatform.isFreeBSD) then "no" else if withBundledLLVM then "in-tree" else "system"}"
+  ] ++ optionals (withBundledLLVM && useLLVMHost) [
     "--enable-use-libcxx"
   ];
 
@@ -239,7 +238,7 @@ in stdenv.mkDerivation (finalAttrs: {
     [source.vendored-sources]
     directory = "vendor"
     EOF
-  '' + lib.optionalString (stdenv.hostPlatform.isFreeBSD) ''
+  '' + lib.optionalString (stdenv.buildPlatform.isFreeBSD) ''
     # lzma-sys bundles an old version of xz that doesn't build
     # on modern FreeBSD, use the system one instead
     substituteInPlace src/bootstrap/src/core/build_steps/tool.rs \
@@ -257,12 +256,19 @@ in stdenv.mkDerivation (finalAttrs: {
     file python3 rustc cmake
     which libffi removeReferencesTo pkg-config xz
   ]
+  # splicing isn't working here - saying llvmPackages.libcxx here does not give you libcxx for the build platform.
+  # e.g. if you're doing linux -> freebsd -> freebsd cross, the libcxx here would be linked against freebsd libc.so.7
+    ++ optionals (!withBundledLLVM && useLLVMBuild) [
+      llvmSharedForBuild.stdenv.cc.libcxx
+      pkgsBuildBuild.llvmPackages.libunwind
+    ] ++ optional (!withBundledLLVM) llvmSharedForBuild.lib
     ++ optionals fastCross [ lndir makeWrapper ];
 
   buildInputs = [ openssl ]
     ++ optionals stdenv.hostPlatform.isDarwin [ libiconv Security zlib ]
     ++ optional (!withBundledLLVM) llvmShared.lib
-    ++ optional (useLLVM && !withBundledLLVM && !stdenv.targetPlatform.isFreeBSD) [
+    ++ optional (!withBundledLLVM && (useLLVMHost || useLLVMTarget)) llvmPackages.libcxx
+    ++ optionals (useLLVMHost && !withBundledLLVM && !stdenv.hostPlatform.isFreeBSD) [
       llvmPackages.libunwind
       # Hack which is used upstream https://github.com/gentoo/gentoo/blob/master/dev-lang/rust/rust-1.78.0.ebuild#L284
       (runCommandLocal "libunwind-libgcc" {} ''
@@ -305,7 +311,7 @@ in stdenv.mkDerivation (finalAttrs: {
 
   passthru = {
     llvm = llvmShared;
-    inherit llvmPackages;
+    inherit llvmPackages llvmSharedForBuild llvmSharedForHost llvmSharedForTarget;
     inherit (rustc) tier1TargetPlatforms targetPlatforms badTargetPlatforms;
     tests = {
       inherit fd ripgrep wezterm;