rust: cargo: Use rustc and cargo built on Build for cross-compiling

pkgs/development/compilers/rust/1_72.nix

@@ -57,4 +57,4 @@ import ./default.nix {
   rustcPatches = [ ];
 }
 
-(builtins.removeAttrs args [ "pkgsBuildTarget" "pkgsBuildBuild" "pkgsBuildHost" "llvmPackages_16" "llvm_16"])
+(builtins.removeAttrs args [ "pkgsBuildTarget" "pkgsBuildHost" "llvmPackages_16" "llvm_16"])

pkgs/development/compilers/rust/cargo_cross.nix

@@ -0,0 +1,14 @@
+{ runCommand, stdenv, lib, pkgsBuildBuild, makeShellWrapper, rustc, ... }:
+
+runCommand "${stdenv.targetPlatform.config}-cargo-${lib.getVersion pkgsBuildBuild.cargo}" {
+  # Use depsBuildBuild or it tries to use target-runtimeShell
+  depsBuildBuild = [ makeShellWrapper ];
+
+  inherit (pkgsBuildBuild.cargo) meta;
+} ''
+  mkdir -p $out/bin
+  ln -s ${pkgsBuildBuild.cargo}/share $out/share
+
+  makeWrapper "${pkgsBuildBuild.cargo}/bin/cargo" "$out/bin/cargo" \
+    --prefix PATH : "${rustc}/bin"
+ ''

pkgs/development/compilers/rust/default.nix

@@ -15,12 +15,15 @@
 , buildPackages
 , newScope, callPackage
 , CoreFoundation, Security, SystemConfiguration
+, pkgsBuildBuild
 , makeRustPlatform
 }:
 
 let
   # Use `import` to make sure no packages sneak in here.
   lib' = import ../../../build-support/rust/lib { inherit lib; };
+  # Allow faster cross compiler generation by reusing Build artifacts
+  fastCross = (stdenv.buildPlatform == stdenv.hostPlatform) && (stdenv.hostPlatform != stdenv.targetPlatform);
 in
 {
   lib = lib';
@@ -48,7 +51,10 @@ in
       # Like `buildRustPackages`, but may also contain prebuilt binaries to
       # break cycle. Just like `bootstrapTools` for nixpkgs as a whole,
       # nothing in the final package set should refer to this.
-      bootstrapRustPackages = self.buildRustPackages.overrideScope (_: _:
+      bootstrapRustPackages = if fastCross
+      then pkgsBuildBuild.rustPackages
+      else
+        self.buildRustPackages.overrideScope (_: _:
         lib.optionalAttrs (stdenv.buildPlatform == stdenv.hostPlatform)
           (selectRustPackage buildPackages).packages.prebuilt);
       bootRustPlatform = makeRustPlatform bootstrapRustPackages;
@@ -61,7 +67,7 @@ in
         version = rustcVersion;
         sha256 = rustcSha256;
         inherit enableRustcDev;
-        inherit llvmShared llvmSharedForBuild llvmSharedForHost llvmSharedForTarget llvmPackages;
+        inherit llvmShared llvmSharedForBuild llvmSharedForHost llvmSharedForTarget llvmPackages fastCross;
 
         patches = rustcPatches;
 
@@ -72,11 +78,11 @@ in
         inherit Security;
         inherit (self.buildRustPackages) rustc;
       };
-      cargo = self.callPackage ./cargo.nix {
+      cargo = if (!fastCross) then self.callPackage ./cargo.nix {
         # Use boot package set to break cycle
         rustPlatform = bootRustPlatform;
         inherit CoreFoundation Security;
-      };
+      } else self.callPackage ./cargo_cross.nix {};
       cargo-auditable = self.callPackage ./cargo-auditable.nix { };
       cargo-auditable-cargo-wrapper = self.callPackage ./cargo-auditable-cargo-wrapper.nix { };
       clippy = self.callPackage ./clippy.nix {

pkgs/development/compilers/rust/rustc.nix

@@ -15,6 +15,10 @@
 , wezterm
 , firefox
 , thunderbird
+# This only builds std for target and reuses the rustc from build.
+, fastCross
+, lndir
+, makeWrapper
 }:
 
 let
@@ -87,13 +91,13 @@ in stdenv.mkDerivation rec {
     # (build!=target): When cross-building a compiler we need to add
     # the build platform as well so rustc can compile build.rs
     # scripts.
-    ] ++ optionals (stdenv.buildPlatform != stdenv.targetPlatform) [
+    ] ++ optionals (stdenv.buildPlatform != stdenv.targetPlatform && !fastCross) [
       (rust.toRustTargetSpec stdenv.buildPlatform)
 
     # (host!=target): When building a cross-targeting compiler we
     # need to add the host platform as well so rustc can compile
     # build.rs scripts.
-    ] ++ optionals (stdenv.hostPlatform != stdenv.targetPlatform) [
+    ] ++ optionals (stdenv.hostPlatform != stdenv.targetPlatform && !fastCross) [
       (rust.toRustTargetSpec stdenv.hostPlatform)
     ])}"
 
@@ -132,6 +136,37 @@ in stdenv.mkDerivation rec {
     "--set rust.jemalloc"
   ];
 
+  # if we already have a rust compiler for build just compile the target std
+  # library and reuse compiler
+  buildPhase = if fastCross then "
+    runHook preBuild
+
+    mkdir -p build/${rust.toRustTargetSpec stdenv.hostPlatform}/stage0-{std,rustc}/${rust.toRustTargetSpec stdenv.hostPlatform}/release/
+    ln -s ${rustc}/lib/rustlib/${rust.toRustTargetSpec stdenv.hostPlatform}/libstd-*.so build/${rust.toRustTargetSpec stdenv.hostPlatform}/stage0-std/${rust.toRustTargetSpec stdenv.hostPlatform}/release/libstd.so
+    ln -s ${rustc}/lib/rustlib/${rust.toRustTargetSpec stdenv.hostPlatform}/librustc_driver-*.so build/${rust.toRustTargetSpec stdenv.hostPlatform}/stage0-rustc/${rust.toRustTargetSpec stdenv.hostPlatform}/release/librustc.so
+    ln -s ${rustc}/bin/rustc build/${rust.toRustTargetSpec stdenv.hostPlatform}/stage0-rustc/${rust.toRustTargetSpec stdenv.hostPlatform}/release/rustc-main
+    touch build/${rust.toRustTargetSpec stdenv.hostPlatform}/stage0-std/${rust.toRustTargetSpec stdenv.hostPlatform}/release/.libstd.stamp
+    touch build/${rust.toRustTargetSpec stdenv.hostPlatform}/stage0-rustc/${rust.toRustTargetSpec stdenv.hostPlatform}/release/.librustc.stamp
+    python ./x.py --keep-stage=0 --stage=1 build library/std
+
+    runHook postBuild
+  " else null;
+
+  installPhase = if fastCross then ''
+    runHook preInstall
+
+    python ./x.py --keep-stage=0 --stage=1 install library/std
+    mkdir -v $out/bin $doc $man
+    makeWrapper ${rustc}/bin/rustc $out/bin/rustc --add-flags "--sysroot $out"
+    makeWrapper ${rustc}/bin/rustdoc $out/bin/rustdoc --add-flags "--sysroot $out"
+    ln -s ${rustc}/lib/rustlib/{manifest-rust-std-,}${rust.toRustTargetSpec stdenv.hostPlatform} $out/lib/rustlib/
+    echo rust-std-${rust.toRustTargetSpec stdenv.hostPlatform} >> $out/lib/rustlib/components
+    lndir ${rustc.doc} $doc
+    lndir ${rustc.man} $man
+
+    runHook postInstall
+  '' else null;
+
   # The bootstrap.py will generated a Makefile that then executes the build.
   # The BOOTSTRAP_ARGS used by this Makefile must include all flags to pass
   # to the bootstrap builder.
@@ -179,7 +214,8 @@ in stdenv.mkDerivation rec {
   nativeBuildInputs = [
     file python3 rustc cmake
     which libffi removeReferencesTo pkg-config xz
-  ];
+  ]
+    ++ optionals fastCross [ lndir makeWrapper ];
 
   buildInputs = [ openssl ]
     ++ optionals stdenv.isDarwin [ libiconv Security ]
@@ -188,7 +224,7 @@ in stdenv.mkDerivation rec {
   outputs = [ "out" "man" "doc" ];
   setOutputFlags = false;
 
-  postInstall = lib.optionalString enableRustcDev ''
+  postInstall = lib.optionalString (enableRustcDev && !fastCross) ''
     # install rustc-dev components. Necessary to build rls, clippy...
     python x.py dist rustc-dev
     tar xf build/dist/rustc-dev*tar.gz