Refactor nix flake to allow cross compiling

flake.nix

@@ -8,43 +8,77 @@
   };
 
   outputs = { self, nixpkgs, utils, naersk, rust-overlay }:
-    utils.lib.eachDefaultSystem (system:
-      let
+    let
+      supportedSystems = [ "x86_64-linux" "aarch64-linux" ];
+      buildTargetsBySystem = {
+        "x86_64-linux" = "x86_64-unknown-linux-musl";
+        "aarch64-linux" = "aarch64-unknown-linux-musl";
+      };
 
-        pkgs = import nixpkgs {
-          inherit system;
-          overlays = [ (import rust-overlay) ];
-        };
+      eachCrossSystem = callback: (builtins.foldl'
+        (overall: buildSystem: overall // {
+          ${buildSystem} = (builtins.foldl'
+            (inner: hostSystem: inner // {
+              "cross-${hostSystem}" = callback buildSystem hostSystem;
+            })
+            { default = callback buildSystem buildSystem; }
+            supportedSystems
+          );
+        })
+        {}
+        supportedSystems
+      );
 
-        toolchain = pkgs.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml;
+      mkPkgs = buildSystem: hostSystem: import nixpkgs {
+        system = buildSystem;
+        overlays = [
+          (import rust-overlay)
+          (final: prev: {
+            rust-toolchain = prev.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml;
+          })
+        ];
+      } // (if buildSystem == hostSystem then {} else {
+        # The nixpkgs cache doesn't have any packages where cross-compiling has
+        # been enabled, even if the target platform is actually the same as the
+        # build platform (and therefore it's not really cross-compiling). So we
+        # only set up the cross-compiling config if the target platform is
+        # different.
+        crossSystem.config = hostSystem;
+      });
 
+      mkRustEnv = pkgs: {
+        OPENSSL_STATIC = "1";
+        OPENSSL_LIB_DIR = "${pkgs.pkgsStatic.openssl.out}/lib";
+        OPENSSL_INCLUDE_DIR = "${pkgs.pkgsStatic.openssl.dev}/include";
+        CARGO_BUILD_TARGET = buildTargetsBySystem.${pkgs.stdenv.hostPlatform.system};
+        CARGO_BUILD_RUSTFLAGS = "-C target-feature=+crt-static";
+      };
+    in
+    {
+      packages = eachCrossSystem (buildSystem: hostSystem: let
+        pkgs = mkPkgs buildSystem hostSystem;
         naersk-lib = pkgs.callPackage naersk {
-          cargo = toolchain;
-          rustc = toolchain;
+          cargo = pkgs.rust-toolchain;
+          rustc = pkgs.rust-toolchain;
         };
-
-        opensslEnv = {
-          OPENSSL_STATIC = "1";
-          OPENSSL_LIB_DIR = "${pkgs.pkgsStatic.openssl.out}/lib";
-          OPENSSL_INCLUDE_DIR = "${pkgs.pkgsStatic.openssl.dev}/include";
-        };
-
+        rustEnv = mkRustEnv pkgs;
       in
-      {
-        defaultPackage = naersk-lib.buildPackage ({
+        naersk-lib.buildPackage ({
           src = ./.;
           doCheck = false;
           nativeBuildInputs = [ pkgs.pkgsStatic.stdenv.cc ];
-          CARGO_BUILD_TARGET = "x86_64-unknown-linux-musl";
-          CARGO_BUILD_RUSTFLAGS = "-C target-feature=+crt-static";
-        } // opensslEnv);
+        } // rustEnv)
+      );
 
-        devShell = pkgs.mkShell ({
+      devShells = eachCrossSystem (buildSystem: hostSystem: let
+        pkgs = mkPkgs buildSystem hostSystem;
+        rustEnv = mkRustEnv pkgs;
+      in
+        pkgs.mkShell ({
           nativeBuildInputs = [
             pkgs.stdenv.cc
             pkgs.openssl
-            toolchain
-
+            pkgs.rust-toolchain
             pkgs.nmap # ncat
           ];
           shellHook = ''
@@ -54,6 +88,6 @@
               export DOMANI_CONFIG_PATH=config-dev.yml
             fi
           '';
-        } // opensslEnv);
-      });
+        } // rustEnv));
+    };
 }

rust-toolchain.toml

@@ -1,4 +1,7 @@
 [toolchain]
 channel = "nightly-2023-07-23"
 components = [ "rustfmt", "rustc-dev", "clippy", "cargo" ]
-targets = [ "x86_64-unknown-linux-musl" ]
+targets = [
+    "x86_64-unknown-linux-musl",
+    "aarch64-unknown-linux-musl"
+]